Plan 2 - Prescott Cover

Watson Lake Reservoir Management Plan

November 2020

Prepared for

Prepared by

4600 E. Washington Street, Suite 600

Phoenix, Arizona 85034

Public Works Department

433 N. Virginia Street

Prescott, AZ 86301

Watson Lake Reservoir Management Plan

City of Prescott

Public Works Department

Prescott, Arizona

Submitted to:

City of Prescott

Prescott, Arizona

Submitted by:

Wood Environment & Infrastructure Solutions, Inc.

Phoenix, Arizona

November 2020

Project No. 3720156008

City of Prescott Contract # 2015-184

4600 East Washington Street, Suite 600

Phoenix, Arizona 85034-1917

Tel: (602) 733-6000

Fax: (602) 733-6100

www.woodplc.com

November 16, 2020

Project No. 3720156008

Contract No. 2015-184

City of Prescott

Public Works Department

430 North Virginia Street

Prescott, Arizona 86301

Attn: Ben Burns, City of Prescott

Matt Killeen, City of Prescott

Re: Watson Lake Reservoir Management Plan

City of Prescott

Public Works Department

Prescott, Arizona

Section 303(d) of the Clean Water Act (CWA) requires that states compile a list of surface waters that do

not meet applicable water quality standards (WQS). The Arizona Department of Environmental Quality

(ADEQ) then must develop Total Maximum Daily Loads (TMDLs) for waterbodies on the 303(d) List. TMDLs

set the amount of the given pollutant(s) that the waterbody can withstand without creating an impairment

of that surface water’s designated beneficial use(s). The City of Prescott (City) is named in two TMDLs,

identified as:

• Watson Lake TMDL: Total Nitrogen, Dissolved Oxygen (DO), pH & Total Phosphorus Targets -

Finalized February 2015 (Open File Report OFR-14-03)

• Final Upper Granite Creek Watershed Escherichia coli (E. coli) TMDL November 2015 (Open

File Report 14-08)

The Upper Granite Creek Watershed drains into Watson Lake; therefore, it’s impaired status is of concern

and must be taken into consideration when addressing the Watson Lake TMDL.

Since 2015, Wood Environment & Infrastructure Solutions, Inc (Wood), has been supporting the City by

performing a wide range of activities. the Watershed Pollutant Reduction Plan (WPRP) and Watson Lake

Reservoir Lake Management Plan (LMP). The objective of the LMP is to identify scenarios and actions the

City can take to achieve cost-effective reductions in the target pollutants. Significant activities performed

to support this effort include:

• developing a TMDL Action Plan;

• planning and performing lake water, lake sediment, street dirt, and watershed E.coli sampling

and analysis;

• developing a lake sediment profile;

• developing a Watershed Water Quality Model using Loading Simulation Program (LSPC) and

the SIMplified Particulate Transport Model (SIMPTM);

4600 East Washington Street, Suite 600

Phoenix, Arizona 85034-1917

Tel: (602) 733-6000

Fax: (602) 733-6100

www.woodplc.com

• developing a hydrodynamic and water quality model of Watson Lake Reservoir using CE-

QUAL-W2 Version 3.72 computer program (W2); and

• delivering a series of Technical Memos documenting the progress of activities performed.

The culminating effort of the aforementioned activities (as well as several others not listed above), is the

development of this LMP and the Upper Granite Creek Watershed Pollutant Reduction Plan (WPRP). These

Plans do not seek to re-present previous work efforts, but rather provide guidance to the City as a result

of previous activities.

Should you have any questions regarding this draft report, please do not hesitate in contacting the

undersigned.

Respectfully submitted,

Wood Environment & Infrastructure Solutions, Inc.

Reviewed by:

Rebecca Sydnor, PE Seth Jelen, PE, CFM, BCEE, CWRE

Project Manager Principal Engineer

Watson Lake Reservoir Management Plan

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TABLE OF CONTENTS

Page

1.0

 BACKGROUND ................................................................................................................................... 1

1.1 Designated Uses, Recreation, and Impact ................................................................................................. 1

1.2 Water Quality Issues .......................................................................................................................................... 2

1.3 Plan Organization ............................................................................................................................................... 2

2.0 BASIC LIMNOLOGY AND LAKE MANAGEMENT ............................................................................ 2

2.1 Lake Morphometry and Hydrology ............................................................................................................. 3

2.1.1 Climate and Elevation ....................................................................................................................................... 3

2.1.2 Dimensions and Water Levels ........................................................................................................................ 3

2.1.3 Watershed Characteristics and Lake Water Sources .............................................................................. 3

2.2 Physical Factors .................................................................................................................................................... 4

2.2.1 Natural Circulation ............................................................................................................................................. 4

2.2.2 Temperature Stratification and Dissolved Oxygen ................................................................................. 4

2.2.3 Evaporation ........................................................................................................................................................... 5

2.2.4 Residence Time ................................................................................................................................................... 5

2.2.5 Sedimentation ..................................................................................................................................................... 5

2.3 Chemical Composition ...................................................................................................................................... 6

2.3.1 Inorganic Minerals .............................................................................................................................................. 6

2.3.2 Nutrients and Eutrophication ......................................................................................................................... 8

2.3.3 Metals and Other Contaminants ................................................................................................................... 9

2.4 Biological Components ...................................................................................................................................10

2.4.1 Algae and Weeds ............................................................................................................................................. 10

2.4.2 Submerged, Floating, and Emergent Weeds ......................................................................................... 14

2.4.3 Zooplankton ...................................................................................................................................................... 19

2.4.4 Benthos ............................................................................................................................................................... 19

2.4.5 Fish ........................................................................................................................................................................ 20

2.4.6 Other Organisms ............................................................................................................................................. 20

2.5 Lake Monitoring ................................................................................................................................................22

2.5.1 Parameters ......................................................................................................................................................... 22

2.5.2 Monitoring Equipment .................................................................................................................................. 22

2.5.3 Test Methods, Data Handling, and Interpretation ............................................................................... 23

3.0 LAKE MANAGEMENT STRATEGIES FOR WATSON LAKE ...................................................... 24

3.1 Circulation and Aeration. ...............................................................................................................................24

3.2 Types of Aeration ..............................................................................................................................................24

3.2.1 Bottom Diffuse Aeration ............................................................................................................................... 24

3.2.2 Floating Vertical Water Circulators ........................................................................................................... 25

3.2.3 Hypolimnetic Aeration ................................................................................................................................... 26

3.2.4 Nanobubble Aeration .................................................................................................................................... 26

3.3 Nutrient Reduction and Inactivation .........................................................................................................27

3.3.1 Watershed Management .............................................................................................................................. 27

3.3.2 In-lake Methods ............................................................................................................................................... 27

3.3.3 Sediment Removal .......................................................................................................................................... 28

3.4 Submerged Weed Management ................................................................................................................30

3.4.1 Weed Harvesting ............................................................................................................................................. 30

3.4.2 Chemical Applications ................................................................................................................................... 31

3.5 Algae Management ..........................................................................................................................................33

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3.6 Nuisance and Vector Insect Management ..............................................................................................34

3.6.1 Mosquitoes ........................................................................................................................................................ 34

3.6.2 Midge Flies ......................................................................................................................................................... 35

3.6.3 Bacteria ................................................................................................................................................................ 35

3.7 Lake Monitoring ................................................................................................................................................36

3.7.1 Monitoring Equipment .................................................................................................................................. 36

3.7.2 Monitoring Locations ..................................................................................................................................... 36

3.7.3 Recommended Testing ................................................................................................................................. 37

3.8 Integrated Management Approach ...........................................................................................................38

4.0 FIELD REFERENCE GUIDE FOR WATSON LAKE ............................................................................ 39

5.0 REFERENCES ..................................................................................................................................... 39

LIST OF FIGURES

Figure 1 Aquatic nitrogen cycle

Figure 2 Aquatic phosphorus cycle

Figure 3A Oligotrophic lake nutrient dynamics

Figure 3B Eutrophic lake nutrient dynamics

Figure 4 White Amur

Figure 5 Weed harvester

Figure 6 Zooplankton forms

Figure 7 Mosquito life cycle

Figure 8 Mosquito life forms

Figure 9 Aquatic midge life cycle

Figure 10 Midge life forms

Figure 11 Lake sampling equipment

Figure 12 Bottom diffuse aeration

Figure 13 Floating aerator

Figure 14 Hypolimnetic aeration

Figure 15 Hypolimnetic aerator-Speece Cone

Figure 16 Nanobubble aeration configuration

Figure 17 Watermilfoil and coontail

Figure 18 Common blue-green algae of Watson Lake

Figure 19 Sampling locations

LIST OF TABLES

Table 1 Trophic status comparison

Table 2 Beneficial and detrimental effects of aquatic plants

Table 3 Factors impacting blue-green algae blooms

Table 4 Common algaecides

Table 5 Herbicide information

Table 6 Herbicide active ingredients

Table 7 Sampling locations

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APPENDIX

Appendix A Field Reference Guide

ACRONYMS

A&Wc aquatic and wildlife (cold water)

A&We aquatic and wildlife (ephemeral)

A&Wedw aquatic and wildlife (effluent-dependent water)

A&Ww aquatic and wildlife (warm water)

AAC Arizona Administrative Code

ADEQ Arizona Department of Environmental Quality

ADOT Arizona Department of Transportation

AF acre-feet

AgI agricultural irrigation

AgL agricultural livestock watering

AMA Active Management Area

APHA American Public Health Association

AZGF Arizona Game and Fish Department

City City of Prescott

CWA Clean Water Act

DO dissolved oxygen

DWS domestic water source

E.coli Escherichia coli

FBC full-body contact

FC fish consumption

LMP Watson Lake Reservoir Lake Management Plan

PBC partial-body contact

TMDL Total Maximum Daily Loads

WEF Water Environment Federation

WIP Watershed Improvement Plan for the Upper Granite Creek Watershed

WQS water quality standards

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1.0 BACKGROUND

Section 303(d) of the Clean Water Act (CWA) requires that states compile a list of surface waters that do

not meet applicable water quality standards (WQS). Total Maximum Daily Loads (TMDLs) must be

developed for waterbodies on this list (the 303(d) List). TMDLs set the amount of the given pollutant(s)

that the waterbody can withstand without creating an impairment of that surface water’s designated

beneficial use(s).

In 2004, Watson Lake Reservoir (Lake) was listed as water quality impaired for high nitrogen, low dissolved

oxygen (DO), and high pH; subsequent TMDL development added phosphorus loading to the Lake’s

pollutants of concern. Additionally, Granite Creek was listed for low DO in 2004, and was listed for

Escherichia coli (E. coli) bacteria in 2010. Miller Creek was also listed for E. coli at that time. Butte Creek and

Manzanita Creek have since been added in the 2012/14 303(d) list, also for E. coli. Aspen Creek, North

Fork of Miller Creek, Banning Creek, Government Canyon, Slaughterhouse Gulch, North fork of Granite

Creek, and two unnamed tributaries (AZ15060202-3333 [known locally as the Virginia St Wash], and

AZ15060202-3313 [known locally as Ackers East]) were listed for E. coli in 2016. As each of these creeks

drains into Granite Creek which ultimate drains into Watson Lake. Because of this connection, their

impaired status is of concern and must be factored into this Watson Lake Reservoir Lake Management

Plan (LMP).

The Arizona Department of Environmental Quality (ADEQ) has since finalized a TMDL document

addressing nutrients within Watson Lake Reservoir. As a stakeholder in this TMDL, the City of Prescott

(City) is required to implement measures to reduce the amount of these pollutants of concern entering

the Lake from the City’s stormwater discharges. This LMP is intended to comply with that requirement.

1.1 Designated Uses, Recreation, and Impact

ADEQ develops WQS for surface waters of the State, including lakes and reservoirs, and conducts

monitoring to determine whether or not those standards are being met. These WQS are codified in Title

18, Chapter 11 of the Arizona Administrative Code (AAC.) and vary across the state depending on each

waterbody’s designated beneficial uses. Designated uses, as promulgated in AAC R18-11-104, are: full-

body contact (FBC), partial-body contact (PBC), domestic water source (DWS), fish consumption (FC),

aquatic and wildlife (cold water) (A&Wc), aquatic and wildlife (warm water) (A&Ww), aquatic and wildlife

(ephemeral) (A&We), aquatic and wildlife (effluent-dependent water) (A&Wedw), agricultural irrigation

(AgI), and agricultural livestock watering (AgL).

Watson Lake has the following designated uses: A&Ww, FBC, FC, AgI, and AgL. The Lake provides a

number of recreational uses, including boating, fishing, hiking, and bird watching; however; full body

contact recreation is not permitted.

Of the beneficial uses for which the Lake has been designated, two are considered recreational in purpose,

FC and FBC. According to AAC R18-11-101.20, FC refers to “the use of a surface water by humans for

harvesting aquatic organisms for consumption.” AAC R18-11-101.21 characterizes FBC, or full-body

contact, as “the use of a surface water for swimming or other recreational activity that causes the human

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body to come into direct contact with the water to the point of complete submergence.” It should be

noted that while the Lake retains the FBC designation, the City continues to post the Lake with “No

swimming” signs.

Recreational designated uses are typically applied to protect public health (i.e., against human illness from

ingestion of or immersion in water). However, by their very nature, recreational designated uses also result

in human impact on the environment, and the 2012 Watershed Improvement Plan for the Upper Granite

Creek Watershed (WIP) found high levels of recreation to be a source of elevated E. coli in the watershed.

The presence of algae and submerged aquatic weeds directly impact the recreational uses, and low DO

could be impacting game fish populations within the Lake. Key characteristics that affect the degree of

impairment of the Lake include, but are not limited to, lake shape (morphometry), shoreline complexity,

depth, and water level fluctuations, thermal stratification, historic and current nutrient loading, submerged

weed and algae growth and decay, and composition and quantity of bed sediment.

1.2 Water Quality Issues

Watson Lake Reservoir has elevated concentrations of nitrogen and phosphorus that stimulate excessive

algae and submerged weed growth during the warmer months. Uptake of carbon dioxide from the water

by the aquatic plants results in significant pH increases. Death, settling, and ultimately decomposition of

the biomass exert a heavy oxygen demand in the deeper waters, with oxygen concentrations approaching

zero at the lake bottom. The Lake is currently under a TMDL for low DO near the benthic layer, pH

excursions above the 9.0 SU limit, and excessive nutrient concentrations (total nitrogen and total

phosphorus). Aside from these issues, the water quality at Watson Lake is suitable for A&Ww, FBC, FC, AgI,

and AgL activities.

1.3 Plan Organization

This LMP is divided into two main parts. Section 2.0 presents physical, chemical, and biological

information on lakes and reservoirs, as well as specific information pertaining to Watson Lake Reservoir.

Section 3.0 provides details for those lake management practices that the City can implement to achieve

compliance with the above-referenced TMDL. Section 4.0 introduces the Field Reference Guide

(Attachment A) which lake operators can utilize/reference when performing Lake activities. Section 5.0

contains references utilized to develop this Plan. All figures mentioned within this Plan are contained in

the Figures section following Section 5.0.

2.0 BASIC LIMNOLOGY AND LAKE MANAGEMENT

This section provides information on basic limnological principles and processes that impact water quality

of lakes and reservoirs. These factors and processes must be reasonably understood to provide lake

owners and operators with a knowledge base upon which they may depend to identify and analyze issues,

identify and evaluate potential solutions, and select the optimal management methods that can improve

water quality and preserve the designated uses of the surface water.

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2.1 Lake Morphometry and Hydrology

The morphometric (shape and structure) characteristics of the lake and the hydrology of the watershed

exert a strong influence on the aesthetic and environmental character of a reservoir. In situations of

dammed confluences, such as Watson Lake, the lake morphology is dependent on the existing geologic

structure. Steep, smooth rock walls near the dam allow for rapid water depth increases, whereas the

opposite end of the reservoir has a much shallower grade. This allows for a variety of biological

production, both flora and fauna, throughout the lake.

2.1.1 Climate and Elevation

Watson Lake is located in Prescott, Arizona at an elevation of 5,100 feet above sea level. The weather

ranges between 50 and 90°F during summer months and between 20 and 60°F in the winter months.

Strong summer storms contribute to annual precipitation of approximately 13-19 inches, with an average

annual snowfall of 20 inches . Although there is some snowfall in the area, Watson Lake is not known to

freeze during the winter.

2.1.2 Dimensions and Water Levels

As identified in past studies, Watson Lake occupies 192 surface acres, with a volume of 3,019 acre-feet.

When full, Lake elevation is 5161 ft. The average depth of the Lake is 4.8 meters (15.7 feet).

In general, deep-water lakes (as defined by AAC R18-11-101.16 as having an average depth of more than

6 meters) have a greater assimilative capacity and are less prone to ecosystem imbalances caused by

environmental perturbations than are shallow lakes. In shallower lakes, such as Watson Lake, aesthetic

and water quality degradation appear at an accelerated rate, or in isolated coves within the waterbody. In

terms of plant growth, shallower lakes are usually more productive than deeper lakes because more water

is above the photic zone. The photic zone is that portion of the water column receiving sufficient light for

supporting growth of aquatic plant life. When a lake has a relatively high proportion of the water column

in the photic zone, the amount of organic material accumulated in the sediment is increased. Deeper lakes

can provide a larger sink for deposition of solids arising from external sources as erosion, storm water

runoff, and feed waters, and from internal sources as algae and decaying animals. Shallower lakes do not

have as sufficient sink capacity.

2.1.3 Watershed Characteristics and Lake Water Sources

The Granite Creek watershed upstream of Watson Lake comprises an area of approximately 40 square

miles and varies in elevation between 5,100 and 7,100 feet. Land uses within the watershed include

commercial, residential, recreation, and some grazing.

Source water to Watson Lake includes other surface waters such as Granite Creek, irrigation return water,

groundwater, domestic (potable) water, urban stormwater runoff, and possibly reclaimed and gray water.

Source water is known to include leachate from unsewered areas and leakage from the sanitary sewer

system. Surface, storm, septic leachate, and reclaimed water can contain large amounts of nitrogen and

phosphorus. The Lake is also an area-wide flood water retention and conveyance feature and receives

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direct runoff from park grounds and facilities arising from precipitation or irrigation. Uncontrolled, such

drainage can result in stagnant pools of water, shoreline erosion, and accumulation of sediment in the

lake bottom. For many lakes, the ideal feed water is often not available due to expense or regulation.

Management strategies must be based on the water quality anticipated for the impoundment.

2.2 Physical Factors

Climatology characteristics, especially temperature and wind, can exert an influence on natural

phenomenon and lake processes.

2.2.1 Natural Circulation

Circulation of the lake water is one of the most important components of a lake management strategy.

Horizontal circulation is advantageous because it eliminates problems associated with stagnation of water,

particularly in isolated coves. Circulation can result from natural wind-induced lake currents and wave

action or increased artificially by expelling pumped water vertically at the lake surface. Good circulation

means algae mats are less likely to form, floating debris are less likely to create unsightly accumulations,

and insect infestations are minimized. ,

Vertical circulation can also occur naturally. Most lakes in the southwest are monomictic. That is, the

phenomenon of natural vertical mixing (“lake turn-over”) is initiated only during one period of the year –

usually in the autumn season. During the summer, surface water heats faster and warmer than deeper

water. The different temperatures result in warmer, less-dense water “floating” on top of denser, colder

water. That difference inhibits vertical mixing of the water layers and transfer of dissolved gases and

chemicals between the layers. This condition is called thermal stratification. As autumn approaches,

surface waters cool and eventually equal the temperature of the bottom waters. Following autumn lake

turn-over, dissolved gases and chemicals freely mix in the water column. Watson Lake experiences rapid

vertical mixing between September and October.

2.2.2 Temperature Stratification and Dissolved Oxygen

Most oxygen in a lake is derived from algal photosynthesis and absorption of oxygen from the

atmosphere. Both occur at or near the surface of a lake where it is in contact with the atmosphere and

where sufficient light exists to support algae growth.

When a lake becomes thermally stratified, oxygen is inhibited from mixing into the deeper waters because

of the thermal stratification. Respiration and decomposition in the deep waters can deplete all or most of

the oxygen faster than it can mix from the surface waters. This is especially true at night when oxygen is

not replenished by photosynthesis. Respiration and decomposition rates are also greatest during the

summer when water temperatures are the highest. This can lead to death of fish and zooplankton

invertebrates and is a frequent cause of summer fish kills.

Loss of oxygen in the deep waters and especially above the sediment can result in the formation of

ammonia and sulfide gases that are toxic to aquatic organisms. Anaerobic organisms produce the gases

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as a by-product of respiration. Additionally, the generated ammonia becomes a growth nutrient that can

increase the rate of algae growth.

Thus, it is often beneficial to have a continuously mixed or at least well-oxygenated water column. This

prevents anoxic conditions, diminishes kills of aquatic organisms and ammonia and sulfide releases to the

water, and limits release from the sediment of phosphorus and other undesirable chemical constituents,

thus limiting nutrient recycling.

2.2.3 Evaporation

The evaporation of surface water can play a major role in water loss from a reservoir or lake. However,

because of the climate, the evaporation rate in Prescott is not considered a significant concern.

2.2.4 Residence Time

The hydraulic residence time is the flushing rate of a lake; i.e, approximately how long it takes for water to

move from inflow to outfall, assuming plug flow. Outflows can be natural as to a river or stream or man-

made as pumped removal for irrigation. There is no ideal retention time because of the myriad of physical,

chemical, and biological factors that influence a lake system. In general, relatively long retention times

permit development of more balanced ecosystems better able to assimilate or resist short-term changes

in water quality. However, short duration retention times reduce sedimentation rates and accumulation of

nutrients. From December through February Watson Lake shows weekly inflows about 20 percent; the

last week of February has one large inflow “spike” of almost 120 percent (enough to more than flush the

reservoir) followed by declining inflows that end mid-April. There are additional small inflows mid-July to

mid-August. The City can store up to 4,600 acre-feet (AF) each year in Watson Lake and recharges

between 2,100 to 2,800 AF each year between April 1 and Nov 30. The amount of water recharged

depends upon how much precipitation has fallen. The recharge allowance is up to 3,861.26 AF; however,

this typically is not realized. In addition to the average recharge from Watson Lake, Granite Dells Ranch is

allotted up to 375 AF each year; however, they typically average approximately 300 AF.

2.2.5 Sedimentation

Lakes accumulate bottom sediment as they age. A number of factors influence the sedimentation rate.

Lake sediments are composed mainly of clastic material (sediment of clay, silt, and sand sizes), organic

debris, chemical precipitates, or combinations of these. Depending on watershed activities and

characteristics, materials that are detrimental to the ecological balance of the lake - e.g., excessive

quantities of nutrients, heavy metals, pesticides, oil, and certain bacteria - can be deposited in lake

sediment. These materials are potentially available for regeneration into the lake water and must be

considered in any planning to abate lake pollution. Within the uppermost lake sediments, large volumes

of pore water are often present that may have high concentrations of nutrients and other constituents

and enhance the exchange potential with the lake’s water column. Sedimentation can reduce the volume

of water in a reservoir and encourage growth of algae and submerged weeds. During the course of this

project, sediment volume was approximated to be 414 acre-feet (668,393 cubic yards).

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2.2.5.1 Sedimentation Rate

The relative abundance of each of the components listed above depends upon the nature of the local

drainage basin, the climate, and the relative age of a lake. Some other factors include:

 Algae, weed, and zooplankton production in the lake,

 Chemical composition of the lake

 Shoreline erosion

 Atmospheric deposition

 Inflow loading

 Lake morphometry

2.2.5.2 Benefits of Reduction and Removal

The removal of sediment from the lake bottom can be beneficial. Physical removal of sediment can

 Reduce potential for recycling of nutrients,

 Increase lake depth,

 Increase the volume and proportion of water below the photic zone, and

 Reduce rooted aquatic plants and rooting substrate.

Sediment can be removed from lakes by a number of methods including suction dredging, mechanical

dredging, and by utilizing coffer dams to allow for the use of terrestrial earthmoving equipment (see

Section 3.2.2.

2.3 Chemical Composition

2.3.1 Inorganic Minerals

The mineral composition of the lake can impact productivity, oxygen concentrations, and pH (nutrients

and alkalinity); potential metals toxicity (hardness), and irrigation quality (dissolved solids and ion

composition). For Watson Lake, the nutrient composition is the most important component. Some of the

basic parameters are described briefly below.

2.3.1.1 Turbidity

Turbidity refers to the suspended solids and color that limit how deep light penetrates the water.

Common contributors are soil particles and algae. Fish, zooplankton and algae all can respond rapidly to

changes in their environment resulting from fluctuations in turbidity. Increases in turbidity can clog or

irritate gills of fish. For other aquatic fauna, turbidity may cause suffocation, abrasion, susceptibility to

disease, impaired reproduction, and reduced availability of food resources. Higher turbidity can reducing

the photic zone and thus reduce algae growth. This results in less oxygen produced and less food for

zooplankton.

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2.3.1.2 Hardness

Water hardness is a measure of the amount of calcium and magnesium in the water expressed as calcium

carbonate. Hard waters tend to have greater buffering capacity (see alkalinity below) and can combine

with or precipitate dissolved metals rendering them biologically inactive in terms of toxicity. This is

beneficial in terms of protecting the fish and other aquatic organisms from runoff containing wear metal

(engine wear) contaminants. However, the same chemical reaction decreases the efficacy of metal-based

algaecides and requires higher algaecide application dosages.

2.3.1.3 pH and Alkalinity

Alkalinity is a measure of buffering capacity; the ability to prevent sudden changes in pH as a result of

addition of acids or bases. Components of alkalinity include carbon dioxide, bicarbonates, carbonate, and

hydroxide ions. pH is a measure of the hydrogen ion content of the water. The pH scale is based on

acidity of the water and is recorded on a logarithmic scale from 0 to 14. At pH 7 water is considered

neutral; at pH>7 water is considered basic or alkaline, and at pH <7 water is acidic. The components of

alkalinity are in an equilibrium that is pH-dependent, with the following forms dominating at the following

pH ranges:

pH <4 carbon dioxide

pH 4-8 bicarbonate

pH 8-10 bicarbonate & carbonate (typical range for Watson Lake during growing season)

pH>10 hydroxides

Generally, pH and alkalinity are controlled by the chemical composition of the sediments, source waters,

and runoff that enter the lake. Most aquatic organisms prefer the pH range of 6.5 to 9.0. Rapid pH

changes are characteristic of soft waters with poor buffering capacity (alkalinity). Alkaline or hard water

lakes are usually more productive. Algae remove carbon dioxide from the water via photosynthesis and

cause a shift in the alkalinity equilibrium toward bicarbonate and carbonates. The result is an increase in

water pH. This was clearly seen in the during 2016-2017 Watson Lake sampling period which revealed pH

above 10 in some cases.

Extreme changes in pH may cause stress or mortality in aquatic organisms. At low pH, solubility of

otherwise bound metals such as aluminum increases and the metals concentration may become toxic to

some organisms. At high pH, some chemicals as ammonia convert from a nontoxic ionized form to a

toxic un-ionized (gaseous) form. As with hard water lakes, alkaline waters require higher dosages of

copper-based algaecides for successful algae or macrophyte control.

Photosynthesis removes dissolved carbon dioxide from the water. In so doing, it causes a shift in the

carbonate equilibrium toward bicarbonates and carbonates and forces the pH to rise. Thus, reducing algal

photosynthesis assist in lowering pH. In the desert southwest, application of algaecide is a common

approach to slowly and, at least temporarily, reducing the pH in a reservoir.

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2.3.2 Nutrients and Eutrophication

Eutrophication is the process by which a body of water becomes enriched in dissolved nutrients (such as

phosphates) that stimulate the growth of aquatic plant life usually resulting in the depletion of dissolved

oxygen.

2.3.2.1 Nitrogen

Forms and concentrations of nitrogen (nitrite, nitrate, ammonia, and organic nitrogen) in the lake are

controlled by the chemical and biological processes of the nitrogen cycle. Figure 1 depicts the possible

transformations of nitrogen in the environment. Plants primarily utilize ammonia and nitrate; however,

atmospheric nitrogen can be used directly by some forms of cyanobacteria. Organic nitrogen constitutes

the part of the total nitrogen that is incorporated into the plant and algae mass. The cycle is kept going

by a continuous supply of ammonia that is released during the decomposition of plant and animal matter

and of nitrogen that is fixed by cyanobacteria.

Overproduction of ammonia can harm the fishery. Ammonia exists in equilibrium between the ionized

form (ammonium, NH4+) and the un-ionized or gaseous form (ammonia, NH3). The equilibrium is

dictated by water temperature and pH; the warmer and the higher the pH, the greater the production of

the gaseous form. The gaseous form is toxic to aquatic organisms.

2.3.2.2 Phosphorus

Phosphorus is often the nutrient in the least amount in water systems. Because its concentration often

controls the amount of plant and algae growth in the lake, it is often the limiting nutrient. The sources of

phosphorus in lake systems include the decomposition of plant and animal matter from the sediments,

atmospheric deposition, or runoff into the lake (Figure 2). Much of the absorbed phosphorus in a water

system, 20-60 percent, is excreted by plants and algae. Phosphorus cycles very quickly (as little as 1 to 8

minutes) through the algal cells and bacteria that are found in the upper waters. Phosphorus tends to

compartmentalize, with a portion in the epilimnion where small (<70 um) organisms absorb phosphorus

rapidly and where it is deposited rapidly, and a second section where the phosphorus is taken up by

larger plants and animals and remains in the upper waters for a longer period.

Lakes that are not flow through systems, or have very long water detention times, are known as terminal

lakes. While Watson Lake does experience higher turnover during winter months, there are also periods

where there is very little outflow from the Lake. During these times with little outflow, the lake can

temporarily take on some characteristics similar to terminal lakes. These lakes can suffer from nutrient

accumulation over time due to external inputs such as the water source (nutrient rich reclaimed water),

irrigation run off, storm water, as well as dust and dirt from erosion and storm fallout. Efforts should be

taken to minimize the input of both phosphorus and nitrogen containing materials into the lake system.

Discouraging or avoiding activities that attract waterfowl to the lake (due to high nitrogen and

phosphorus in their waste) and controlling the amount of runoff containing ammonia, urea, and

phosphate-based fertilizers discharging to the lake can help reduce the nutrient load.

Watson Lake Reservoir Management Plan

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Prescott, Arizona November 2020 Page 9

2.3.2.3 Sources

The productivity of a lake is usually determined by the amount of plant matter produced, including

macrophytes (submerged and emergent aquatic plants) and algae. Chlorophyll-a measurements are often

used as a surrogate test to estimate the amount of algae in a lake. Lakes and ponds with high nutrient

levels are called eutrophic and the process of nutrient enrichment is referred to as eutrophication. Those

lakes with few plants and algae and relatively low concentrations of nutrients are termed oligotrophic.

Those lakes with intermediate levels of plant growth and nutrients are termed mesotrophic. The general

characteristics of oligotrophic and eutrophic lakes are provided in the table and figures below. The trophic

status of a lake can be quantified by measuring the types and amount of algae present, the degree of

water clarity, and the amount of phosphorus available for growth of algae. The trophic status of a lake will

be an important means of tracking changes in water quality and the degree of success of corrective

actions.

Interactions of chemical and biological factors that determine the trophic status of a lake are listed in

Table 1.

Table 1

Trophic Status Comparison

Condition Eutrophic Oligotrophic

Productivity High Low

Algae density High Low

Nutrient concentrations High Low

Hypolimnion oxygen content Low High

Sediment nutrient release High Low to none

Organic matter High Low

Light transparency Shallow Deep

Macrophyte density High Low

The growth of biological communities is dependent on the concentration of nitrogen and phosphorus,

which are the primary plant nutrients. The flow charts below (Figures 3A and 3B) diagram the difference

in nutrient loads between an oligotrophic lake and a eutrophic lake.

2.3.3 Metals and Other Contaminants

If concentrations become high enough, dissolved metals can become harmful to invertebrates and fish in

lakes and reservoirs or may adversely impact the health of individuals that are in partial contact with the

water. The physiological availability of the metals can be influenced by the hardness of the water.

Generally, in softer water lakes, such as Watson Lake, a lower proportion of some metals in solution will

be associated with bicarbonates and carbonates, making them less susceptible to precipitation and more

available to aquatic organisms than in a harder water lake.

Other constituents of concern include herbicides, pesticides, and synthetic organic contaminants. They are

listed in the surface water quality standards; however, these contaminants have not been reported as

problematic in Watson Lake and, accordingly, are not specifically addressed in this LMP.

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Prescott, Arizona November 2020 Page 10

2.4 Biological Components

2.4.1 Algae and Weeds

Aquatic plants can be found in all types of surface impoundments. They serve an important role in aquatic

ecosystems. They have a dichotomous relationship (benefits and detriments) with the environment that

must be kept in balance in order to preserve the quality of the water systems for ecological and

recreational uses. Some of the benefits and detriments of aquatic vegetation are listed in Table 2.

Table 2

Beneficial and detrimental effects of aquatic plants

Plant Benefits Possible detriments

Planktonic algae

Oxygen production

Zooplankton food (food chain)

Aesthetic deterioration

Post-bloom fish kills

Filamentous algae Oxygen production

Aesthetic deterioration

Odors

Clog irrigation system

Submerged macrophytes

Improve water clarity (nutrient

uptake)

Oxygen production

Fish habitat and food

Physically obstruct lake

Interfere with fishing and boating

Emergent macrophytes

Wildlife habitat and fish

Improve water quality (nutrient

removal)

Aesthetic enhancement

Loss of lake volume

Insect attraction

Waterfowl attraction

Floating macrophytes Aesthetics (flowering plants only)

Reduce light

Physically obstruct lake

Interfere with fishing and boating

2.4.1.1 Importance

Aquatic plants can be broken into two basic forms: algae composed of unicells, colonies, or filaments, and

vascular plants (macrophytes or aquatic weeds) that include forms that are submerged, emergent, or

floating. Their form and growth habitat are important characteristics in their dispersion and possible

management strategies. Aquatic plants provide dissolved oxygen to the water, absorb nutrients, provide

refuge for fish, create habitat for macroinvertebrates, and supply food for some fish, macroinvertebrates,

and waterfowl. Conversely, aquatic weeds can become a nuisance by obstructing boating and fishing

areas and reducing aesthetic value. They can increase water pH and add organic matter to the sediment

and consume dissolved oxygen during decomposition.

2.4.1.2 Forms of Algae

Algae may be one-celled organism (unicells), colonies (multiple cells held together in a matrix, or

filaments (branched or non-branched chains of cells). This affects how they grow, where they develop, and

what management techniques can be successful.

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There are eight taxa or divisions of algae that are common to Arizona lakes and reservoirs.

Cyanobacteria, sometimes called blue-green algae (Cyanophyta): These algae lack differentiation

in cellular organization (prokaryotic). They are typically filamentous or colonial in form, with

mucilaginous sheaths around the cells. Many filamentous forms have specialized vegetative cells

called heterocysts which can fix (absorb) atmospheric nitrogen. Some are known to produce

dangerous neurotoxins.

Green algae (Chlorophyta): This is a diverse group that includes flagellated, amoeboid,

filamentous, and colonial forms. Asexual and sexual reproduction is common. Some filamentous

forms undergo conjugation.

Golden-Brown algae (Chrysophyta): Chrysophyceae have a golden-brown color because of B-

carotein and xanthophyll carotenoid pigments. Most are unicellular, with one flagellum.

Reproduction is predominantly by cell division.

Baccilariophyceae (diatoms): These have highly ornamented silicious cell walls.

Cryptomonads (Cryptophytes): Most cryptophytes are unicellular and motile. Cells are usually

compressed with two equal length flagella. Reproduction is by longitudinal division.

Dinoflagellates (Pyrrophyta): These are unicellular, flagellated algae. Most have a thick cell wall

with spines. The flagellum is usually associated with transverse and longitudinal furrows in the

cell wall.

Euglenoids (Euglenophyta): Many of these organisms are not planktonic forms, but they can

become planktonic when environmental conditions are right. They have no cell wall and often

change shape, may have 1-3 flagella, and are photosynthetic and heterotrophic (consume pre-

formed food particles).

Chara and Nitella (Charaophyta). These two forms of algae appear to be submerged vascular

plants, but are actually advanced forms of algae. Chara (muskgrass) is prevalent in Arizona’s hard

water lakes, often precipitating carbonate on its thallus to become crusty and resistant to

chemical control.

2.4.1.3 Cyanobacteria (Blue-green Algae) and Algal Toxins

Toxin-producing blue-green algae are often a concern in lakes. A number of algae species have been

identified as potential toxin formers including representatives from the following genera: Microcystis,

Nodularia, Nostoc, Oscillatoria, Anabaena, Aphanizomenon, Cylindrospermum, Gloeotrichia, Phormidium,

Schizothrix, Scytonema, Synechocystis, Tolypothrix, and Trichodesmium. Not all species in these genera

produce toxins, and in some cases, only certain strains produce the toxins and only under specific

environmental conditions. In most reported cases, the toxins are associated with mortality of livestock

animals that consume large numbers of the algae under bloom conditions. However, some evidence

indicates the toxins may limit the growth of other algae and zooplankton. Factors affecting cyanobacteria

(blue-green algae) blooms are summarized in Table 3.

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City of Prescott

Prescott, Arizona November 2020 Page 12

Table 3

Factors impacting cyanobacteria algae blooms

Environmental factor Impact on bloom

Nitrogen Low N:P ratio favors blue-greens because heterocyst-forming algae

can grow in low-N conditions

Phosphorus Typically need high concentrations; however, in collected data from

Watson Lake, cyanobacteria appeared to grow in lower concentrations.

Temperature Optimum near 35C for some species but can grow in cooler water

Light Low light often favorable but many can migrate to preferred depth

Micronutrients Iron and molybdenum important

pH Tolerate high pH; many can photosynthesize with bicarbonate

Toxins May reduce zooplankton predation

Morphometry Shoreline coves create quiescent waters favoring blue-green growth

Lakes capable of recycling nutrients from sediment favor blue-greens

The golden alga, Prymnesium parvum, has recently been the cause of numerous fish kills in central

Arizona urban lakes. The alga, once considered to be a species found in brackish, cold waters has adapted

to warmer and less saline environments. The trigger for toxin production and release is unknown.

Because the toxins destroy exposed cells, they can attack the naked cells in gill tissue of fish and mollusks.

When the gill cells are ruptured, oxygen cannot be absorbed and hemorrhaging, lethargy, loss of motor

control, asphyxiation, and death occur within hours. P. parvum has not been reported in northern Arizona;

however, it is moving north and westward and might become a concern in the future.

2.4.1.4 Management of Algae

Mitigation methods for limiting cyanobacteria formation in lakes include:

 Artificial circulation (de-stratification, hypolimnetic aeration, or layer aeration) to prevent release of

nutrients from sediment

 Phosphorus precipitation or inactivation (alum or ferric chloride treatment)

 Light attenuation (application of lake dyes)

 Biomanipulation (adding herbivorous fish to change the community structure)

 Watershed management to reduce nutrient inputs (especially phosphorus) to lakes

 Algaecide application to reduce population density and lower pH

 Lake level increase or flushing

Light attenuation, algaecide applications, biological controls, and lake level and flushing are discussed

below. The remaining management methods are discussed in later sections.

While unlikely to be appealing, lake dyes are an option and can be added to the water to absorb

photosynthetically active radiation (PAR). Two products are currently Environmental Protection Agency

(EPA) registered for use (Admiral® and Aquashade®). Products are water soluble and have no adverse

effects on aquatic organisms. The dye should be applied to waters with greater than a 3-foot depth to be

effective. The dye decreases PAR by about 30 percent.

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Prescott, Arizona November 2020 Page 13

While unlikely to be appealing to the City, algaecides can be applied, usually to the upper 3 feet of water,

to management various algal forms. Application can be by spray, subsurface injection, prop-wash, or

spreader (granular forms). In some cases such as shallow waters, algae growth occurs on the sediment

surface and application by weighted hose or using a density increasing additive is required. Products

contain different active ingredients and additives that impact genera and forms it may control. Some

products have water use restrictions following application, such as for drinking water supplies, swimming,

and irrigation. The product label should always be consulted before making any application.

A brief summary of common algaecides is presented in Table 4.

Table 4

Common algaecides

Algaecide Trade names ® Plants affected

Mixed copper ethanolamine

Cutrine Plus

Clearigate

Captain

Planktonic algae

Filamentous algae

Chara and Nitella

Mixed copper ethanolamine with surfactant

Captain XTR

Cutrine Ultra

Copper sulfate pentahydrate with aluminum

sulfate

SeClear Planktonic algae

Filamentous algae

Copper citrate and gluconate Algimycin

Planktonic algae

Filamentous algae

Pithophora

Copper sulfate pentahydrate in acid Earthtec Planktonic algae

Mixed copper ethanolamine with diquat

dibromide

Cutrine+Reward/Tribune

Filamentous algae

Sodium peroxycarbonate

Green Clean Pro

Phycomycin

Planktonic algae

Filamentous algae

Endothall dimethylamine salt Hydrothol 191

Flumioxazin

Cutrine Plus

Clearigate

Captain

Herbivorous fish can also be stocked in a lake to consume submerged weeds and sometimes filamentous

algae, but no fish species is truly capable of filter-feeding on microscopic planktonic algae. Species regularly

used include tilapia (Tilapia spp.) and the White Amur (Ctenopharyngodon idella). Tilapia can overpopulate

lakes and compete with game species. They are also cold-sensitive and die at temperatures below 55 F and

thus cannot be used in Watson Lake. White Amur (grass carp, Figure 4) are considered an exotic species.

They require a license from Arizona Game and Fish Department (AZGF), must be 12-14-inch minimum size,

and there must be no possibility of ingress or egress from the lake. They are thermally tolerant. Stocking

densities from 10 to 50 fish per acre are typically planted.

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Prescott, Arizona November 2020 Page 14

Lake level adjustment and flushing is sometimes an option when source water is available. Flushing the

lake can serve multiple purposes including removing nutrients and suspended algae from the water column

and decreasing the time for algae to grow, among many others. Much of the volume in Watson Lake is

displaced during winter runoff, and some volume is displaced during summer monsoon runoff in July to

August.

2.4.2 Submerged, Floating, and Emergent Weeds

2.4.2.1 Forms

Floating plants include those that float on the surface and are rooted on the lake bottom. Others can be

free-floating on the water surface and derive their nutrients directly from the water through cell walls or

through a well-developed root system that penetrates into the water. Water lilies and duckweed are

common examples.

Emergent plants are either found growing upright along the damp shoreline or anchored to the lake

bottom through their root systems, but have a substantial portion of the plant body emerging out of the

water. Cattail would be a typical example.

A submerged plant essentially grows under water. It can often send up flowering parts above the water

for seed dissemination or have vegetative parts reach the surface in shallow reservoirs. Horned pondweed

(Zanechellia palustris), brittle naiad (Najas minor), and Eurasian watermilfoil (Ceratophyllum spicatum) are

common local examples.

2.4.2.2 Management Methods

The same list of management methods as described for algae (Section 2.4.1.4) applies to aquatic weeds,

and submerged forms in particular. The described fish species will usually prefer submerged plants over

filamentous algae. However, they do not consume most types of established emergent vegetation.

Aquatic dyes are effective on submerged weeds but have no effect on established floating or emergent

forms. Chemical herbicides for aquatic weeds contain different formulations than algaecides and are

discussed below. Submerged and floating weeds may also be physically removed and these methods are

also presented below.

The City of Prescott has historically been opposed to the application of herbicides in Watson Lake;

however, chemical herbicides are a viable option for controlling aquatic weeds and may need to be

considered depending on lake conditions. The primary chemical categories for weed control are

presented below.

Copper herbicides contain copper that is either available or chelated with an organic compound to keep it

in solution and prevent rapid precipitation in the water column. Copper is very effective for algae control

and some submerged macrophytes as southern naiad and macroscopic algae (Chara and Nitella). Mixed

with diquat (2 parts copper to 1 part diquat), it provides an effective agent against many shoreline plants

including filamentous algae. However, copper can be very toxic to fish.

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Prescott, Arizona November 2020 Page 15

2,4-D works on broadleaf species. It is systemic, requiring a relatively short contact time. It is usually not

effective on pondweeds, but is effective on Elodea, milfoils, coontail, and hydrilla. However, it should not

be used on lakes subject to water contact or consumption because of irrigation restrictions.

Diquat is a contact herbicide that works very quickly and causes a rapid die-off. It is not effective in killing

roots, rhizomes, or tubers, thus it requires repeated applications. It combines with particulates (especially

clays) and dissolved organic matter which limits its effectiveness in some waters. It is tank-mixed with

copper for edge treatments (see above).

Endothall is a contact herbicide which is unaffected by particulates or organic matter. It should not be

used in combination with copper compounds.

Flumioxazin is a relatively new aquatic contact herbicide for selective control of tough invasive and

nuisance plants such as cabomba, watermeal, Eurasian watermilfoil, water lettuce, duckweed, and giant

salvinia. The chemical dissipates quickly from the water column and does not accumulate in the sediment.

It is pH sensitive, working best at pH 7, and requiring maximum label rate when pH exceeds 8.0 SU.

Fluridone product trade names include Sonar® and Whitecap®. Fluridone is a slow-acting systemic

herbicide used to control underwater plants. It may be applied as a pellet or as a liquid. Fluridone can

show good control of submersed plants where there is little water movement and an extended time for

the treatment. Its use is most applicable to whole-lake or isolated bay treatments where dilution can be

minimized. It is slow-acting and may take six to twelve weeks before the dying plants fall to the sediment

and decompose. Granular formulations of fluridone are proving to be effective when treating areas of

higher water exchange or when applicators need to maintain low levels over long time periods. Some

native aquatic plants, especially pondweeds, are minimally affected by low concentrations of fluridone.

Glyphosate is a systemic herbicide. It is only used on shoreline emergent species such as cattail.

Triclopyr-TEA: There are two formulations of triclopyr. It is the TEA formation of triclopyr that is registered

for use in aquatic or riparian environments. Triclopyr, applied as a liquid, is a relatively fast-acting,

systemic, selective herbicide used for the control of Eurasian watermilfoil and other broad-leaved species

such as purple loosestrife. Many native aquatic species are unaffected by triclopyr.

Imazapyr: This systemic broad spectrum, slow-acting herbicide, applied as a liquid, is used to control

emergent plants like spartina, reed canarygrass, and phragmites and floating- leaved plants like water

lilies. Imazapyr does not work on underwater plants. Although imazapyr is a broad spectrum, non-

selective herbicide, a good applicator can somewhat selectively remove targeted plants by focusing the

spray only on the plants to be removed.

Imazamox: Imazamox is the common name of the active ingredient ammonium salt of imazamox

pyridinecarboxylic acid. It was registered with EPA in 2008 and is currently marketed for aquatic use as

Clearcast® and Imox® It is a liquid formulation that is applied to submerged vegetation by broadcast

spray or underwater hose application and to emergent or floating leaf vegetation by broadcast spray or

foliar application. There is also a granular version.

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Prescott, Arizona November 2020 Page 16

Penoxsulam: Penoxsulam is a liquid used for large-scale control of submerged, emergent and floating-leaf

vegetation. Penoxsulam must remain in contact with plants for around 60 days. Because of this long

contact period, penoxsulam is likely to be used for larger-scale or whole-lake treatments and should not

be used where rapid dilution can occur such as spot treatments or moving water.

Information on the various products and their use is presented in Table 5.

Table 5

Herbicide information

Active

ingredient

Mode of action Environmental fate Advantage Disadvantage

Copper Photosynthesis and

cell growth inhibitor.

Cu2+ is primary toxic

form.

Broad-spectrum

contact herbicide

Highly water

soluble with no

degradation.

Strong particle

and DOC

affinity

causes rapid

sediment deposition.

Transport occurs

between water and

sediment

Cost-effective

Relatively safe for

applicator

Moderately

effective on some

forms of

submerged weeds

Toxic to sensitive

fishes and other non-

target organisms

Sediment

accumulation/persisten

Ineffective at

cold

temperatures

Endothall Inhibition of

messenger RNA

activity.

Decreasing rate of

respiration and lipid

metabolism,

inhibiting protein

synthesis and

interfering with

normal

cell division.

Rapidly degraded in

water. Its half-life is 4

to 7days for

dipotassium endothall

and about 7 days for

technical endothall in

surface water.

Moderately to

highly effective on

submersed and

floating plants

Limited toxicity

to fish

Non-selective

Potentially toxic

to aquatic fauna

Water use restriction

Rapid action

Selective contact

herbicide

Diquat dibromide Causes superoxide to

be generated during

photosynthesis,

that damages cell

membranes and

cytoplasm. Leads

to desiccation.

Non-selective

contact herbicide.

Water column

concentrations

typically drop below

detection within days

to weeks after

application. This

results from binding

to particles and

sediment and

retention

in plant tissue.

Moderately to

highly effective on

submersed and

floating plants

Limited toxicity

to fish.

Rapid action.

Potentially toxic

to zooplankton.

Inactivated

particulates.

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Table 5

Herbicide information

Active

ingredient

Mode of action Environmental fate Advantage Disadvantage

Fluridone inhibits production of

carotene, which

enhances degradation of

chlorophyll and inhibits

photosynthesis

Selective systemic

herbicide,

Photodegrades; low

and experiments

indicated low potential

to bioaccumulate or

biomagnify.

Glyphosate Inhibits a key enzyme

that plants and bacteria

use to make amino

acids (EPSP synthase),

Systemic

Once glyphosate

enters the water

column, it is quickly

adsorbed to soil

particles. Microbial

degradation begins

Immediately,

Moderately to

highly effective on

emergent and

floating plants.

Rapid action and low

toxicity.

Inactivated by

suspended

particulates.

No time

restrictions for

water use.

Triclopyr Mimics the plant

growth hormone

auxin (indole acetic

acid), causes

uncontrolled and

disorganized plant

growth that leads to

plant death.

Selective systemic.

Hydrolysis occurs

rapidly with half-lives

in water of 2.8-14.1

hours. Photolysis is

the primary

breakdown process in

water.

Microbial degradation

occurs in soil with a

soil half-life of 30-90

days.

Effect ion

floating and

submersed

plants.

Effective n difficult

to control species

Low toxicity

to aquatic

fauna.

Fast acting.

Non-selective.

Time delays for

recreational use of

water.

Flumioxazin inhibition of

protoporphyrinogen

oxidase, an enzyme

important in the

synthesis of

chlorophyll. Contact

herbicide.

Flumioxazin degrades

rapidly in water and

soil. Dissipation

occurs by a

combination of

hydrolysis and

microbial oxidation.

Low use rate.

Low toxicity

to aquatic

fauna.

Efficacy reduced in

high pH waters.

Imazapyr Imazapyr kills plants by

preventing the

synthesis of certain

amino acids produced

by plants but not

animals.

Non-selective systemic.

Imazapyr can be

highly mobile or

persistent in soils,

dependent on soil

characteristics. The

primary form of

degradation in water

is photodegradation

with a half- life of

approximately 2

days.

Non-toxic to fish

and most aquatic

flora and fauna.

Non-selective.

Persistent in soils.

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Table 5

Herbicide information

Active

ingredient

Mode of action Environmental fate Advantage Disadvantage

2,4-D Hormone that

stimulates stem

elongation & nucleic

acid/protein synthesis,

stimulating

uncontrolled growth

until death.

Affects enzyme

activity/respiration/c

ell division.

Post emergent

systemic herbicide.

Rapid hydrolysis to

2,4 D acid, then binds

to sediments.

2,4 D DMA < 2,4 D

BEE in sediments.

Bioaccumulation

not expected.

Moderately to

highly effective on

emergent,

submerged and

floating plants.

Fairly fast acting.

Potential toxicity

to aquatic fauna.

Time delays for

recreational use of

water.

Imazamox Systemic herbicide that

moves throughout the

plant tissue and

prevents

plants from producing

a necessary enzyme,

acetolactate synthase

Dissipation studies in

lakes indicate a half-

life ranging from 4 to

49 days with an

average of 17 days.

Herbicide breakdown

doesn’t occur in

deep, poorly-

oxygenated water

where there is

no light. In this part

of a lake, imazamox

will tend to bind to

sediment rather than

breaking

down, with a half-life

of approximately 2

years.

Fast acting Dos not

harm animals.

May be used

immediately post-

application for

fishing, boating,

and swimming.

Potable water use

restriction.

Limited data on

adverse effects on

desirable aquatic

species.

Penoxsulam Systemic herbicide that

moves throughout the

plant tissue and

prevents production of

a necessary enzyme,

acetolactate synthase

(ALS)

Very mobile. but not

persistent in aquatic

environment

Slow mode of

action allows for

whole-lake

applications. No

restrictions.

Slow mode of

action allows for

dilution in small

areas

Cutting and harvesting: Aquatic plants can be reduced by cutting and harvesting the vegetative material

by mechanical means. Some disturbance of the sediment is expected. Plants are cut, mechanically

collected and placed on shore to dry prior to disposal. A wide range of techniques is available from

manual to highly mechanical. For large lakes, a weed harvester, capable of cutting and collecting the

material, as shown in Figure 5, would be applicable.

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Prescott, Arizona November 2020 Page 19

Mechanical removal provides a highly flexible control of plants. Location and timing can be selected and

disruption to recreational activities minimized. Other incidental debris can also be removed

simultaneously. Adverse impacts include habitat disruption for fish and benthic macroinvertebrates,

possible spread of the plants by fragmentation and re-rooting, and possible increase in lake turbidity.

Removal is non-selective, with any plant in the path subject to cutting and removal. Plants may also

become re-established if the growing season is sufficiently long.

Drawdown or dewatering a portion of the lake supporting submerged weeds is also a physical

management technique. The plant material dies and dries during solar exposure. The dried biomass may

be removed mechanically or left to be incorporated into the lake sediment upon refilling. The disruption

to use of the lake and aquatic fauna is significant.

2.4.3 Zooplankton

2.4.3.1 Importance and Forms

The zooplankton of lakes is composed of microscopic or nearly microscopic crustaceans, including

members of the rotifers, cladocerans, copepods, and ostracods (see examples below and Figure 6). These

organisms function as consumers of algae, bacteria, and other organic particulates and as a food source

for immature fish.

Rotifers are loricate (possess a shell). They are particulate feeders, consuming algae cells, bacteria, and

detritus (dead organic matter). They are easily identified by the cilia around the mouth that sweeps in

particulate food.

Cladocerans, Daphnia being the most common, are indiscriminate filter feeders, and are preyed upon by

the predatory zooplankton species. They consume any algae cell of a size that can be collected by the

filtering apparatus. They can be identified in water samples by their large, dark eye or straight swimming

motion, and eggs on back.

Copepods have three forms – cyclopoid, calanoid, and harpactacoid. These can be easily differentiated by

the size of their antennae and body segments (cyclopoids-short antennae; calanoids-antennae about as

long as body; harpactacoid-short antennae, extra long body). Copepods are essentially filter feeders,

straining out algae cells. However, some of the larger forms can be size-selective predators; some are

cannibalistic. They can be identified in a water sample by their zigzag swimming motion and females

carry their eggs laterally.

Ostracods are usually a minor part of the zooplankton and usually found in shallow waters near shore.

They are barely visible to the naked eye and resemble small clams (often called seed shrimp).

2.4.4 Benthos

The benthos is composed of bottom dwelling organisms. The microscopic portion of the benthic layer at

the lake bottom is primarily composed of protozoa and bacteria. When it is warm enough and with

sufficient oxygen present, these organisms provide the bulk of the breakdown of organic matter in the

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aquatic ecosystem. The larger, macroinvertebrate portion includes fly larvae, beetle larvae, dragonfly

larvae, etc., that serve as a food source for bottom feeding fishes. The plant component may consist of

filamentous algae and diatoms. In terms of lake management and possible manipulation, the animal

portion of the benthos is most important.

2.4.5 Fish

Fish surveys at Watson Lake have found a moderate variety of game fish present within the Lake,

including largemouth bass, sunfish, bluegill, crappie, catfish, and rainbow trout. The abundance of aquatic

weeds provides ample protective cover for larvae and fingerlings and foraging opportunities for adults.

2.4.6 Other Organisms

2.4.6.1 Birds and Waterfowl

Birds and waterfowl are attracted to standing waterbodies. The birds are popular among bird watchers

and typical lake users. The lake in turn provides food sources and nesting areas for the waterfowl.

Although popular with the citizenry, birds and waterfowl can reduce water quality. The birds produce an

enormous amount of fecal matter that can add nitrogen, phosphorus, and bacteria to the lake waters, soil

recreational and picnic areas, and add feathers and oils to the surface waters.

2.4.6.2 Nuisance and Vector Insects

It is often impossible to prevent occurrence or totally eliminate insect populations in and around a lake.

However, understanding their life histories of the organisms provides a basis for management techniques.

The following sections provide information about the life cycles of these semi-aquatic organisms.

Mosquitoes are unlikely to breed in the lake; however, mosquitoes may breed in associated irrigated areas

where water inadvertently pools or where runoff or storm water accumulates. Additionally, poorly

maintained park grounds and maintenance facilities can become a breeding site for mosquitoes.

Mosquitoes are not only a nuisance from their bites but pose a potential threat to public health. Many

mosquito species are vectors of viruses that can impact human health.

Most mosquitoes typically breed in areas of moderate to dense vegetation and relatively stagnant waters.

Both stagnant-water and flood-water forms of mosquitoes exist. Many species prefer organically rich

habitats. The generalized life cycle of the mosquito has four basic stages: egg, larva, pupa, and adult

(Figures 7 and 8).

Egg: Most mosquitoes lay eggs singularly or in rafts on the surface of the water. The rafts can

contain between 100 and 400 eggs. Other mosquitoes (flood water forms) lay eggs on rocks and

vegetation in wait of submergence by rainfall or flooding. Eggs usually hatch within two to three

days of being laid or submerged by water.

Larvae: Upon hatching, small wiggling larvae swim to the surface of the water to begin breathing

through their siphon. They feed on minute organic particulate matter and bacteria. The larvae go

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through four molts (skin-shedding) to accommodate growth during the next two to 16 days,

depending on species. The organism during each of these stages is called an instar.

Pupa: Following the fourth instar, the mosquito develops into a pupa. The pupa does not eat.

Within the pupa the mosquito develops over the next two days. When fully developed, the pupa

skin splits and the adult fly emerges.

Adult: The adult generally rests on the surface of the water until it is strong enough to fly away.

The mature adult males will feed on nectar while the females will search for blood meals to

nourish their eggs. Adults may fly from a few hundred yards to 15 miles in a night. Adults may live

from two to nine weeks, depending upon species, and some females may produce up to three

batches of eggs.

Midge flies typically encountered in Arizona are not biting insects. However, they tend to swarm, produce

a nuisance buzzing, fly into eyes and mouths, congregate in eves of homes and buildings, and make a

mess when large swarms die. The midge fly life cycle takes 10 to 28 days to complete depending upon

species (Figures 9 and 10). Their larvae inhabit the bottom sediments of lakes. They tend to be more

prevalent in organically rich waters with large amounts of mucky sediment. After several instars, the pupa

emerges from the mud and the adult fly leaves the water. The swarms are part of the mating ritual.

Females lay hundreds of eggs that are deposited on the surface of the water in a jelly-like case which later

sinks to the lake bottom.

Management efforts for midge flies are similar to those for mosquitoes, but these organisms will invade a

water body regardless of the degree of movement of the water column.

2.4.6.3 Bacteria

Waterfowl and birds are a source of E. coli bacteria. Whether added to water or shore, fecal deposits

containing bacteria create a potential human health risk from incidental contact (e.g., fishing, boating,

picnicking, etc.). Children, less likely to be careful where they play and how well they wash prior to eating,

are at greatest risk.

2.4.6.4 Invasive Species

Aquatic invasive species present a number of different risks to both the proper and desired functioning of

a waterbody. High amounts of aquatic vegetation can prevent swimming, sport fishing, and boating.

Invasive fish, snails, and mussels can quickly overwhelm the natural balance of the aquatic ecosystem and

have detrimental effects on the infrastructure and local aquatic species. The exclusion of aquatic invasive

species can include programs to address both emergent and submerged vegetation as well as invasive

aquatic fauna. Despite that Eurasian watermilfoil is considered an invasive form in other parts of the

country AZGF does not currently list Watson Lake as having aquatic invasive species present.

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2.5 Lake Monitoring

Monitoring the water quality of the lake is an essential component of any lake management plan. The

data provide guidance on what actions are required, insight into the best management or mitigation

practice, and ultimately provide the ruler for measuring success of any corrective actions.

2.5.1 Parameters

For most lakes that contain fish and at which fishing and boating are common, water quality monitoring

incorporates the following general areas: (a) chemical-physical parameters, (b) nutrients, (c) algae and

weed density and identification, (d) midge density, (e) bacteria density, and (f) metals. A typical list of

parameters is shown below

 Temperature and oxygen profile by depth

 pH

 Secchi disk depth

 Nitrate-N

 Ammonia-N

 Total Kjeldahl N

 Total N

 Total phosphorus

 Algae identification and count

 Chlorophyll-a

 Midge larvae density (sediment grab)

 E. coli bacteria

 Metals, hardness and alkalinity (Ag, As, Be, Ba, Cd, Cr, Cu, Hg, Pb, Sb, Se. Tl, Zn)

For dendritic lakes (meaning lakes that receive drainage from many contributing drainage systems), more

than a single sample location is usually required to understand variations in the water body. However,

composite samples can be used as simple tracking related to compliance limits. Sampling frequency is

usually monthly, except for metals that are usually checked annually.

2.5.2 Monitoring Equipment

The following is a description of recommended equipment used for lake and insect monitoring (see

Figure 11). Other equipment may be needed based on lake conditions and measurements needed.

2.5.2.1 Water Sampling

Shallow (0.5 m) grab samples can be collected manually or using a standard water sampler such as a Van

Dorn, Kemmerer, or Alpha bottle. Composite samples should include equal volume collections for each

designated area that are combined in a single container.

Vertical (depth) profile sampling will require a temperature-oxygen meter with cables of sufficient length

to allow releasing and maintaining the probe at all water depths. For Watson Lake this is about 15 meters.

Golden algae samples should consist of multiple surface grabs, especially in down-wind areas.

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2.5.2.2 Sediment Sampling

Lake sediment should be collected with an Ekman or Ponar dredge. The Ekman dredge is spring loaded

and dropped to the lake bottom. The jaws are closed by releasing a messenger (weight) that is connected

to the lanyard. The Ponar dredge is for soft sediment and closes when the unit is lifted from the lake

bottom.

2.5.2.3 Midge Fly Sampling

Midge larvae are collected from the lake bottom using an Ekman dredge. The standard dredge has a

surface area of approximately 1/40 square meter. The sediment is hand dispersed or organisms are

floated in a sugar solution. The number of midge fly larvae recovered x 40 represents the number of

larvae per square meter. Typically, midge counts <400 per square meter do not result in nuisance

complaints from lake visitors.

2.5.2.4 Mosquito Sampling

Mosquito larvae are sampled manually with a dipper in standing water locations around the lake or in

stagnant areas of the lake itself. Five dips at a single location are considered representative of one

sampling event.

Adult mosquitoes are collected using EVS (Encephalitis Vector Survey) carbon dioxide traps. The traps

consist of a battery operated fan that draws mosquitoes into a collection net. The upper portion of the

device holds the bait which is dry ice (carbon-dioxide attractant). Traps are set at a 6 ft height in late

afternoon and collected the following morning. Mosquito counts, species identification, and encephalitis

screening can be performed on collected specimens.

2.5.3 Test Methods, Data Handling, and Interpretation

Analytical methods used for examining water samples should be approved by the American Public Health

Association (APHA), EPA, Water Environment Federation (WEF) or other recognized authority. Biological

samples should be examined by experienced biologists or entomologists. When regulatory compliance is

necessary, submitting the chemistry samples to an internal or external State-licensed laboratory that uses

approved methods and has an established and accepted quality assurance program is recommended.

Chain of custody, sample preservation, and holding time procedures should be followed.

Hard copies of all field notes and data, and laboratory reports should be maintained. Electronic files are

suitable and usually preferred for data analysis. Chemical and biological data should be tracked on a

temporal basis by spreadsheet and graphics.

Data interpretation should be made by a qualified lake manager or limnologist. The data interpretation

will influence and direct any additions or changes to the lake management strategies or techniques being

employed.

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3.0 LAKE MANAGEMENT STRATEGIES FOR WATSON LAKE

Based on the findings of the Watson Lake modeling efforts, supplemental aeration and nutrient reduction

are the major management practices recommended for Watson Lake. Other management methods

related to algae and weed growth will improve pH and support the primary practices. Some additional

management practices will support sustainability of the lake for recreational and aesthetic benefits.

Please note that recognition of equipment, material manufacturers,

or suppliers is for example only and does not constitute endorsement

of their products by Wood or the City.

3.1 Circulation and Aeration.

With typical winds in the area, horizontal circulation is not an issue at Watson Lake. Some protected coves

likely exist, but for the most part, the lake appears to have sufficient horizontal flow patterns as shown by

similarity between observation sites. As a monomictic reservoir, Watson Lake vertically circulates during

the autumn and winter and begins to form vertical stratification in late spring and summer. The deeper

water (hypolimnion) rapidly loses oxygen through decomposition of biomass and respiration in the water

column above the sediment. The loss of oxygen (anoxia) causes two significant adverse effects: (1)

restriction of fish, zooplankton, invertebrates and other aerobic aquatic life and (2) release of phosphorus

and gases such as ammonia that can be re-circulated into the photic zone and stimulate algae growth and

eventually cause an increase in pH. Adequate oxygenation is especially important to maintain a healthy

fishery during the warmer months when the oxygen saturation level in the deeper waters of the lake

decreases.

Mechanical vertical circulation or aeration can reduce algae growth directly by moving suspended algae in

and out of the photic zone, thereby reducing its productivity. Vertical mixing also provides an aerobic

zone over the sediment and limits release and redistribution of phosphorus. Aeration helps limit the

release from the sediment of undesirable dissolved gases such as hydrogen sulfide and ammonia.

3.2 Types of Aeration

A number of different types of aeration systems exist with one of two functions based on their mode of

action: (a) complete vertical circulation to deliver oxygen throughout the water column by destroying the

thermocline and thermal stratification and (b) to deliver oxygen to the hypolimnion directly and

maintaining thermal stratification. These are described below.

3.2.1 Bottom Diffuse Aeration

The term aeration is commonly used to describe submersed diffused-air aeration systems that can help

oxygenate the lower depths of a waterbody that cannot be reached by floating aerators or surface

fountains. These systems use an onshore compressor to pump air through subsurface tubes to

submerged bubble diffusers. An air flow rate of 1.3 cu-ft/min minimum is required for each diffuser. As

the bubbles rise, they carry the low-oxygen water from the bottom towards the surface, where it mixes

with the oxygen-rich surface water and atmospheric oxygen before sinking back to the bottom of the

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water column (Figure 12). This vertical mixing, even if localized, can help to increase dissolved oxygen

throughout the entire deep, low-oxygen part of the lake through lateral mixing.

Complete or partial air lift or bubble-plume diffusers would be located on the bottom in areas of the lake

where the water is deep enough that there is low-oxygen hypolimnion water present during the warmer

months with stratified conditions (deeper than 6 meters). Diffusers are not needed in shallow areas where

only highly oxygenated water is present.

The installation and maintenance of a bottom-diffuser aeration system would only have a minor impact to

routine access and recreational use of the lake. Areas of the lake would be closed to boating and fishing

for one to two days while aeration tubing or piping is laid. An electrical source for compressors or blowers

would be needed and the length of air conveyance tubing could be a challenge, depending on where

systems were installed. However, with the lake being mixed laterally, these diffusers do not need to be

installed at the very deepest point farthest from the shore; they can be installed closer to shore as long as

they are below the thermal stratification. The system would require quarterly maintenance of compressors

or blowers and annual maintenance on bottom diffusers, and that might result in partial closures of

affected areas of the lake for a day or two.

Anticipated cost for initial installation of a complete aeration system using bottom diffusers, piston-driven

air compressors or blowers, and weighted airline hoses is $125,000 to $250,000. Power is currently

available at the south boat dock, but not at the north boat dock or dam. Additional costs would apply to

provide power to the aeration system in the northern portion of the Lake. Alternatively, electrical power

could be provided by solar panels where line power is not feasible.

3.2.2 Floating Vertical Water Circulators

Floating vertical water circulators (Figure 13) create vertical water movement by cycling deep oxygen-

poor water to the water-air interface. These circulators float on the water surface and are anchored in

place. They require water depths greater than 1 meter and would be anchored in water depths greater

than 5 meters to be effective. Solar or shore-powered versions are available. Each unit has a footprint of

approximately 16 feet in diameter. The ability to install different draft tubes could allow for water

circulation from the bottom of the deepest part of the reservoir. There would be some negative impact on

boating and other recreational uses in the lake because some units represent a large diameter obstruction

that would need to be marked and avoided, possibly with an additional safety buffer around the unit also

blocked to boating and other uses.

The vertical water circulators would float on the surface in areas of the lake where the water is deep

enough that there is low-oxygen hypolimnetic water below them during the warmer months with

stratified conditions. These are not needed in shallow areas where only high-oxygen epilimnion water is

present.

The anticipated cost for initial installation of all four (4) floating vertical water circulators is $190,000 to

$250,000. This assumes the installed units are solar powered and have no electrical requirement from

land.

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3.2.3 Hypolimnetic Aeration

With hypolimnetic aeration (Figure 14), the oxygen demand of deep water is compensated by oxygen

from the atmosphere or an external oxygen supply without destroying the lake's natural stratification.

Thus the deep water becomes aerobic, the phosphate dissolution is reduced significantly and the

mineralization of sediments improves while stratification and habitat variations are maintained. The

systems are appropriate for lakes wishing to maintain a two-tier fishery by maintaining a cold-water

habitat for certain fish species.

In these systems air is blown into a diffuser at the bottom of a recirculation tube and entrains water

upward through the tube. The water is re-oxygenated by contact with the air bubbles, then returns to the

bottom where it discharges through radial outlets. Excess air is discharged through the top or a vent pipe.

The anticipated cost for initial installation is in the range of $100,000 to $200,000.

3.2.3.1 Speece Cone

A down-flow bubble contactor (Speece Cone) is a hypolimnetic aeration system that uses the change in

water velocity that occurs in different diameter pipes to ensure complete diffusion of pure oxygen

bubbles. Water flows down through a cone; as the cone widens, the water slows. At the same time,

oxygen bubbles are injected into the bottom of the cone (Figure 15). The bubbles rise against the

counter-flowing water until the point that the velocity of the down-flowing water equals the speed of the

bubbles rising. At this point, the bubbles hover in the water flow as they slowly diffuse away. There is no

off-gas and thus no loss of expensive oxygen. The oxygenation system is housed on land and a diffuser

system is placed at the bottom of the lake. Speece Cones have been installed and are successfully

operating in several California reservoirs, including those with summer algae blooms of Aphanizomenon.

Reductions in nitrogen, phosphorus and algae have been reported.

Speece cones appear to be a highly effective but expensive aeration alternative. Depending on pump size,

distribution locations, and use of generated or delivered oxygen, capital costs could be in excess of

1 million dollars.

3.2.4 Nanobubble Aeration

In this relatively new technology, water is pumped from the lake to an on-shore air or oxygen generator

and gas diffusion system (Figure 16) that produces ultra-fine bubbles (nanobubbles less than 1 um

diameter). Both shoreline and solar-powered floating systems are available. The oxygen infused water is

returned to the lake. As a result of their small size and extremely large total surface area, nanobubbles

have no natural buoyancy, follow Brownian movement, and transfer oxygen throughout the water column.

This unique behavior enables nanobubbles to provide a homogenous distribution of oxygen throughout

an entire body of water. They also have a strong negative charge and are attracted to organic molecules

with positive surface charge. When they connect with positively charged metal ions, pollutants and

dangerous cyanotoxins, nanobubbles render them inactive. Nanobubbles also remain within the water

column for a longer period of time over alternative aeration methods.

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Some benefits of nanobubble treatment include (a) saturation of water with over 50,000 times more

oxygen than diffuse aeration, (b) helps mitigate nutrient recycling from the sediment, (c) neutralizes

toxins, (d) helps sustain growth of beneficial bacteria and desirable microbes, and (e) reduces

accumulation of anaerobic bottom muck and sediments.

The anticipated cost for initial installation of approximately ten (10) solar-powered nanobubble

generators is $450,000 to $550,000 (for all units). This does not include electrical connections for non-

solar systems.

3.3 Nutrient Reduction and Inactivation

3.3.1 Watershed Management

Only reducing the loading of nutrients into Watson Lake from Granite Creek truly treats the cause of high

nutrient levels (particularly phosphorus) that support excessive growth rates of algae and weeds. The

Granite Creek Watershed Pollutant Reduction Plan (developed alongside this Plan) describes ways to

reduce phosphorus from the developed environment towards levels needed to limit growth of algae in

the lake.

3.3.2 In-lake Methods

The successful growth and propagation of aquatic weeds and algae are dependent on adequate nutrient

concentrations within the water column and sediment. Precipitation of dissolved and suspended nutrients

from the water column or inactivation (immobilization) of nutrients in the bottom sediment using

chemicals can reduce algae and aquatic macrophyte growth. Nitrogen is very difficult to manage because

it can be replenished from the atmosphere by nitrogen fixing organisms such as cyanobacteria (also called

blue-green algae); therefore, phosphorus is usually the target element. Typical forms of phosphorus in

surface waterbodies include inorganic phosphates, iron-bound phosphorus and inorganic phosphorus.

Nutrient inactivating chemicals can bind and capture each form of bioavailable phosphorus. Lower doses

can be used to just target the water column phosphorus or higher dosages can be applied to also

inactivate releases from the sediment.

The inactivating material is applied by boat throughout the water column in a series of applications

spanning a few days to a few weeks depending on the treatment protocol. Water quality parameters can

affect the suitability and dosage requirements of some products. Nutrient inactivation agents work

through chemical precipitation; they are widely used in lakes and they are not toxic like herbicides or

algaecides.

The types of products that fall into this category include aluminum sulfate (alum), buffered aluminum

sulfate (Baraclear® or similar), or lanthanum-impregnated clay (Phoslock®). Liquid alum can be applied

directly from a boat or barge, but is relatively expensive because of the limited solubility of alum in water

and the resulting low concentration in the liquid as supplied. Dry forms are usually preferred and mixed as

a suspension in lake water and applied through a manifold system from a boat or barge. The specific

reaction of alum and water will also generally lower pH. For high dosage applications, addition of a buffer

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may be necessary to maintain a stable pH. Although there are numerous chemicals that can be used,

sodium aluminate is typically added as it also contributes aluminum to the dose.

Nutrient inactivating agents have been shown to be effective in locking phosphorus in the lake sediment

for five or more years between applications. Some precautions must be considered. Benefits may be

short-lived when inflows contain high concentrations of phosphorus (as with Watson Lake). Although

buffered alum should control cyanobacteria (blue-green algae) and some filamentous algae, it may not

eliminate rooted aquatic plants that may still obtain some phosphorus from the deeper layers of lake

sediment. Additionally, application of alum in the relatively shallow areas of the lake may not be effective.

Effectiveness may be short-lived where area is covered with macrophytes during the summer and their

decay will contribute phosphorus to the water.

Two (2) applications per year may be required to address seasonal phosphorus-laden inflows. One

application would be targeted for late April or May, before the start of warm-weather algae growth, to

address phosphorus that was already in the reservoir (e.g. released from bottom sediment) or in winter

inflow. The second application would be applied soon after the monsoon season to help bind phosphorus

washed into the reservoir. The use and application of these materials would have minimal impact on the

recreational use of the lake.

Cost for a single application of alum is dosage dependent and jar testing is usually recommended to

identify the optimum dosage. Costs can range from $500 to $1,000 per acre. The addition of lanthanum-

impregnated clay would be the costliest, at an estimated $1,000 or more per acre. Cost is calculated based

on water quality data with a total removal of 407 lb of phosphorus from the water column (or 2.45 lbs per

surface acre, based on reducing phosphorus concentrations to under 3 μg/L). If applied to the entire lake,

the cost would be approximately $96,000 - $192,000 per application.

3.3.3 Sediment Removal

Dredging is the process of the physical removal of sediment from the bottom of a lake or reservoir. This

has the benefit of reducing a major in-lake source of nutrients (recycled phosphorus in particular),

increasing the depth of the lake or reservoir, and thereby reducing the area of shallow lake water that

receives sufficient light penetration to support submerged weed growth, and in some cases directly

removing submerged weeds and their substrate. The benefit of dredging can be realized for 5 to 10 or

more years depending on how much new sediment and nutrients wash into the lake and settle to the

bottom. Selective dredging of the areas suffering from the most sedimentation may also provide benefits,

particularly to reduce rooted weed growth, and would cost less than a full-scale dredging effort. Due to

water rights and Active Management Area (AMA) recharge considerations, there may be limitations and

significant coordination efforts required for lake draining and dredging activities to be feasible.

Because the annual inflow of water is large relative to the lake volume, dredging alone is unlikely to

significantly reduce levels of phosphorus in the lake unless inflowing phosphorus is also reduced (at the

watershed level) or is bound or inactivated upon entering the lake before it is used for algae growth. Lake

modeling results supported this relationship. In addition, the large annual inflow means that dredging is

not a once-only activity because the large inflow brings large amounts of new sediment and nutrients into

the lake that settle to the bottom where they would eventually need to be re-dredged.

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The spatial distribution and depth of high-phosphorus sediment deposits would need to be better

quantified for an improved understanding of cost and benefit, particularly of selective dredging. Lake

sediment appears to be a large potential reservoir of phosphorus, and small reductions in the sediment

area in contact with the lake is unlikely to achieve large reductions in lake phosphorus; large-scale

dredging might be needed.

Dredging efforts would have a major impact on the normal operation and recreational availability of the

lake. Relative to other lake management techniques, dredging is one of the most disruptive alternatives in

terms of aesthetics and interruption to lake operation and recreation uses. Dredging can temporarily

reduce oxygen concentrations, increase turbidity, and encourage algal blooms by upwelling some of the

sediment during the removal operation. It would likely require portions of the lake to be closed to

boating and fishing in the vicinity of dredging activities. However, the disruption is only for the duration

of the dredging, unlike some other measures that involve ongoing effects.

The depth to which the lake, or a section of the lake, is dredged is also of importance. Dredging shallow

areas to a depth deeper than the photic zone (2 meters) will reduce the amount of lake bottom available

for aquatic weed growth. A greater depth may need to be dredged because the lake level can vary from

year to year. The Sediment Study indicates an even distribution of nutrients throughout the sediment

depths. Dredging activities that do not dredge to below the bottom of the high-nutrient sediment may

not reduce nutrient redistribution to the water as it will merely expose a new layer of nutrients to the

sediment-water interface.

3.3.3.1 Sediment Removal Methods

The most common forms of sediment removal in large lakes include direct removal by mechanical

equipment and vacuum/suction (hydraulic) dredges. Dredging involves draining the lake or a portion of

the lake to the lowest level possible, targeting material dried by the drawdown, and excavating the

material with conventional land equipment. Wet dredging may use a shovel-like apparatus to scoop the

material from a completely filled lake and deposit the sediment in a holding bin. Hydraulic dredging uses

a suction line to essentially vacuum up the material and pump it to shore. The main benefit of hydraulic

dredging is the ability to retain the existing water level throughout the removal process. This avoids

disruption of recreational activities and the fish community. Dredging also eliminates odors associated

with exposed lake beds but there is increased lake traffic and noise associated with the activity. If the

sediment slurry is not dried on site, it may be difficult to find a disposal location and relocation may

require sealed transport containers to comply with Arizona Department of Transportation (ADOT)

regulations.

On-site dewatering activities, when used in conjunction with dredging, may help reduce the overall cost of

sediment removal and disposal. Dewatering on site may allow for disposal at a nearby facility and use of

more traditional hauling equipment. Costs are reduced because the weight of the removed water is no

longer included in the hauling and disposal cost. The removed liquid is redirected back to the lake or an

evaporation pit.

Dewatering on site could be achieved by two different methods. First, specialized roll-off style containers

that allow for the draining of bulk materials could be employed. The advantage of this option is that the

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dewatering container could also serve as the transport container and reduce a step and associated time

between dewatering and transport. The second option is to use specially designed dewatering bags

(GeoTubes). The tubes achieve a very high level of solids compaction, and are picked up, transported, and

disposed at the completion of dredging operations. Dewatering presents issues including the odors while

the process is undertaken and the need for a large staging area for dewatering.

A realistic cost of dredging should include costs of monitoring, dredging equipment, labor, and transport

of and disposal of dredged material. Monitoring costs include pre-dredge screening of the sediment to

determine quality (hazardous waste characterization), post-dredge monitoring of the lake and staging or

disposal site, and in-lake monitoring to fulfill any regulatory or permit requirements.

Costs for suction dredging are estimated at $25,000 to $75,000 per acre plus approximately $25 to $50

per cubic yard. The Sediment Study revealed approximately 414 acre-feet (668,393 cubic yards) of

sediment volume. Based on the cost and volume, costs for dredging the full volume are approximately

$17 to $34 million. Cost is influenced by variables such as engineering and permitting, mobilization,

allowable run times, transport distance, disposal, and de-watering management. A partial dredging of

selected areas, especially the commonly vegetated shallows, may be advisable based on the high cost of

the activity and the minimal impact on nutrient dynamics in the lake.

3.4 Submerged Weed Management

In-lake phosphorus inactivation and watershed reduction would decrease availability of a primary nutrient

required for growth and propagation of aquatic weeds. Dredging in the shallow portion of the lake that

supports submerged weeds would also be applicable as the process can reduce rooting substrate and

simultaneously remove vegetative material with the sediment if the work is performed during the

summer. Deepening the shallow portion of the lake may also decrease the portion of the lake that

becomes choked with vegetation.

Other management options specifically for submerged aquatic vegetation are discussed below. The area

covered by weeds during the summer is approximately 30 acres.

3.4.1 Weed Harvesting

Harvesting may be an affordable method of seasonal removal of submerged weeds. A harvester may be

purchased and operated by the City or the work may be contracted out to a professional weed

management provider. Based on the climate and observations made during the TMDL study, weed growth

appears limited to the period of April through September. A harvesting event in June or July could

remove sufficient vegetation to provide recreational and aesthetic relief until the plants begin to die back

in October. Adequate shoreline exists for dumping and allowing the vegetation to dry prior to hauling and

disposal. Removal of submerged weeds could make more nutrients available to algae, so this condition

would require monitoring and possible initiation of other algae mitigation efforts.

Weed harvesting should be done on a trial basis. Because the species of concern tend to fragment during

mechanical activities, it is possible there would be significant re-growth from those fragments. Should this

occur, another means of management will be required.

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Purchase cost of a weed harvester ranges from about $85,000 to $150,000. Operational, per-acre cost

ranges from $500 to $3,000, including labor and disposal. Disposal costs could be reduced or eliminated if

removed material were used as a fertilizer or soil amendment instead of being disposed.

3.4.2 Chemical Applications

Although the City has expressed a desire to avoid the use of chemical pesticides, herbicides, and

algaecides in Watson Lake, aquatic herbicides may be safely used to reduce the density of the plants

during the summer. Because the growing season at Watson Lake is relatively short, a single herbicide

application or several small, area-specific applications could decrease plant density for the duration of the

summer growing season and provide recreational relief to fishermen and boaters.

During lake monitoring activities, several submerged aquatic weed species were identified. They included

Coontail (Ceratophyllum spicatum), Sago Pondweed (Stuckinea pectinatus), Curly-leaf Pondweed

(Potamogeton crispus), and Eurasian watermilfoil (Myriophyllum spicatum). Chara, an advanced form of

algae resembling an aquatic weed and often managed as one, was also present. See Figure 17 for

examples of watermilfoil and coontail weeds.

Recommended herbicide choices are provided below for each of the resident species, with effectiveness

indicated as good (G) or excellent (E). Liquids are usually applied by sub-surface injection using weighted

hoses. Granular products are mechanically spread across the lake surface and sink to the bottom where

the weeds are located and rooted. Label directions must be followed including any restrictions to use, and

application should be in accordance with any ordinances, including a Pesticide General Permit.

Applications must be made by a Department of Agriculture Pest Management Division licensed aquatic

applicator.

Table 6

Herbicide choices

Species Product and active ingredient. Active

ingredient

Form Effect

Coontail

Navigate, Weedar-64 2,4-D Liquid G

Cutrine Plus, Captain, Clearigate* Copper Liquid G

Reward, Tribune Diquat

dibromide

Liquid E

Aquathol K or Super K Endothall Liquid, granule E

Sonar Fluridone Liquid, granule E

Clipper Flumioxazin Granule G

Sago pondweed

Reward, Tribune Diquat

dibromide

Liquid E

Cutrine Plus, Captain, Clearigate* Liquid G*

Reward, Tribune Liquid G

Aquathol K or Super K Endothall Liquid, granule E

Sonar Fluridone Liquid, granule E

Clipper Flumioxazin Granule G

Galleon SC Penoxsulam Liquid G

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Table 6

Herbicide choices

Species Product and active ingredient. Active

ingredient

Form Effect

Curly-leaf

Pondweed

Reward, Tribune Diquat

dibromide

Liquid G

Cutrine Plus, Captain, Clearigate* Copper Liquid G

Aquathol K or Super K Endothall Liquid, granule E

Sonar Fluridone Liquid, granule E

Clearcast Imazamox Liquid G

Clipper Flumioxazin Granule G

Eurasian

watermilfoil

Cutrine Plus, Captain, Clearigate* Copper Liquid G

Navigate, Weedar-64 2,4-D Liquid G

Aquathol K or Super K Endothall Liquid, granule E

Sonar Fluridone Liquid, granule G

Reward, Tribune Diquat

dibromide

Liquid E

Clearcast Imazamox Liquid G

Galleon SC Penoxsulam Liquid E

Renovate Triclopyr Liquid E

Clipper Flumioxazin Granule G

Chara

Cutrine Plus Granular Copper Granule E

Copper sulfate crystals Cooper Granule E

Aquathol K or Super K Endothall Liquid, granule G

*can be tank mixed with diquat dibromide

If used, applications should be made to a limited area, and never more than one-third of the lake surface

area at a time to avoid depletion of oxygen in the water column that results from plant decomposition.

Applications are usually based on surface area for granular products and volume of treated lake water

(acre-ft) for liquids. Because liquids can disperse, a sinking and/or sticking adjuvant can be added to the

liquid to help keep the chemical on and around the plants. By using the adjuvants, the number of acre-

feet treated can be reduced and cost minimized. Liquids are usually pumped from a boat using a

weighted hose to better direct the herbicide to the weed bed.

Typical herbicide applications range in cost from $75 to 100 per labor hour plus chemical costs that range

from as little as $75 to as much as $500 per acre depending on weed and product selected.

Lake dyes have been successful at reducing light penetration in lakes and reducing algae and submerged

weed growth in small reservoirs. Longevity can be a month or more with no outflow. Because the entire

lake must be dyed, such applications would require approximately 600 gallons of lake dye. Materials cost

would be approximately $30,000 per application.

3.4.3 Biological Management of Aquatic Weeds

Stocking the lake with White Amur would be the most environmentally sound management technique for

weed removal; this would involve no chemicals, no disturbance, and would offer up to 10 or more years

effectiveness or until they are fished out. The City would need to negotiate with AZGF to agree upon a

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means to satisfy the no ingress or egress requirement. AZGF has relented most recently at Rainbow Lake

where planting of White Amur has significantly reduced weed growth and distribution. An initial stocking

density of 30 fish per acre is recommended.

White Amur cost $6 to $12 dollars each depending on quantity purchased. Assuming the lake would be

stocked at one time, the cost would be about $35,000 - $70,000. This cost does not include the capital

cost that might be required to satisfy the no ingress or egress requirement such as fish screens at or near

the Lake inlet and outlet. A cost cannot be estimated until specific requirements and specifics for such

measures are negotiated with AZGF. Restocking approximately 10 to 15 percent of fish per year should be

anticipated.

3.5 Algae Management

Reducing the loading of nutrients into Watson Lake from Granite Creek truly addresses the cause of high

nutrient levels (particularly phosphorus) that support excessive algae during the summer. The Watershed

Pollutant Reduction Plan (developed alongside the Lake Management Plan) describes ways to reduce

nitrogen and phosphorus from the developed environment towards levels that would be needed to

inhibit algae blooms in the Lake. In-lake management efforts as aeration, phosphorus inactivation, and

dredging will help reduce algae production. These practices and their benefits have been described

previously.

The City has expressed a desire to avoid the use of pesticides, herbicides, and algaecides in Watson Lake.

Algal densities exceeding ADEQ target ranges for total algae and cyanobacteria (blue green algae)

percent composition are sometimes exceeded causing elevated pH and potential toxin formation issues.

Aquatic algaecides may be judiciously used to reduce the density of nuisance and potentially harmful

forms during the summer. It is simply another management tool that could be considered.

The primary problematic and high-density (biovolume) algae that have been identified in Watson Lake

during the summer are listed below (see Figure 18):

Cyanobacteria (blue-green algae) Others

Oscillatoria (filament) Closterium (green unicell)

Gloeocystis (colony) Chroomonas (cryptophyte unicell)

Anabaena (filament) Schroedria (green unicell)

Lyngbya (filament) Fragilaria (diatom unicell)

Microcystis (filament) Rhizoclonium (green filament)

Aphanizomenon (filament)

Coelosphaerium (colony)

Gloeotrichia (colony)

All the blue-green algae listed are represented by species that are potential toxin producers. The major

pathways for human toxin exposure are by ingestion, skin contact, and inhalation. Because no swimming

is allowed at Watson Lake, the major pathway, ingestion, is unlikely to occur. However, incidental contact

with water and inhalation of aerosols are always possible. When concentrations of blue-green algae

approach or exceed the ADEQ Targets for Lakes and Reservoirs for PBC and A&Ww (20,000 units/mL or 50

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percent of total count), action may be considered. Toxin testing is recommended to determine if there is a

public health concern. A few out-of-state specialty laboratories (such as Green Water Laboratories

Cyanolab) are available for testing. Test strips can be used for routine monitoring; however, during

blooms and peak recreational periods, using quantitative test methods (EPA method 546 ELISA or

LCMS/MS) is recommended.

The major algaecides for blue-green algae management are peroxide-based algaecides and the copper-

based algaecides containing a surfactant. For green algae and cryptophytes, any of the copper-based

products are appropriate and cost effective. In the unlikely case of a diatom bloom, an acidic copper

algaecide works well to fracture the silica cell wall of the algae and allow the copper to better penetrate

the cells.

Materials should be applied to the upper three feet of the water column in the area of the algal bloom,

and over an area never to exceed one-third of the total surface area of the reservoir. Application costs

range from $65 to $1,000 per acre-ft, depending on the severity of the algae bloom and product choice.

Labor cost for a licensed applicator usually ranges from $75 to $100 per hour.

Lake dyes have been successful at reducing light penetration in lakes and reducing algae and submerged

weed growth in small reservoirs. Longevity can be a month or more with no outflow. Because the entire

lake must be dyed, such applications would require approximately 600 gallons of lake dye. Materials cost

would be approximately $30,000 per application.

3.6 Nuisance and Vector Insect Management

Although not directly involved with the Watson Lake TMDL, management of the lake and its immediate

surrounding helps preserve its recreational benefits. The shoreline around Watson Lake provides an ample

number of locations for mosquito propagation. A high level of soft sediment throughout the lake also

provides good habitat for the propagation of midge flies and other aquatic-life cycle insects. These insects

are part of the lake and watershed ecosystem, but at times can reach nuisance or public health concern

numbers.

3.6.1 Mosquitoes

Adult mosquitoes may cause nuisances around the shoreline. Because mosquitoes are vectors of several

arboviruses, elimination of breeding habitat and reduction of numbers are important. Although fogging

with adulticides can be performed for immediate short-term relief, it is not recommended for Watson

Lake because of public safety and environmental concerns.

Mosquitoes are not expected to breed in the lake proper. Mosquitoes prefer shallow, stagnant water with

vegetative cover and absence of predatory fishes. An open-water lake does not meet these criteria.

However, small isolated rocky coves at the edge of the lake and terrestrial pools in vegetated areas along

the lake shoreline can serve as breeding habitats. Should such pools be identified by presence of

mosquito larvae, one or more the following actions may be taken:

 Seek and identify breeding habitat if adult numbers become problematic

 Drain terrestrial pools

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 Add mosquito eating fish (Gambusia) to large semi-permanent pools

 Add monomolecular film to terrestrial pools

 Apply larvicide

If monitoring of the sites is desired, traps can be placed in the late afternoon and collected the following

morning, Suggested action levels for mosquitoes are >30 Culex mosquitoes per trap, >30 Aedes

mosquitoes per trap, or >300 floodwater (Aedes, Anopheles, and Psorophora) mosquitoes per trap. Some

laboratories can perform encephalitis screening on the collected mosquitoes, including West Nile and

Dengue Viruses.

These mitigation responses pose minimal environmental or public health threat, including toxicity of

management chemicals. Monomolecular films are synthetic oils that spread out and cover pools. Because

the films lower the surface tension of the water, the mosquito larvae cannot hang at the surface to

breathe through their siphons and essentially suffocate. Larvicides include reduced-risk bacteria-based

products containing Bacillus thuringiensis and B. sphaericus endospores or spinosad (soil bacteria toxin).

Methoprene, a low toxicity product, is also an excellent choice for spot applications. The products come in

granular form and could be manually broadcast into standing water. The methoprene and spinosad

products have formulations that can be effective for up to 150 days. Examples include Vectobac® and

Aquabac® (bacteria), Natular® (spinosad), and Altosid® (methoprene).

3.6.2 Midge Flies

Midge flies will inhabit the lake sediments. They are far less likely to occur in any temporary pools on land.

Algae and bacteria provide an adequate food source. Midges found in Arizona do not bite, but can be

major nuisances especially at sunrise and sunset when they swarm as part of their mating routine. They

tend to fly into eyes, ears, and open mouths.

As with mosquitoes, management should target the larval form. A list of possible responses to excessive

midge fly populations is provided below:

 Use terrestrial traps to monitor adult numbers and dredged sediment samples to monitor larvae

density

 Stock bottom-feeding fishes such as sunfish

 Apply larvicide

The methoprene and spinosad products used for mosquito management are also highly effective on

midge larvae. The Bacillus products are also frequently used because they are relatively inexpensive, but

their efficacy is short-term and reduced. Application of the products can vary from $60 to $350 per acre

plus labor of $75 to $100 per hour for a licensed applicator.

3.6.3 Bacteria

If a need to manage bacteria in Watson Lake emerges, trying to manage bacteria density and E. coli

specifically will be difficult in the large watershed and natural setting of Watson Lake. Watershed Best

Management Practices will serve as the primary management strategy to prevent bacteria from washing

into the lake. However, some in-lake methods are available and are listed below:

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 Remove aquatic vegetation to reduce waterfowl numbers

 Remove soft anaerobic sediments that might harbor E. coli

 Maintain a strict cleanup policy for pet owners

 Redirect local runoff away from the lake wherever possible

Some copper-based algaecides are labeled for temporary reduction of bacteria concentrations in water

(e.g. Earthtec) while others are oxidizing agents and will coincidentally reduce bacteria densities (e.g.,

Phycomycin, Green Clean). High application rates of up to six gallons product per acre-ft (~$100 per acre-

ft of treated water volume) are required. Therefore, chemical applications should be considered for

emergency response only. The PBC standard (575/100 CFU/100 mL) may be established as an action level.

3.7 Lake Monitoring

An ongoing lake monitoring program is recommended both to monitor water quality and compliance

with TMDL goals. Ancillary monitoring of nuisance and vector insects may be part of the program as a

service to recreational users of the lake in terms of quality of life and public health. The program may be

operated by the City, using a contract ADHS-licensed laboratory and City personnel for field sampling and

testing; or the entire monitoring program may be contracted to a suitable vendor as long as a licensed

laboratory is used. A specialty lab may be required for midge and mosquito identification. Licensed

laboratories employ acceptable quality assurance practices, which is needed if the data will be reviewed

and used by both the City and ADEQ for decision making.

3.7.1 Monitoring Equipment

If City personnel collect samples and take field measurements, the following equipment will be required.

 Two-person boat with motor and safety equipment

 Temperature-oxygen meter with 50-ft sensor cable

 pH/ORP meter, electrodes, and calibration solutions

 Secchi disk and marked rope (at least 20 ft long)

 Sub-surface sample collection device (Alpha, Kemmerer, Van Dorn or equivalent)

 Ekman dredge

 Wisconsin plankton net (80 um)

 GPS Unit

 Sample bottles and preservatives (typically provided by laboratory)

 EVS mosquito traps, dry ice (OPTIONAL)

 Mosquito dipper (OPTIONAL)

 New Jersey Light Traps (OPTIONAL)

3.7.2 Monitoring Locations

Water samples should be collected from three to four locations on the lake. Suggested locations, using

same locations as was used in past Wood and ADEQ sampling efforts. Suggestion site locations are: near

dam (VRWAT-A), open deep water (VRWAT-B), cove (VRWAT-C) and shallow water (VRWAT-SO) (see

Table 7 and Figure 19).

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Table 7

Sampling Locations

SITE NAME SITE ID LATITUDE

Degrees

LONGITUDE

Degrees

Watson Lake - At Dam VRWAT-A 34.5952778 -112.4166389

Watson Lake - Mid Lake VRWAT-B 34.5905556 -112.4158333

Watson Lake - Mid Lake VRWAT-C 34.5875000 -112.4191667

Watson Lake - At Boat Ramp VRWAT-BR 34.5930833 -112.4192500

Watson Lake - South End Revised VRWAT-SO-REV 34.584861 -112.420111

Sediment would be collected at the same locations. If optional adult midge and mosquito monitoring is

performed, the two boat dock locations would be reasonable locations. Should a cyanobacteria (blue-

green algae) bloom occur, toxin testing can be limited to the specific area of concern.

3.7.3 Recommended Testing

To monitor progress and adjust lake management activities when needed, on-going sampling and

analysis are highly recommended. Testing should be completed monthly at each sampling location. The

test frequency exceptions are the heavy metal screen that may be tested one time per year and midge

and mosquito adult collections and mosquito dipping (all on shore) that may be conducted during peak

season (May to August) or as needed. Weekly pH, temperature, and dissolved oxygen (surface and lake

bottom) is also recommended particularly during the peak growth season (June through September). A

subsurface (0.5-m deep) sample is acceptable for water samples unless otherwise stated. Midge fly,

mosquito, and vertical temperature and oxygen profiles maybe limited to March through October. All

testing should be in accordance with APHA, WEF, or EPA methods and all holding times should be met.

Field data should be recorded in a designated water-proof field book. Chain of custody procedures

should be followed.

The following parameters are recommended for routine testing:

 Temperature and DO profile by depth (field measurement)

 Secchi disk depth (field)

 pH

 Alkalinity

 Hardness

 Turbidity

 Oxidation-reduction potential (water above and below thermocline and superficial sediment)

 Nitrate+nitrite-N

 Ammonia-N

 Total Kjeldahl N

 Total N

 Total phosphorus

 Dissolved phosphorus

 Dissolved Silica

 Dissolved in

inorganic carbon

 Algae identification (to genus) and individual counts

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 Chlorophyll-a

 E. coli bacteria

 Metals (Ag, As, Be, Ba, Cd, Cr, Cu, Hg, Pb, Sb, Se, Tl, Zn)

 Adult midges, total

 Midge larvae density (sediment grab, OPTIONAL)

 Adult mosquitoes, total and species counts; encephalitis screens as appropriate (OPTIONAL)

 Algal toxin testing (OPTIONAL)

3.8 Integrated Management Approach

Based on the information above, it is clear that no single management technique will result in achieving

the TMDL limitations for nutrients, pH, and dissolved oxygen, and there are other important factors and

conditions that need to be managed that go beyond the TMDL. Therefore, an integrated management

plan should be used that combines a number of techniques to achieve success. An integrated plan also

allows for staging of activities and initiation of some activities while delays might occur for funding,

construction, or installation.

The integrated approach for managing water quality at Watson Lake should incorporate most of the

suggested activities in this document. All have a purpose and can assist in achieving TMDL goals as well

as improving the lake for recreational uses. An example of how an integrated management approach may

work for Watson Lake is provided below.

 Immediately initiate the Lake Monitoring Program

 Apply phosphorus inactivation chemicals in late April (after winter inflows) through late June.

Depending on Lake response, additional application after summer monsoons in late August to early

September (before lake turn-over) may also be suggested.

 Select and install hypolimnetic aeration system

 Harvest submerged weeds in mid-June to early July as an interim, trial measure

 Because re-growth may occur and the method deemed unreliable, contracting the service would

be advisable during this trial period before purchasing a harvester

 Determine cost specifics for dredging the shallow portions of the lake where weed growth occurs

 Negotiate with AZGF for use of White Amur for aquatic vegetation management

 Install fish barriers or other mechanisms of ingress and egress protection if White Amur are

approved

 Implement dredging if use of White Amur is rejected



Apply algaecides sparingly and herbicides as an interim activity to manage excess plant growths

during the summer, especially to reduce cyanobacteria (blue-green algae) that produce toxin

 Monitor midges and mosquitos for quality of life maintenance and public health protection

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4.0 FIELD REFERENCE GUIDE FOR WATSON LAKE

This Field Reference Guide, contained in Appendix A, is a stand-alone document designed for use by lake

operators during their day to day activities. It includes a trouble shooting guide that refers to common

lake management problems, lists items to check and verify, and offers appropriate corrective action

choices. The Field Reference Guide also includes photographs of common resident plant and animal

species of Watson Lake, so organisms can be readily identified. The following Field Reference Guide for

Watson Lake is provided for interim use and should be considered a living document. Because

management strategies laid out in this LMP have not yet been implemented, the contents of the Field

Reference Guide could change considerably. As management methods are accepted and implemented,

the Field Reference Guide should be modified to address applicable procedures relative to them.

5.0 REFERENCES

Amos, W.H. 1969. Limnology. An Introduction to the freshwater environment. La Motte Chemical Products

Company, Chestertown, Maryland.

Applied Biochemists. 1987. How to identify and control water weeds and algae. J.C. Schmidt (ed.). Mequon,

Wisconsin.

Aquaplant. Texas A&M University. https://aquaplant.tamu.edu/plant-identification/

Arizona Department of Environmental Quality. 2014. Watson Lake TMDL: Total nitrogen, DO, pH, and total

phosphorus targets. OFR-14-03.

Beutel, M.W and A.J. Horne. 1999. A reviewof the effects of hypolimnetic oxygenation on lake and reservoir

water quality. Lake and Reservoir Management 15(4): 285-297.

Bold, H. and M.J. Wynne. 1985. Introduction to the algae. Prentice Hall, Inc., Englewood, Cliffs, NJ. 720 pp.

Bryany, C.B. 1969. Aquatic weed harvesting. Weeds, Trees and Turf.

Carpenter, S.J., J.F. Kitchell, and J.R. Hodgson. Surgery for ailing lakes-the art of dredging. Lakeline, Sept.

1989.

Castro, J and F. Reckendor. 1995. Effects of sediment on the aquatic environment. Working Paper No. 6.

United States Department of Agriculture.

Chapman, V.J. 1964. The algae. St. Martin's Press, New York, NY. 472 pp.

Cole, G.A. 1983. Textbook of limnology. C.V. Mosby Company, St. Louis, MO.

Cook, D.C., B. Welch, S.A. Petersen, and P.R. Newroth. 1992. Restoration and management of lakes and

Reservoirs. 2nd Ed. Lewis Publishers, Ann Harbor, MI.

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Czarnecki, D.B. and D.W. Blinn. 1978. The diatoms of the Colorado River. Strauss and Kramer. 181 pp.

Dillard, G.E. 1989. Freshwater algae of the southeastern United States. Vol. 1-6. J. Cramer, Berlin, Germany.

163 pp.

Flock, G., J. Taggart, and H. Olem. Organizing lake users: A practical guide. Terrene Institute, Washington,

D.C.

Gangstad, E.O. 1982. Weed control methods for recreational facilities management. CRC Press, Boca

Raton, FL.

Olem. H. and G. Flock (eds). 1990. Lake and reservoir restoration guidance manual. 2nd Ed. EPA 440/4-90-

006. North American Lake Management Society, Madison WI..

Goldman, R.G. and A.J. Horne. 1983. Limnology. McGraw-Hill Book Company, New York, NY.

Holdren, H.C., W. Jones, and J. Taggart. 2001. Managing lakes and reservoirs. N. American Lake Management

Society, Madison WI.

Horne, A.J. and C.R. Goldman. 1994. Limnology, 2nd Ed. McGraw-Hill Co., New York.

Horne, A J. and M. Beutel. 2019. Hypolimnetic oxygenation 3: an engineered switch from eutrophic to a

meso-/oligotrophic state in a California reservoir, Lake and Reservoir Management, DOI

10.1080/10402381.2019.1648613

IFAS, Center for Aquatic Plants, University of Florida, Gainesville, FL. 115 pp.

Kortmann, R.W. 1989. Aeration technology and sizing methods. Lakeline, Jan. 1989.

Langeland, K.A. 1992. Training manual for aquatic herbicide applicators in the Southern United States.

Madsen, J.D. Advantages and disadvantages of aquatic plant management. 2000. Lakeline. 20(1), pp 22-34.

McComas, S. 1990. Weed harvesting easy with small cutters. Lakeline, May 1990.

Mc Comas, S. 1993. Lake smarts. Terrene Institute, Washington, D.C. 215 pp.

Mobley, M. 2019. Hypolimnetic oxygenation of water supply reservoirs using bubble plume diffusers. Lake

and Reservoir Management. Vol:35(3).

Morris, E.M. and R.D. Clayton. 2006. Best management practices for aquatic vegetation management in

lakes. 11th Triennial National Wildlife and Fisheries Extension Specialists Conference. .

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North Carolina State University. 2006. Pond Management Guide. North Carolina State Fisheries and Pond

Management Extension.

Nürnberg, G.H. Hypolimnetic withdrawal as a lake restoration technique: determination of feasibility and

continued benefits. Hydrobiologia (2019). https://doi.org/10.1007/s10750-019-04094-z

Nürnberg, G.H. R. Hartley, and E. Davis. 2018. Hypolimnetic withdrawal in two North American lakes with

anoxic phosphorus release from the sediment. Water Research: 21(8):923-928.

O’Connor, P.J. and K.K. Garvey. 2001. Aquatic pest control. University of California Pub. 3337.

Patrick, R. and C.W. Reimer. 1966. The diatoms of the United States. Monographs ANS No. 13, Vol. 1.

Academy of Natural Sciences, Philadelphia, PA.

Patrick, R. and C.W. Reimer. 1975. The diatoms of the United States. Monographs ANS No. 13, Vol. 2

Academy of Natural Sciences, Philadelphia, PA.

Perry, K. Missouri Pond Handbook. Missouri Department of Conservation.

Phillips, N, M. Kelly, J. Taggart, and R. Reeder. 2000. The Lake Pocket Book. Terrene Institute. Alexandria, VA.

Prescott, G.W. 1978. How to know the freshwater algae. William C. Brown Publishers, Dubuque, IA. 348

pp.

Prescott, G.W. 1962. Algae of the Western Great Lakes. William C. Brown Publishers, Dubuque, IA. 977 pp.

Rivers, I. 1978. Algae of the Western Great Basin. Desert Research Institute, Pub. 50008. University of

Nevada, Las Vegas, NV. 390 pp.

Singleton, V.V. and J.C, Little. 2006. Designing hypolimnetic aeration and oxygenation systems-a review.

Environ. Sci. Technol. 40(34):7512-7520.

Smith, G.M. 1950. Freshwater algae of the United States. McGraw-Hill Book Company, Toronto, Canada.

719 pp.

Sze, P. 1986. The biology of the algae. William C. Brown Publishers, Dubuque, IA. 251 pp.

Tetratech. 2012. Watson Lake TMDL receiving water modeling. Arizona Department of Environmental

Quality.

United States Environmental Protection Agency. 1990. The Lake and Reservoir Restoration Guidance

Manual, 2nd Ed. EPA-440/4-90-006. Office of Water, Washington DC.

Varnner, M. 2003. North American birds. Paragon Publishing, Bath, UK., 384 pp.

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Wagner, K.J. 2004. The Practical Guide to Lake Management in Massachusetts: A comparison to the Final

Generic Environmental Impact Report on Eutrophication and Aquatic Plant Management in

Massachusetts. Department of Environmental Protection, Commonwealth of Virginia.

Wagner, K. 2019. Advances in hypolimnetic aeration. Lake and Reservoir Management: 35(3).

Warner, D. 1977. The biology of the diatoms. University of California Press, Berkley, CA. 498 pp.

FIGURES

Watson Lake Reservoir Management Plan

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Figure 1 Aquatic nitrogen cycle

Figure 2 Aquatic phosphorus cycle

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Figure 3A Oligotrophic lake nutrient dynamics

Figure 3B Eutrophic lake nutrient dynamics

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Figure 4 White Amur

Figure 5 Weed harvester

Figure 6 Zooplankton forms (left to right: rotifer, cladoceran, copepod, and ostracod)

Watson Lake Reservoir Management Plan

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Figure 7 Mosquito life cycle

Figure 8 Mosquito life forms (left to right: larva, pupa, adult)

Watson Lake Reservoir Management Plan

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Prescott, Arizona November 2020 Figures

Figure 9 Aquatic midge life cycle

Figure 10 Midge life forms (left to right: larva and adult)

Figure 11 Lake sampling equipment

(left to right: Van Dorn bottle, Kemmerer bottle, Ekman dredge, Ponar dredge, EVS trap, New

Jersey light trap)

Watson Lake Reservoir Management Plan

City of Prescott

Prescott, Arizona November 2020 Figures

Figure 12 Bottom diffuse aeration (Kasco)

Figure 13 Floating aerator (Solarbee)

Watson Lake Reservoir Management Plan

City of Prescott

Prescott, Arizona November 2020 Figures

Figure 14 Hypolimnetic aeration (from McAliley, HDR Inc.)

Figure 15 Hypolimnetic aerator-Speece Cone

Watson Lake Reservoir Management Plan

City of Prescott

Prescott, Arizona November 2020 Figures

Figure 16 Nanobubble aeration configuration

Figure 17 Watermilfoil (left) and coontail (right)

Figure 18 Common blue-green algae of Watson Lake (left to right: Gloeocystis, Microcystis,

Aphanizomenon, Gloeotrichia, Lyngbya.)

Watson Lake Reservoir Management Plan

City of Prescott

Prescott, Arizona November 2020 Figures

Figure 19 Sampling Locations

APPENDIX A

FIELD REFERENCE GUIDE

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 i

FIELD REFERENCE GUIDE FOR WATSON LAKE

TABLE OF CONTENTS

Page

Part 1: MONITORING PROGRAM ....................................................................................................................... 1

Part 2: TROUBLE SHOOTING ............................................................................................................................... 4

Part 3: AQUATIC PLANT IDENTIFICATION AND CONTROLS ................................................................... 8

Part 4: AQUATIC INSECT MANAGEMENT ..................................................................................................... 16

Part 5: FISHERY ....................................................................................................................................................... 18

Part 6: WATERFOWL IDENTIFICATION ........................................................................................................... 21

Part 7: MAINTENANCE ......................................................................................................................................... 22

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-1

Part 1: MONITORING PROGRAM

Locations

Sampling Locations

SITE NAME

SITE ID

LATITUDE

Degrees

LONGITUDE

Degrees

Watson Lake - At Dam

VRWAT-A

34.5952778

-112.4166389

Watson Lake - Mid Lake

VRWAT-B

34.5905556

-112.4158333

Watson Lake - Mid Lake

VRWAT-C

34.5875000

-112.4191667

Watson Lake - At Boat Ramp

VRWAT-BR

34.5930833

-112.4192500

Watson Lake - South End Revised

VRWAT-SO-REV

34.584861

-112.420111

Parameters

0.5 m

grab

Vertical

profile

Month

Monthly

Annually

Temperature

Oxygen

Temperature and Oxygen profile at 1.0

m intervals

Mar to Oct

Transparency

Secchi disk

Alkalinity

Total hardness

Turbidity

Ammonia-N

Nitrate+nitrite-N

TKN

Phosphorus, total

Phosphorus, dissolved

Chlorophyll-a

Pheophytin-a

Algae Identification

Algae count by genus

Zooplankton count

Aquatic macrophyte inspection

Bottom survey

Midge density (sediment)

Bottom survey

Mar to Oct

E. coli

Mar to Oct

Waterfowl count/ID

Visual

Metals (13 priority pollutants)

Mosquito (adults) (OPTIONAL)

Shore

Midge flies(adult) (OPTIONAL)

Shore

*0.5 m and above sediment

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-2

Field Survey Form

Date ________________ ........................................................................... Analysts ____________

Sampling Locations

SITE NAME

SITE ID

LATITUDE

Degrees

LONGITUDE

Degrees

Watson Lake - At Dam

VRWAT-A

34.5952778

-112.4166389

Watson Lake - Mid Lake

VRWAT-B

34.5905556

-112.4158333

Watson Lake - Mid Lake

VRWAT-C

34.5875000

-112.4191667

Watson Lake - At Boat Ramp

VRWAT-BR

34.5930833

-112.4192500

Watson Lake - South End Revised

VRWAT-SO-REV

34.584861

-112.420111

Field Measurements

Constituent

Unit

VRWAT-

SO-REV

Temperature

Dissolved oxygen

mg/L

ORP

Secchi disk depth

Vertical Profiles

Depth (m)

VRWAT-A

VRWAT-B

VRWAT-C

VRWAT-BR

VRWAT-SO-

REV

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-3

Vegetation location, density, species: _____________________________________________

Sample

Collection

□

Inorganics

□

Bacteria

□

Metals

□

Bacteria

□

Nutrients

□

Midges

□

Chlorophyll

□

Other

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-4

Part 2: TROUBLE SHOOTING

Common Problems and Causes

Problem

Possible causes

Possible corrective actions

pH too high

Too much algae

• Add fresh water

• Apply algaecide or dye

• Reduce nutrients

• Check aeration system operation

Odor present

Stagnant areas

• Check aeration system operation

• Add permanganate or calcium nitrate

Dead fish

• Check aeration system operation

Low dissolved oxygen

• Check aeration system operation

Turbidity, transparency, or

suspended solids too high

Planktonic algae

• Apply algaecide

• Check nutrient levels

• Add alum, nutrient inactivation

Storm water/runoff

• Wait for settling of non-biological

solids

Low dissolved oxygen

Aeration system malfunction

• Check aeration system operation

• Increase system run time

• Increase number of diffusers

Algae population crash

• Improve algae management

• Increase algaecide application

frequency with decreased dosage.

Organic accumulation at lake

bottom

• Check for runoff entry points and

eliminate

• Improve algae management Dredge

lake bottom

Phosphorus concentration

too high

Runoff or fertilizer inputs

• Add alum; nutrient inactivation

• Check with Watershed Pollutant

Reduction Plan

Recycling from sediment

• Check aeration system operation

• Add alum; nutrient inactivation

• Dredge lake bottom

Aeration system malfunction

• Check aeration system operation

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-5

Problem

Possible causes

Possible corrective actions

Fish kill

Oxygen depletion

• Check aeration system operation

Ammonia toxicity

• Check aeration system operation

• Reduce algae density

• Add nitrifying bacteria

• Investigate nutrient load from

watershed

Metals toxicity

• Check for change in metals levels and

water hardness in source waters

(especially copper)

• Increase alkalinity or hardness

• Eliminate corroding materials in

distribution system

• Check for runoff entry points and

eliminate

Fish disease

• Analyze fish and use antibiotic foods

• Remove infected fish population

Submerged aquatic

macrophytes too dense

Nutrient levels too high

• Check for runoff entry points and

eliminate

• Change fertilizer application and

irrigation protocol

Transparency too great

• Apply dyes

• Apply herbicide

Emergent aquatic

vegetation present

Nutrient levels too high

• Check for runoff entry points and

eliminate

• Change fertilizer application and

irrigation protocol

• Apply alum for phosphorus

inactivation

Bank erosion

• Stabilize shoreline and remove

sediment

Contamination or

transplantation

• Educate citizens

• Apply herbicide

Too much transparency

• Consider adding lake dye

Mosquitoes present

Stagnant water

• Identify species, locate breeding site,

apply larvicide

• Stock with top and bottom, larvae-

eating fish

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-6

Problem

Possible causes

Possible corrective actions

Algae mats at water

surface

High nutrient levels

• Check for and eliminate runoff entry

points

• Change fertilizer application and

irrigation protocol

• Apply alum for phosphorus

inactivation

Bottom growths surfacing

• Apply algaecide or dye

Too many birds

Migration

• Wait/cannot remove migratory

waterfowl

• Limit nesting areas

• Install decoys

• Apply repellents

• Remove food sources

Feeding with human food

• Educate public

• Relocate waterfowl

Bacteria levels too high

Storm water runoff

• Check for and eliminate runoff entry

points

Irrigation runoff

• Change irrigation protocol

Bird feces

• Limit waterfowl

Animal feces

• Require bagging of dog wastes or

restrict pets near lake

Lake depth

loss/sedimentation

Erosion

• Stabilize shoreline and remove

deposits

• Check for and eliminate runoff entry

points

Excess algae growth

• Reduce growth rate with algaecides

or dyes

Midge flies becoming a

nuisance

Organic sediments

• Increase appropriate fish stocks

• Apply larvicide

• Dredge sediments

Chlorophyll-a or algae

density too high

Nutrient levels too high

• Check for runoff entry points and

eliminate

• Change fertilizer application and

irrigation protocols

• Apply alum for phosphorus

inactivation

• Apply algaecides or dyes

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-7

Water Quality

Action Levels and Responses

Parameter

Trigger

Importance

Possible actions

Dissolved oxygen

<6 mg/L @ 1m

<2 mg/L @ bottom

TMDL limits

Fish kills

Nutrient cycling

• Increase aeration system

run time

9.0 SU or higher

TMDL

Ammonia toxicity

• Apply algaecide to reduce

algae

Transparency

<12 inches

Aesthetics

• Apply algaecide to reduce

algae

• Apply light reducing dye

• Flocculate solids with

aluminum sulfate

Ammonia

>0.5 mg/L and pH>9

and temp >28 C

Ammonia toxicity

• Increase aeration system

run time

• Add nitrifying bacteria

Chlorophyll-a

>10 ug/L

TMDL

Aesthetics

• Apply algaecide to reduce

algae

• Inactivate nutrients

• Consider light reducing

dye addition

Algae Count

>5 x 10

cells/mL

Aesthetics

• Apply algaecide to reduce

algae

• Inactivate nutrients

• Consider light reducing

dye addition

Golden algae

Present

Fish kills

• Apply potassium

permanganate to oxidize

toxin

• Apply algaecide to reduce

algae

• Consider light reducing

dye addition

Midge fly density

>500/m

in sediment

Public nuisance

• Stock additional bottom

feeding fish

• Apply larvicide

E. coli bacteria

>575 per 100 mL

Public health

• Apply bactericide

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-8

Part 3: AQUATIC PLANT IDENTIFICATION AND CONTROLS

Common phytoplankton and algae (40x-1000x magnification)

Microcystis

Spirulina

Anabaena

Oscillatoria

Chroococcus

Coelastrum

Chlamydomonas

Aphanizomenon

Selenastrum

Pediastrum

Scenedesmus

Carteria

Golenkinia

Chlorella

Euglena

Trachelomonas

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-9

Chroomonas

Cryptomonas

Peridinium

Glenodinium

Diatoma

Synedra

Chaetoceros

Cyclotella

Navicula (type 1)

Navicula (type 2)

Gleocystis

Prymnesium

Common Weeds- Floating Macrophytes

Duckweed (Lemna sp.)

Water lily (Nymphia sp.)

Pennywort (Hydrocotyle sp.)

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-10

Common Weeds-Emergents

Bulrush (Scirpus sp.)

Bulrush (Eleocharis sp.)

Cattail (Typha sp.)

Common Weeds-Submerged Macrophytes

Sago Pondweed

(Stuckenia pectinatus)

Eurasian watermilfoil

(Myriophyllum spicatum)

Horned pondweed

(Zannichellia palustris)

Brittle naiad

(Najas minor)

Coontail (Ceratophyllum sp.)

Chara sp.

Widgeon grass

(Ruppia maritima)

Curly-leaf pondweed

Potomogeton crispus

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-11

Herbicide Information

Chemical

Exposure

required

Advantages

Disadvantage

Best uses

Plants

controlled

and response

time

Relative

Persistence in

Environment

Bio-

accumulation

Risk

Copper, chelated

18-72 hr

inexpensive,

rapid action

non-

degradable,

but inactive in

sediment

Algae and

macrophytes in

lakes and ponds

broad

spectrum;

7-10 days

Low, rate of

sediment

accumulation

outpaces rate

of copper

accumulation

Low

2,4-D

18-72 hr

inexpensive;

systemic

perceived as

toxin

lakes;

watermilfoils

broad-leaf;

5-7 days

Low

Diquat

12-36 hr

rapid; little

drift

does not

affect

underground

plant parts

Shorelines and

localized areas;

submersed and

marginal plants

broad

spectrum;

7 days

Low, absorbs

into sediment

and is

inactivated

Low

Endothall

12-36 hr

rapid; little

drift

does not

affect

underground

plant parts

Shorelines and

localized areas;

submersed

plants; algae

(Hydrothol)

broad

spectrum;

7-14 days

Moderate,

present 30-60

days in

environment

before

microbial

degradation

Low

Flumioxazin

20 hr

low dosage;

few

restrictions

Works poorly

at higher pH

small lakes;

submersed or

floating plants

broad

spectrum 1-3

weeks

Low, quick

microbial

degradation

Low

Fluridone

30-60 days

low dosage;

few

restrictions

long contact

time required

small lakes;

submersed

plants

broad

spectrum;

30-90 days

Moderate to

High, present

50-75 days

Low, EPA

tolerance level

set at 0.5 ppm

in fish tissue

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-12

Chemical

Exposure

required

Advantages

Disadvantage

Best uses

Plants

controlled

and response

time

Relative

Persistence in

Environment

Bio-

accumulation

Risk

Glyphosate

n/a

systemic

slow; does not

control

submersed

plants

emergent and

floating plants

only

broad

spectrum;

7-10 days

Moderate to

High,

environmental

persistence is

currently

under strong

debate

Moderate,

metabolites

persist in new

plant material

after initial

treatment

Sodium

peroxycarbonate

30 min

Contact,

extremely

fast-acting

No residual

Suspended and

filamentous

(string) algae

broad

spectrum;

1 hr

Very low,

degraded in

less than 30

minutes

Very low

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-13

Common Aquatic Plant Control Product Dosages (refer to label before using)

Chemical

Trade names

Manufacturers

Max.

application

rate

Max.

water

conc.

Application

notes

Copper,

chelated

Cutrine Plus

Cutrine Ultra

Applied

Biochemists/Arch

0.6-3.0

gal/acre-ft

Some also

available as

granular

1.0 mg/L

algaecide and

herbicide

Komeen

Nautique

Applied

Biochemists/Arch

Earthtec

Earth Science

Laboratories

Clearigate

Applied

Biochemists/Arch

K-Tea

SePro.

Captain

SePro.

2,4-D

Navigate

Applied

Biochemists/Arch

100-200

lb/A

0.5 gal/acre

2.0 mg/L

systemic;

excellent on

milfoils

Aqua-Kleen

Cerexagri

Diquat

Reward

Syngenta

2 gal/acre

2.0 mg/L

contact

herbicide

Weed Plex Pro

Sanco Industries

Harvester

Applied

Biochemists/Arch

Endothall

Aquathol K

Aquathol Super K

Hydrothol 191

UPI

1.3 gal/acre

5.0 mg/L

binds with

particulates

and loses

effectiveness

Flumioxazin

Clipper

Valent

6-12 oz/A

100-400

ppb

pH sensitive

Fluridone

Sonar

SePRO

13 gal/acre

5.0 mg/L

fish may be

sensitive

Glyphosate

AquaMaster

Monsanto

2 gal/acre

0.2 mg/L

Emergents

and floating

plants only

AquaPro

SePRO

Peroxy-

carbonate

Alimycin

Green Clean Pro

Applied

Biochemists

Biosafe Systems

200 lb/Aft

n/a

Algae

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-14

PLANT SUSCEPTIBILITY TO VARIOUS AQUATIC HERBICIDES

Plant

Endothall

(Aquathol)

Copper

Diquat

Endothall

(Hydrothol)

Fluridone

Glyphosate

Flumioxazin

2,4-D

Sodium

peroxycarbonate

Algae

Filamentous algae

Suspended algae

Chara

Submersed macrophytes

Elodea

Bladderwort

Brittle naiad

Coontail

Curlyleaf

pondweed

Eurasian

watermilfoil

Horned pondweed

Sago pondweed

Southern naiad

Waterstargrass

Free-floating macrophytes

Duckweed

Watermeal

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-15

Plant

Endothall

(Aquathol)

Copper

Diquat

Endothall

(Hydro

thol)

Fluridone

Glyphosate

Flumioxazin

2,4-D

Sodium

peroxycarbonate

Rooted floating macrophytes

American

pondweed

Spatterdock

Water lily

Emergent plants

Arrowhead

Bulrush

Cattail

Spikerush

Water smartweed

Water Use Restrictions (in days after herbicide application)

Chemical

Drinking

Swimming

Fish

consumption

Animal

drinking

Irrigation

Copper

Diquat

1-3

1-5

Endothall

7-25

Fluridone

7-30

Glyphosate

Flumioxazin

2,4-D

Watson Lake Reservoir Management Plan – Field Reference Guide

City of Prescott

Prescott, Arizona November 2020 A-16

Part 4: AQUATIC INSECT MANAGEMENT

Larva and Adult Midges and Mosquitos

Midge larva

Mosquito larva

Adult midge

Adult mosquito

Mosquito and Midge Management Products

Product

Active ingredient

(A.I.)