U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 03– 4011

Water Quality and the Effects of Changes 

in Phosphorus Loading to Muskellunge Lake, 

Vilas County, Wisconsin

Prepared in cooperation with the Muskellunge Lake Association
Prepared in cooperation with the Muskellunge Lake Association

Water-Resources Investigations Report 03–4011

Middleton, Wisconsin: 2003

U.S. Department of the Interior

U.S. Geological Survey

Water Quality and the Effects of Changes in 

Phosphorus Loading to Muskellunge Lake, 

Vilas County, Wisconsin

By Dale M. Robertson, William J. Rose, and David A. Saad
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

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Contents

Abstract.......................................................................................................................................................... 1

Introduction ................................................................................................................................................... 1

Muskellunge Lake and its watershed ....................................................................................................... 1

Data-collection sites and techniques ....................................................................................................... 3

Lake-water quality........................................................................................................................................ 5

Water temperature and dissolved oxygen..................................................................................... 5

Nutrient concentrations.................................................................................................................... 6

Chlorophyll a concentrations........................................................................................................... 6

Water clarity ....................................................................................................................................... 8

Trophic state indices ......................................................................................................................... 8

Sources of water and phosphorus to Muskellunge Lake ...................................................................... 9

Water.................................................................................................................................................... 9

Phosphorus ....................................................................................................................................... 13

Effects of changes in phosphorus loading ............................................................................................. 15

Summary ...................................................................................................................................................... 17

References................................................................................................................................................... 17

Figures

 1–2. Maps showing:

 1. Muskellunge Lake, Wisconsin, and surrounding area, with surface-water 

and ground-water contributing areas identified .................................................................... 2

 2. Data-collection sites for Muskellunge Lake, Wisconsin, and watershed 

boundaries ................................................................................................................................... 4

 3–6. Graphs showing:

 3. Water temperature and dissolved oxygen distributions near the deep hole 

in Muskellunge Lake, Wisconsin, on March 21, 2001 (with aeration) and 

March 12, 2002 (without aeration) ............................................................................................ 5

 4. Dissolved oxygen distribution in several locations in Muskellunge Lake, 

Wisconsin, on March 21, 2001, with aeration system in operation..................................... 6

 5. Concentrations of total phosphorus and chlorophyll a, and Secchi depths

near the deep hole in Muskellunge Lake, Wisconsin............................................................ 7

 6. Trophic state indices based on surface concentrations of total phosphorus 

and chlorophyll a, and Secchi depths collected near the deep hole in 

Muskellunge Lake, Wisconsin................................................................................................... 8

 7. Schematic of the hydrologic cycle of Muskellunge Lake, Wisconsin................................ 9

 8. Daily average lake stages (measured and calculated) of Muskellunge 

Lake, Wisconsin......................................................................................................................... 10

 9. Ground-water-flow paths to Muskellunge Lake, Wisconsin.............................................. 12

 10–11. Graphs showing:

 10. Water budget for Muskellunge Lake, Wisconsin, November 1, 2000 to 

October 31, 2001......................................................................................................................... 13

 11. Phosphorus budget for Muskellunge Lake, Wisconsin, November 1, 2000 

to October 31, 2001 .................................................................................................................... 15
Tables

 1. Monthly water budget for Muskellunge Lake, Wisconsin, November 1, 2000 

to October 31, 2001....................................................................................................................... 10

 2. Monthly phosphorus budget for Muskellunge Lake, Wisconsin, November 1, 2000 

to October 31, 2001....................................................................................................................... 14

 3. Estimated phosphorus concentrations in Muskellunge Lake from 

various phosphorus-loading models contained in the Wisconsin 

Lakes Modeling Suite.................................................................................................................. 16

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Conversion Factors and Abbreviated Water-Quality Units

 Multiply By To Obtain

 inch (in.) 25.4 millimeter

 inch per day(in/d) 25.4 millimeter per day

 inch per year (in/yr) 25.4 millimeter per year

 foot (ft) 3.2808 meter

 foot per day (ft/d) 3.2808 meter per day

 mile (mi) 1.6093 kilometer

 acre .004047 kilometer squared

 acres-foot (acre-ft) 1233.4818 cubic meter

 pound (lb) .4536 kilogram

 cubic foot per second (ft3

/s) .02583 cubic meter per second

 Temperature, in degrees Celsius (°C) can be converted to degrees Fahrenheit (°F) 

 by use of the following equation:

 °F = [1.8 x °C] + 32.

Water year: Water year is defined as the period beginning October 1 and ending September 30, designated by the 

calendar year in which it ends.

Abbreviated water-quality units used in this report: Chemical concentrations and water temperature are given in 

metric units. Chemical concentration is given in milligrams per liter (mg/L) or micrograms per liter (μg/L). Milligrams 

per liter is a unit expressing the concentra tion of chemical constituents in solution as weight (milligrams) of solute per 

unit volume (liter) of water. One thousand micrograms per liter is equivalent to one milligram per liter. For concentrations less than 7,000 mg/L, the numerical value is the same as for concentrations in parts per million.
Acknowledgments

Technical Reviewers

Herbert S. Garn, Supervisory Hydrologist, U.S. Geological Survey, Middleton, Wis.

Richard Heleniak, Muskellunge Lake Committee, Eagle River, Wis.

Art Horton, Muskellunge Lake Committee, Eagle River, Wis.

Jennifer K. Wudi, Water Resources Specialist—Lake Management Coordinator, Wisconsin

Department of Natural Resources, Rhinelander, Wis.

Project Coordinators

Tom Fountaine, Muskellunge Lake Committee, Eagle River, Wis.

Chester Moore, Muskellunge Lake Committee, Eagle River, Wis.

Editorial and Graphics

Michael Eberle, Technical Publications Editor, U.S. Geological Survey, Columbus, Ohio

Michelle M. Greenwood, Cartographer, U.S. Geological Survey, Middleton, Wis.

Leah N. Hout, Editor, U.S. Geological Survey, Columbus, Ohio

Susan Z. Jones, Editorial Assistant, U.S. Geological Survey, Middleton, Wis.

Jennifer L. Bruce, Publications Assistant, U.S. Geological Survey, Middleton, Wis.

Chester Zenone, Hydrologist (Retired), U.S. Geological Survery, Reston, Va.

Data Collection

Stephanie M. Berg, Student Trainee, U.S. Geological Survey, Middleton, Wis.

Jeffrey J. Hannig, Hydrological Technician, U.S. Geological Survey, Merrill, Wis.

Richard Heleniak, Muskellunge Lake Committee, Eagle River, Wis.

Daniel L. Olson, Hydrological Technician, U.S. Geological Survey, Middleton, Wis.

Elissa D. Roerish, Hydrological Technician, U.S. Geological Survey, Middleton, Wis.

Approving Official

Dorothy H. Tepper, Reports Improvement Advisor, U.S. Geological Survey, Reston, Va.

vi
Abstract

Muskellunge Lake is a productive, eutrophic lake 

because of high nutrient loading. Historical data indi cate 

that water quality has only slightly degraded since the 

early 1970s, possibly because of phosphorus input from 

effluent from septic systems. A detailed phosphorus budget 

for the lake indicated that most of the phosphorus comes 

from natural sources—ground water and surface water 

flowing through relatively undeveloped areas surrounding 

the lake. Modeling results indicated that the natural input 

of phosphorus was sufficient to maintain the lake’s eutrophic condition. Analysis of sediment cores confirmed that 

only small changes in nutrient and algal concentrations 

have occurred over the past 100 years; however, the analysis indicated that the macrophyte community has increased 

over this time period. The aeration system, installed to 

alleviate winter anoxia, maintains aerobic conditions 

throughout the main bays of the lake.

Introduction

Muskellunge Lake is in Vilas County, Wisconsin, 

about 6 mi west of Eagle River (fig. 1). In the early 1960s, 

the lake was described as being moderately productive 

(relatively high concentrations of nutrients and algae, 

and relatively high fish populations), with clear water 

of low transparency, and bottom material consisting 

predominantly of muck (Black and others, 1963). In 

many shallow, productive lakes like Muskel lunge Lake, 

dissolved oxygen concentrations throughout the lake are 

near saturation when the lake freezes; however, as winter 

progresses, oxygen is consumed. If ice and snow cover is 

present for a sufficient period of time, dis solved oxygen 

throughout the lake can drop below critical concentrations (usually considered below 2 mg/L) and cause fish 

kills (this is often referred to as a “winterkill”). Winterkill 

occasionally occurs in Muskellunge Lake. In the interest 

of protecting and improving water quality and the lake’s 

fishery, the Muskellunge Lake Association was established 

in 1991. Beginning in 1992, the Association began collecting water-quality data as part of the Wisconsin Department 

of Natural Resource’s (WDNR) Self-Help Citizen Lake 

Monitoring Program (Wisconsin Department of Natural 

Resources, 2002). To prevent winterkills from occurring 

in Muskellunge Lake, the Association, with partial support 

from the Musky Clubs Alliance of Wisconsin, installed 

an aeration system in the fall of 1995 (the location of the 

aerator is shown in fig. 1) and operated it every winter 

except the winter of 2001–02. 

Continuing concern over degrading water quality in 

the lake led to a cooperative effort with the U.S. Geological Survey (USGS) beginning in 2000 to evaluate the 

water quality of the lake in terms of long-term trends, to 

evaluate the effectiveness of the aerator in maintaining 

adequate dissolved oxygen concentration throughout the 

lake, to quantify the sources of phosphorus to the lake, and 

to determine how the lake would respond to changes in 

phosphorus loading. This report presents the results of 

this effort.

Muskellunge Lake and its 

Watershed 

Muskellunge Lake has a surface area of 272 acres and 

volume of 2,520 acre-ft. The lake has a maximum depth 

of 19 ft and mean depth of 9.3 ft. Most of the water enters 

the lake through ground water and precip itation; however, 

a small tributary on the southeast side of the lake and overland flow episodically dis charge water to the lake. Outflow 

from the lake is to Muskellunge Creek, which leaves the 

west side of the lake and flows to Little St. Germain Lake, 

about 3 mi downstream. 

Water Quality and the Effects of Changes in Phosphorus 

Loading to Muskellunge Lake, Vilas County, Wisconsin

By Dale M. Robertson, William J. Rose, and David A. Saad
2 Water Quality and the Effects of Changes in Phosphorus Loading to Muskellunge Lake, Vilas County, Wisconsin

Snipe Lake

Eagle River

Muskellunge Creek

Forest/shrubland

Wetland

Grassland

Water

Land use/land cover

Surface-water contributing area

Ground-water contributing area

Road

Aerator

EXPLANATION

45o58'30"

89o

24' 89o21'

45o

56'30"

Muskellunge

Lake

0 .5 1 MILE

0 .5 1 KILOMETER

Land use/land cover data from WISCLAND

(Lillesand and others, 1998).

Nearshore G

contributing area

Southeast tributary

contributing area

WISCONSIN

Eagle River

Figure 1. Muskellunge Lake, Wisconsin, and surrounding area, with surface-water and ground-water contributing areas 

identified.
The area of the watershed contributing surface 

water to Muskellunge Lake, defined by examining USGS 

topographic maps, is about 550 acres (fig. 1). The area 

contributing ground water to the lake, as deter mined 

from a ground-water-flow model constructed for the area 

(described later in this report), is much larger than the area 

contributing surface water and is about 2,200 acres (fig. 1). 

The area is predominantly forest and wetlands, although 

areas of low-density residential development are increasing. The soils in the area consist mainly of well-drained 

sand and sandy loams and are thought to be naturally high 

in phosphorus content (Wisconsin Department of Natural 

Resources, 1985).

Data-Collection Sites and 

Techniques

Data describing the water quality of the lake were 

collected in 1960 by the Wisconsin Conservation Department, from 1973 to 1975 by the WDNR, in 1984 by the 

U.S. Environmental Protection Agency, from 1992 to 1999 

by the Muskellunge Lake Association, and from June 2000 

to March 2002 by the USGS as part of this study. Generally, three measurements have been used to describe water 

quality: near-surface concentration of total phosphorus 

(an indicator of the amount of nutrients in the lake), nearsurface concen tration of chlorophyll a (an indicator of the 

amount of algae in the lake), and Secchi depth (an indicator of water clarity). Lake water-quality properties were 

usually measured during May through October near the 

deep hole of the lake (fig. 2). During each sampling in this 

study, except during March 2002, the USGS col lected 

Secchi depth, depth profiles of water temperature, dissolved oxygen, specific conductance, and pH with a multiparameter instrument; near-surface samples (collected 

with a Van Dorn sampler) were analyzed for concentrations of total phosphorus and chlorophyll a and nearbottom samples (collected just above the sediment/water 

interface with a Van Dorn sampler) for concentrations of 

total phosphorus. During spring overturn in 2001, surfacewater samples were analyzed for a full suite of waterquality constituents (U.S. Geological Survey, Wisconsin 

District Lake-Studies Team, 2002). During the March 

2002 sampling, only profile data were collected. 

Additional depth-profile measurements of temperature and oxygen were made at three secondary sampling 

locations (near the center of each of the bays; fig. 2) 

during March 2001, when oxygen depletion should be 

most extreme, to assess the ability of the aeration system 

to maintain oxygenated conditions throughout the main 

areas of the lake. These measurements are compared with 

a dissolved oxygen profile collected in late winter in 2002 

when the aerator was not operated to examine the overall 

effects of the aeration system. 

Lake water-quality data collected by the USGS as 

part of this study were published in the annual USGS data 

report series, the most recent being “Water Quality and 

Lake-Stage Data for Wisconsin Lakes, Water Year 2001” 

(U.S. Geological Survey, Wisconsin District Lake-Studies 

Team, 2002). 

Inflow measurements were made and water samples 

(analyzed for total phosphorus) were collected approximately twice per month from the unnamed tributary to the 

lake at the culvert on Musky Road (here after referred to as 

the “southeast tributary”; fig. 2). Inflow was not continuously measured in the southeast tributary; therefore, daily 

flows were estimated from continuous flow measured 

at a nearby gaged stream (Muskellunge Creek at Birchwood Drive just upstream from Little St. Germain Lake). 

Coinciding measure ments from the southeast tributary 

and Muskellunge Creek at Birchwood Drive were used 

to develop a rela tion between flows at the two sites. The 

regression relation and the daily flows from Muskellunge Creek were then used to estimate daily flow in the 

southeast tributary. Temporally varying offsets were then 

applied to all estimated flows to force estimated flows 

to equal those measured in the southeast tributary. Daily 

phosphorus concentrations in the southeast tributary were 

obtained by linearly interpolating between measured 

concentrations. 

Daily lake stages and surface-water outflow were 

estimated from continuous (15-minute-interval) waterelevation measurements recorded near the dam at the 

outlet to Muskellunge Creek (fig. 2). Stream flow was 

measured approximately monthly and used to develop a 

stage-discharge relation for the dam, which was then used 

in conjunction with stage data to calculate flow from the 

lake. Outflow data were pub lished in the USGS report, 

“Water-Resources Data, Wisconsin—Water Year 2001” 

(Waschbusch and oth ers, 2002). Water samples were collected approximately monthly and analyzed for total phosphorus. Daily phosphorus concentrations were estimated 

by linearly interpolating between measured concentrations.

Eight small, shallow piezometers (small-diameter 

observation wells) were installed around Muskel lunge 

Lake to help define areas contributing ground water to the 

lake and determine the phosphorus con centrations in the 

ground water entering the lake. The ground-water gradient 

Data-Collection Sites and Techniques 3
4 Water Quality and the Effects of Changes in Phosphorus Loading to Muskellunge Lake, Vilas County, Wisconsin

Snipe Lake

Muskellunge Creek

Musky Road

Surface-water contributing area

Road

Deep-hole site

Secondary sampling site

Outflow and precipitation site

Inflow site

Aerator

EXPLANATION

45o57'46"

89o

23'45" 89o21'48"

45o

56'22"

Muskellunge

Lake

0 .5 MILE

0 .5 KILOMETER

Nearshore

contributing area

Southeast tributary

contributing area

Deep-hole

 site

North site

Northwest site

Southwest site

Figure 2. Data-collection sites for Muskellunge Lake, Wisconsin, and watershed boundaries.
(determined from water elevation in the piezometer and the 

water elevation of the lake) and phosphorus concentrations 

in the ground water in each piezometer were measured 

three times during the study (October 2000, June 2001, 

and August 2001).

All chemical analyses of water samples were done 

by the Wisconsin State Laboratory of Hygiene in accordance with standard analytical procedures described in 

the “Manual of Analytical Methods, Inorganic Chemistry 

Unit” (Wisconsin State Laboratory of Hygiene, 1993).

Lake-Water Quality

Water-quality data collected in Muskellunge Lake 

confirmed that the lake was very productive and had soft 

water with moderate alkalinity (36 μg/L). The pH in the 

lake varied from about 6.7 in winter and near the bottom 

during stratified periods in summer to about 8 near the 

surface during mid to late summer. Lakes with good fish 

populations and productivity generally have a pH between 

6.7 and 8.2 (U.S. Geological Sur vey, Wisconsin District 

Lake-Studies Team, 2002).

Water Temperature and Dissolved Oxygen

Thermal stratification in the lake was usually very 

weak, with seldom more than 2 or 3o C difference from the 

surface to the bottom. Weak stratification developed during 

summer, but stratification usually broke down episodically 

during cooling periods or periods of high winds. During winter, weak thermal strat ification was also present 

throughout the lake, even during the operation of the 

aeration system (fig. 3).

Although thermal stratification during summer was 

very weak, dissolved oxygen concentrations in the deepest 

water (usually only below 12 ft) occasionally dropped 

below 0.5 mg/L as a consequence of decom position of 

dead algae that settled from the surface and the biochemical oxygen demand of bottom sediment. 

To determine how well the aeration system maintained oxygenated conditions throughout the lake, oxygen 

profiles were collected near the deep hole and in each of 

the main bays of the lake (fig. 2) in late winter (March 21, 

2001), when oxygen depletion was expected to be most 

extreme (fig. 4). At the deep-hole sampling site, a short 

distance from the aerator, dissolved oxygen concentrations 

were above 9 mg/L even just above the bottom. These 

concentrations were well above the Wisconsin state standard of 5 mg/L for dissolved oxygen in warm-water lakes 

(Shaw and others, 1993). Dissolved oxygen profiles in the 

main bays of the lake demonstrated that concentrations 

were above 2 mg/L throughout the lake but often below 

the 5 mg/L state standard. As a rule of thumb, the critical 

level of dissolved oxygen is about 2 mg/L for most game 

fish native to warmwater lakes, and levels below 1 mg/L 

for extended periods of time are lethal (Michigan Department of Natural Resources, 2002). To demonstrate what 

Lake-Water Quality 5

March 21, 2001 - Aeration

0

2

4

6

8

10

12

14

16

0 2 4 6 8 10 12 14

Temperature

Dissolved

oxygen

Bottom

Ice

March 12, 2002 - No aeration

0

2

4

6

8

10

12

14

16

0 2 4 6 8 10 12 14

Dissolved Temperature

oxygen

Bottom

Ice

DEPTH, IN FEET BELOW SURFACEDEPTH, IN FEET BELOW SURFACE

TEMPERATURE, IN DEGREES CELSIUS AND

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

TEMPERATURE, IN DEGREES CELSIUS AND

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

Figure 3. Water temperature and dissolved oxygen distributions near the deep hole in Muskellunge Lake, Wisconsin, on March 21, 

2001 (with aeration) and March 12, 2002 (without aeration).
6 Water Quality and the Effects of Changes in Phosphorus Loading to Muskellunge Lake, Vilas County, Wisconsin

oxygen concentrations would have been like if aeration 

was not used, these data were compared with those from 

late winter 2001–02 (fig. 3). In winter 2001–02, the aerator 

was not operated because of a late freeze and a moderately 

warm winter with little snow cover. Even with the conditions that favored relatively high dissolved oxygen concentrations in winter 2001–02, concentrations were less than 

5 mg/L below about 4 ft beneath the ice surface and less 

than 2 mg/L below about 6 ft beneath the ice surface 

(fig. 3). This oxygen depletion resulted in a moderately 

small zone (about 4 ft thick) from just below the ice to 6 ft 

below the surface that was habitable by fish. The beneficial effects of the aerator and its importance can be seen 

by comparing the dissolved oxygen profiles from March 

2001 (with aeration) with that from March 2002 (without 

aeration). 

Nutrient Concentrations

Phosphorus and nitrogen are essential for plant 

growth and are the nutrients that usually limit algal growth 

in Midwestern lakes. High nutrient concentrations can 

cause high algal populations (blooms) and can therefore be 

the cause of eutrophication (that is, accelerated aging and 

increased productivity) of lakes. The ratio of near-surface 

concentrations of total nitrogen to total phosphorus is commonly used to determine which of these nutrients should 

limit algal productivity in a lake. The specific value of this 

ratio that determines which nutrient is potentially limiting 

varies under different conditions such as water temperature, light intensity, and nutrient deficiencies (Correll, 

1998); however, a ratio greater than about 12:1, by weight, 

usually indicates that phosphorus is the potentially limiting 

nutrient. The average nitrogen-to-phosphorus ratio in Muskellunge Lake ranged from 15:1 to 20:1; therefore, phosphorus should usually be the nutrient limiting algal growth 

in the lake and the nutrient to focus on when considering 

management efforts to improve water quality.

Phosphorus concentrations in Muskellunge Lake 

have generally ranged from about 20 to 80 μg/L (fig. 5). 

Phosphorus concentrations greater than about 17–20 μg/L 

indicate eutrophic conditions. Concen trations were lowest 

in late fall, winter, and spring, and highest in late summer. 

Surface concentrations com monly increase throughout 

summer in shallow lakes because of the release of phosphorus from the sediments in deep areas of the lake that 

occurs during short stratified periods when these areas 

are depleted of oxygen. This phosphorus is subsequently 

mixed with the rest of the water column during periods of 

destratification. Short stratified periods occur in Muskellunge Lake because the lake is relatively shallow with 

respect to its surface area. There was a slight increase 

in phosphorus concentrations from a summer average 

concentration of 30–40 μg/L in about 1996 (the earliest measurements to about 1996 were relatively stable) 

to about 40–50 μg/L in 2000–01. Higher concentrations 

were, however, measured from 1997 to 1999. The average 

surface concentration in summer (June through August) of 

2001 was 49 μg/L. 

Chlorophyll a Concentrations

Chlorophyll a is a photosynthetic pigment found in 

algae and other green plants. Its concentration, therefore, 

is commonly used as a measure of the density of the algal 

population in a lake. Concentrations greater than about 

7–10 μg/L indicate eutrophic conditions and greater than 

20–30 μg/L are usually consid ered to be associated with 

algal blooms. Chlorophyll a concentrations ranged from 

5–10 μg/L in early sum mer to 30–50 μg/L in late summer, with the highest concentration of 91 μg/L measured 

in August 1999 (fig. 5). The average concentration in the 

summer of 2001 was 19 μg/L.

 March 21, 2001

0

2

4

6

8

10

12

14

16

0 2 4 6 8 10 12 14

Ice

N and NW sites SW site

DEPTH, IN FEET BELOW SURFACE

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

Deep-hole site

Figure 4. Dissolved oxygen distribution in several locations 

in Muskellunge Lake, Wisconsin, on March 21, 2001, with 

aeration system in operation.
Lake-Water Quality 7

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70

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JULY 1972

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Wisconsin Department of Natural Resources

Environmental Protection Agency

Self-help volunteers

U.S. Geological Survey

Summer average

TOTAL PHOSPHORUS, 

IN MICROGRAMS PER LITER

CHLOROPHYLL A,

IN MICROGRAMS PER LITER SECCHI DEPTH, IN FEET

Figure 5. Concentrations of total phosphorus and chlorophyll a, and Secchi depths near the deep hole in Muskellunge Lake, 

Wisconsin.
8 Water Quality and the Effects of Changes in Phosphorus Loading to Muskellunge Lake, Vilas County, Wisconsin

Water Clarity

Water clarity, based on Secchi depth measurements, 

generally ranged from 2 to 6 ft; however, on a few occasions (1992 and 2000) it exceeded 8 ft (fig. 5). Secchi 

depths less than 6 ft indicate eutrophic conditions. Water 

clarity was typically best in spring and fall and decreased 

as summer progressed. The average sum mer Secchi depth 

was about 4 ft and has not changed throughout the period 

of measurements. In 1999, how ever, the average summer 

Secchi depth decreased to 2.6 ft. The average summer 

Secchi depth in 2001 was 3.6 ft.

Trophic State Indices

One method of classifying the water quality or productivity of a lake is by computing water-quality indices 

such as Trophic State Indices (TSIs). These indices, based 

on near-surface concentrations of total phosphorus and 

chlorophyll a and on Secchi depths, were developed by 

Carlson (1977) and modified for Wisconsin lakes by Lillie 

and others (1993). These indices were developed to place 

these three characteris tics on similar scales. Oligotrophic 

lakes (TSIs less than 40) typically have a limited supply of 

nutrients, clear water, low algal populations, low phosphorus concentrations, and the deepest water in the lake 

typi cally contains oxygen throughout the year. Mesotrophic lakes (TSIs between 40 and 50) typically have a 

moderate supply of nutrients, are prone to moderate algal 

blooms, and have occasional oxygen depletions at depth. 

Eutrophic lakes (TSIs greater than 50) are nutrient rich 

with correspondingly severe water-quality problems, such 

as frequent seasonal algal blooms, oxygen depletion in the 

deeper areas of the lakes, and poor clarity. Lakes with 

TSIs greater than 60 are considered hypereutrophic and 

usually have extensive algal blooms during summer. All 

three indices indicate that Muskellunge Lake was usually eutrophic and often hypereutrophic (fig. 6). The TSIs 

indicate that the only time the lake would have been considered mesotrophic was in early summer and late fall. All 

three indices were relatively high in 1997 to 1999. Lower 

values occurred prior to 1997 and after 1999. The higher 

TSI values in 1997 to 1999 may have been caused by 

the longer growing season in these years associated with 

warmer springs and earlier ice-out dates. 

40

45

50

55

60

65

70

Total phosphorus

Secchi depth

Chlorophyll a

Hypereutrophic

Eutrophic

Mesotrophic

JULY 1972

JULY 1974

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JULY 1978

JULY 1980

JULY 1982

JULY 1984

JULY 1986

JULY 1988

JULY 1990

JULY 1992

JULY 1994

JULY 1996

JULY 1998

JULY 2000

JULY 2002

TROPHIC STATE INDEX

Figure 6. Trophic state indices based on surface concentrations of total phosphorus and chlorophyll a, and Secchi depths 

collected near the deep hole in Muskellunge Lake, Wisconsin.
Sources of Water and Phosphorus 

to Muskellunge Lake 

Because productivity in Muskellunge Lake should 

be limited by the input of phosphorus (based on nitrogento-phosphorus ratios), reduction in phosphorus input to 

the lake would be a logical management goal. Most of the 

phosphorus enters the lake associated with the input of 

water. Therefore, to determine where the water and phosphorus originates and how changes in phosphorus loading 

would affect the trophic status of the lake, we first quantified the water and phosphorus budgets of the lake.

Water

The hydrology of Muskellunge Lake can be described 

in terms of components of its water budget (fig. 7). The water 

budget for the lake may be represented by the equation:

∆S = (PPT + SWIn + GWIn) – (Evap + SWOut + GWOut), (1)

where ∆S is the change in the volume of water stored in 

the lake during the period of interest and is equal to the 

sum of the volumes of water entering the lake minus the 

sum of the volumes leaving the lake. Water enters the lake 

as precipitation (PPT), surface-water inflow (SWIn), and 

ground-water inflow (GWIn). Water leaves the lake through 

evaporation (Evap), surface-water outflow (SWOut), and 

ground-water out flow (GWOut).

Each term in the water budget was computed on a 

daily basis from November 1, 2000 to October 31, 2001. 

Changes in lake volume were determined from lake stages 

monitored near the outlet dam (fig. 2), assuming a constant surface area of the lake. Lake stage fluctuated from 

a minimum of 9.94 ft (relative to an arbitrary datum) in 

early September to a maximum of 10.55 ft in late April 

(fig. 8). The lake stage was relatively stable from November through early April, rose rapidly by about 0.3 ft during 

April, decreased gradually throughout summer, and then 

gradually increased starting in September. The lake stage 

at the end of October 2001 was 0.10 ft higher than at the 

beginning of the 1-year period (table 1).

During the ice-free period, precipitation was measured continuously near the outlet dam. For the remainder 

of the period, daily precipitation was interpolated from 

data collected by weather observers in St. Germain, Eagle 

River, and near Lower Buckatabon Lake. The total annual 

precipitation was 30.69 in. (table 1), which is near the 

long-term average for the area.

Surface-water inflow was divided into that from the 

southeast tributary (drainage area of 227 acres) and that 

from the remaining nearshore areas (323 acres). Inflow 

measurements were made approximately two times per 

month from the southeast tributary and used to develop a 

relation with daily flows collected by the USGS at Muskellunge Creek at Birchwood Drive (just upstream of Little 

St. Germain Lake). Daily flows at Birchwood Drive were 

then used to estimate daily inflows from the southeast tributary. The total annual input from the southeast tributary 

was 575 acre-ft, which equates to a change in water level 

in the lake of about 25 in. (table 1). The southeast tributary 

accounts for most of the surface-water input to the lake, 

except during snowmelt.

Surface inflow from the remaining nearshore areas 

was estimated by three different methods for three different periods of the year: non-snow-covered period, winter 

snow-covered period, and spring snowmelt period. A 

0.0015 runoff/rainfall coefficient was used to estimate 

daily runoff during non-snow-covered peri ods. This 

coefficient was the average coefficient determined for 

six intensively monitored, forested plots in Vilas County 

(D. Graczyk, U.S. Geological Survey, written commun., 

2002). For the winter period, it was assumed that there was 

no nearshore runoff from these areas. For the snowmelt 

period, defined as the 30-day period from April 6 through 

May 5, runoff was estimated by comparing the measured 

lake stage with stage calculated using equation 1. During 

this 30-day period, 0.48 ft of water was needed on the lake 

surface to maintain the observed and calculated lake stages 

tracking near and parallel to each other (fig. 8). For simplicity, the total runoff was distributed uniformly through 

time (0.19 in/d), although actual runoff during this period 

Sources of Water and Phosphorus to Muskellunge Lake 9

GWIn GWOut

PPT

SWOut

Ground water

Evap

SWIn SWOut

Figure 7. Schematic of the hydrologic cycle of Muskellunge 

Lake, Wisconsin. Abbreviations are defined in the text.
10 Water Quality and the Effects of Changes in Phosphorus Loading to Muskellunge Lake, Vilas County, Wisconsin

Table 1. Monthly water budget for Muskellunge Lake, Wisconsin, November 1, 2000 to October 31, 2001, in inches on the surface 

of the lake

Month

Inputs Outputs

Precipitation

Southeast 

tributary

Remaining 

nearshore 

areas Ground water Evaporation

Muskellunge 

Creek

Change in 

storage

November 1.77 2.13 0.00 4.91 0.47 7.24 0.07

December 1.27 1.56 .00 5.08 .00 7.27 .02

January 1.22 .77 .00 5.08 .00 6.61 -.02

February 1.43 .60 .00 4.59 .00 6.17 .01

March .81 1.44 .00 5.08 .00 6.75 -.03

April 5.32 5.64 4.80 4.91 .82 19.00 .20

May 3.88 4.58 .96 5.08 3.70 13.03 -.08

June 4.68 3.21 .01 4.91 4.09 11.36 -.07

July 3.36 1.81 .00 5.08 4.21 5.96 -.07

August 1.44 .52 .00 5.08 3.62 4.90 -.17

September 2.86 1.29 .00 4.91 2.80 4.38 .09

October 2.65 1.82 .00 5.08 2.05 5.99 .15

Total, in inches 30.69 25.38 5.78 59.77 21.77 98.65 .10

Total, in acre-feet 695 575 131 1,350 493 2,230 2.30

9.8

10.0

10.2

10.4

10.6

10.8

Calculated stage

Measured stage

LAKE STAGE, IN FEET

NOV 2000

DEC 2000

DEC 2000

JAN 2001

MAR 2001

APRIL 2001

MAY 2001

JUNE 2001

JULY 2001

AUG 2001

SEPT 2001

OCT 2001

Figure 8. Daily average lake stages (measured and calculated) of Muskellunge Lake, Wisconsin.
Sources of Water and Phosphorus to Muskellunge Lake 11

was quite variable. The total inflow from these remaining 

nearshore areas was estimated to be 131 acre-ft, which 

equates to a change in water level of about 6 in.

Evaporation from the lake was estimated from average monthly evaporation-pan data from 12 years of data 

collected at the Rainbow Flowage about 16 mi west of 

Muskellunge lake. Monthly lake/pan coeffi cients computed as part of a study on Vandercook Lake, Vilas County 

(Wentz and Rose, 1991), were used to adjust the pan 

evaporation rates to obtain total lake evaporation. These 

coefficients ranged from 0.75 in May to 1.17 in October. 

No evaporation was assumed to occur when the lake was 

ice covered. About 22 in. of water was estimated to have 

evaporated from the lake during this period (table 1), most 

of which occurred during summer. Surface-water outflow 

consisted of flow over the dam into Muskellunge Creek. 

There was a total annual output of 2,230 acre-ft through 

Muskellunge Creek, which equates to about 99 in. from 

the surface of the lake.

Ground-water gradients from the piezometers 

installed around the lake (fig. 9) and a two-dimensional, 

analytic-element, steady-state, ground-water-flow model, 

GFLOW (Haitjema, 1995), were used to define groundwater-source areas and to allocate ground-water discharge 

to shoreline segments represented by the eight piezometers. A description of the ground-water model is beyond 

the scope of this report, but can be found in Haitjema 

(1995). The GFLOW model covered an area extending approximately 3 to 6 mi around Muskellunge Lake. 

The model was calibrated by varying horizontal hydraulic conductivity and ground-water recharge within an 

expected range until there was a reasonable match between 

measured and simu lated ground-water levels and stream 

base flows in the area. Areal recharge to the ground-water 

system was set equal to 9 in/yr and horizontal hydraulic 

conductivity was set equal to 28 ft/d for the calibrated 

model. These values are similar to those used by Hunt 

and others (1998) for another area in Vilas County. For 

46 water-level targets, the calibrated model had a mean 

difference between measured and simulated water lev els 

of 0.1 ft and mean absolute difference of 3.6 ft. Simulated 

base flows were within 0.5 ft3

/s of measured base flows 

at Muskellunge Creek at Birchwood Drive and Little 

St. Germain Creek at Highway 70.

The ground-water contributing area for Muskellunge 

Lake was delineated using particle-tracking tech niques in 

GFLOW. The area contributing ground water to the lake 

is much larger than that contributing sur face water and 

covers approximately 2,200 acres (figs. 1 and 9). Ground 

water discharges into the lake from all sides except for 

two small areas during certain periods of the year, based 

on hydraulic-head measure ments made in the piezometers 

around Muskellunge Lake (fig. 9). Along a segment of 

the shoreline near the outlet of the lake represented by 

piezometer 3 and along a segment of the south shoreline 

represented by piezometers 5 and 6, ground-water gradients were slight but toward the lake most of the time. For 

a brief 1- to 2-month period in late summer, ground-water 

gradients were slightly negative or away from the lake 

in areas near these piezometers. On an annual basis this 

outflow would be slight. Ground-water flow simulated 

by GFLOW also demonstrated that ground water enters 

the lake from all areas except near piezometers 3 and 4. 

Hence, GWOut was assumed to be negligible and was 

omitted from the water budget (table 1).

A total ground-water input to the lake of 1,350 acreft, which equates to 1.87 ft3

/s or a change in water level 

of about 60 in. (table 1), was computed as the residual 

in the water-budget equation (eq. 1). Ground-water flow 

to the lake was assumed to be uniform throughout the 

year, although in reality there is seasonal variation. The 

separation of the calculated and measured stage graphs 

in figure 8 is likely a reflection of actual seasonal variation 

in ground-water flow to the lake.

GFLOW simulated a total ground-water discharge 

to the lake of 2.48 ft3/s, whereas GWIn, calculated as the 

residual in the water budget, was 1.87 ft3/s. The simulated 

ground-water flux into the lake was calcu lated for eight 

shoreline segments, each corresponding to the length of 

shoreline halfway between adjacent piezometers installed 

around the lake. The model developed for the area did not 

include the southeast trib utary. For this reason, ground 

water that actually discharges to the tributary was simulated as discharge to the lake in the shoreline segments 

near piezometers 5, 6, and 7. It was estimated that about 

75 percent, or 0.59 ft3

/s, of flow from the southeast tributary is base flow (ground-water discharge). Therefore, 

the 0.59 ft3/s was proportionately subtracted from the 

model-indicated flows to shoreline segments represented 

by piezometers 5, 6, and 7. The total ground-water input 

to the lake was then 1.87 ft3/s. The amount of ground 

water entering the lake through each shoreline segment is 

represented by the thickness of the segment lines around 

the lake in figure 9. Ground-water flux into the lake ranged 

from 0.02 ft3

/s (shoreline segment near piezometer 4) to 

0.63 ft3/s (shoreline segment near piezometer 1). More 

than 65 percent of the ground water is estimated to enter 

the lake through the north shore (represented by piezometers 1 and 8). 
Muskellunge Creek

Ground-water contributing area

Ground-water-flow path

Road

Piezometer with identification number

EXPLANATION

Muskellunge

Lake

0 .5 1 MILE

0 .5 1 KILOMETER

Snipe Lake

1

8

2

3

4

5

6

7

4

45o58'30"

89o

24' 89o21'

45o

56'30"

Figure 9. Ground-water-flow paths to Muskellunge Lake, Wisconsin. The relative amount of ground water entering the lake 

is demonstrated by the width of shoreline segments. The relative amount of phosphorus loading per shoreline segment is 

demonstrated by the size of the circle at each piezometer location.

12 Water Quality and the Effects of Changes in Phosphorus Loading to Muskellunge Lake, Vilas County, Wisconsin
Sources of Water and Phosphorus to Muskellunge Lake 13

The complete daily water budget fairly accurately 

simulated the changes in the water level of the lake (fig. 8). 

The largest error in the hydrologic balance of the lake was 

about 0.2 ft. The complete water budget (fig. 10) indicated 

that from November 1, 2000 to October 31, 2001, groundwater inflow was the major source of water to the lake 

(49 percent of the total inputs). Direct precipitation 

contributed 25 percent of the total input, whereas surfacewater inflow from the southeast tributary and other nearshore areas contributed 26 percent of the total input. The 

major loss of water from the lake was through the outlet 

(81 percent), whereas 18 percent was lost through evaporation and 1 percent went into increased storage in the lake.

Phosphorus 

To help define where the phosphorus in Muskellunge Lake originates, a detailed phosphorus budget was 

computed for this 1-year period. Sources of phosphorus to 

the lake include precipitation, surface-water inflow, ground 

water, and contributions from septic systems. The phosphorus concentration in precipitation was assumed 

to be constant at 7 μg/L, a value suggested by Rose (1993) 

for northern Wisconsin. Therefore, direct precipitation 

contributed about 13 lb of phosphorus to the lake during 

the study year (table 2). 

Phosphorus concentrations were measured approximately monthly and more frequently during flow events 

in the southeast tributary. Concentrations were generally 

lowest from October through June (20–40 μg/L) and then 

steadily increased throughout summer, reaching about 

90 μg/L in late August. During a high-flow event in July, 

however, a concentration of 177 μg/L was measured. Daily 

phosphorus concentrations in the southeast tributary were 

obtained by linearly interpolating between measurements. 

The phosphorus concentration in runoff from the remaining nearshore areas that are predominantly forested was 

assumed to be 100 μg/L, based on six intensively monitored, forested plots in Vilas County (D. Graczyk, U.S. 

Geological Survey, written commun., 2002). The amount 

of phosphorus delivered to the lake was then computed by 

multiplying the daily phosphorus concentrations by the 

daily inflow volumes. The total input of phosphorus from 

the south east tributary and remaining nearshore areas was 

estimated to be about 65 and 36 lb, respectively (table 2). 

Over 50 percent of the phosphorus loading from surface 

inflow occurred in April and May. 

Dissolved phosphorus concentrations were measured 

in the eight piezometers installed around the lake (fig. 9) 

three times during the study period. Phosphorus concentrations varied only slightly at each location. The average 

phosphorus concentration was highest on the north side of 

the lake (131 μg/L, piezometer 1). Moderate concentrations were measured on the east and southeast sides of the 

lake (26 to 50 μg/L, piezom eters 5, 6, 7, and 8). Lowest 

concentrations were measured on the west side of the 

lake (13 to 17 μg/L, pie zometers 2, 3, and 4). The input of 

phosphorus was then computed by multiplying the average 

Water inputs

Precipitation

25%

Southeast 

tributary

21%

Nearshore runoff

5%

Ground 

water

49%

Water exports

Evaporation

18%

Storage

1%

Muskellunge 

Creek

81%

Figure 10. Water budget for Muskellunge Lake, Wisconsin, November 1, 2000 to October 31, 2001.
14 Water Quality and the Effects of Changes in Phosphorus Loading to Muskellunge Lake, Vilas County, Wisconsin

Table 2. Monthly phosphorus budget for Muskellunge Lake, Wisconsin, November 1, 2000 to October 31, 2001, in pounds

Month

Inputs Outputs

Precipitation

Southeast 

tributary

Remaining 

nearshore 

areas

Ground 

water

Septic 

systems

Muskellunge 

Creek

November 0.8 3.3 0.0 20.8 5.7 11.1

December .5 2.5 .0 21.5 5.9 9.0

January .5 1.3 .0 21.5 5.9 5.8

February .6 1.1 .0 19.4 5.3 7.9

March .3 2.6 .0 21.5 5.9 12.0

April 2.3 13.9 29.5 20.8 5.7 36.9

May 1.7 9.8 5.9 21.5 5.9 26.7

June 2.0 10.7 .0 20.8 5.7 31.5

July 1.4 9.6 .0 21.5 5.9 20.5

August .6 2.9 .0 21.5 5.9 12.1

September 1.2 4.0 .0 20.8 5.7 13.9

October 1.1 3.7 .0 21.5 5.9 19.1

Total 13.2 65.3 35.6 253.5 69.4 206.4

phosphorus concentration from each piezometer by the 

ground-water flux computed for each shoreline segment. 

Ground water was estimated to contribute about 250 lb 

of phosphorus during the period (table 2). The relative 

amount of phosphorus contributed through each shoreline 

segment is demonstrated in figure 9 by the size of the 

circle around the piezometer number. Annual phosphorus 

loading from ground water ranged from 0.4 lb (shoreline 

segment near piezometer 4) to 172 lb (shoreline segment 

near piezometer 1). Most phosphorus enters from the north 

side of the lake where flow and phosphorus concentrations 

were both highest. Most of this phosphorus appears to be 

from natural sources because the highest concentrations 

were found where there is little development. 

The input of phosphorus from septic systems (M) 

was estimated by use of equation 2 (Reckhow and others, 

1980):

 M = ES

 * (number of capita years) * (1 – SR), (2)

where M is a function of an export coefficient, ES, and a 

soil retention coefficient, SR. In applying equation 2, it was 

assumed that the most likely value for ES was 1.5 lb of 

phosphorus per capita per year. Typical export coefficients 

range from 1.1 lb per capita per year (Reckhow and others, 

1980; Panuska and Kreider, 2002) to 1.8 lb per capita per 

year (Garn and others, 1996). The number of capita years 

was esti mated to be 150 (50 full-year residents and 400 

three-month residents), and the most likely value of SR was 

0.7 (70 percent). The total input from septic tanks was then 

computed to be 69 lb per year. Because inputs from septic 

systems were not directly measured, low and high estimates for this value were obtained by applying high and 

low estimates for SR (0.9 and 0.5). The possible low and 

high estimates of phosphorus from septic systems were 

23 and 83 lb, respectively. These values may overestimate 

the contributions from septic systems because many residents live in areas with little ground-water flow to the lake 

(southwestern side of the lake, fig. 9).

The phosphorus concentration in the water leaving the

lake was measured approximately monthly. Concentrations

in the outflow were similar to that in the lake and were 

generally lowest in winter and early spring (20–30 μg/L) 

and highest in mid summer (45–60 μg/L, in July). Daily 

phosphorus concentrations were obtained by linearly 

interpolating between measurements. The total phosphorus 

exported from the lake was then computed by multiplying 

the daily phosphorus concentrations by the daily flow in 

Muskellunge Creek. The total output of phosphorus from 

Muskellunge Creek was estimated to be 206 lb (table 2).
Effects of Changes in Phosphorus Loading 15

The complete phosphorus budget (fig. 11) indicates 

that 437 lb of phosphorus were input into Muskel lunge 

Lake from November 1, 2000 to October 31, 2001. Input 

from ground water was the major source of phosphorus to 

the lake (58 percent). The secondary source was surfacewater inflow (southeast tributary and other nearshore 

areas), which contributed 23 percent of the total input. 

Most of the phosphorus from surface-water inflow enters 

in the southeast tributary. With the best approximations 

considered, septic systems were estimated to contribute 

about 16 percent of the phosphorus during the 1-year 

study period. With the worse-case assumptions considered, 

septic systems would contribute about 19 percent of the 

total input to the lake. Approximately 47 percent of the 

total phosphorus input to the lake (206 lb) was exported 

through the outlet. The remaining 53 percent of the 

phosphorus input (231 lb) was deposited in the bed 

sediment of the lake. 

Effects of Changes in Phosphorus 

Loading 

Most of the phosphorus entering Muskellunge Lake 

comes from natural sources (ground water flowing through 

undeveloped areas to the north of Muskellunge Lake); 

however, septic systems were estimated to contribute about 

16 percent of the total input. One way to determine how 

this additional phosphorus from septic systems affects the 

water quality of the lake is through the use of empirical 

models. These models relate phosphorus loading to measures describing lake-water quality (such as phosphorus 

and chlorophyll a concentrations and Secchi depth). 

Empirical models contained in the Wisconsin Lake 

Modeling Suite (WiLMS, Panuska and Kreider, 2002) 

were used to assess how phosphorus input from septic systems affects the water quality of Muskel lunge Lake. Of the 

13 models contained in WiLMS, 9 were applicable to the 

hydrologic system of Muskel lunge Lake (table 3). Therefore, the morphometry of the lake, the water and phosphorus loadings for the study period (November 2000 through 

October 2001), and the same hydrology with loading from 

septic sys tems removed were input into these models, and 

then phosphorus concentrations were simulated. Each of 

the models simulates an average phosphorus concentration 

for a specific season (table 3). The average phos phorus 

concentration simulated by the models for these different 

seasons with all phosphorus inputs included was 35 μg/L, 

slightly lower than the average of the seasonally averaged 

data measured in the lake (about 43 μg/L). The models 

were then applied with the phosphorus from the septic 

systems removed. With out input from septic systems, the 

average phosphorus concentration in the lake decreased 

by 4.4 μg/L (a reduction in concentration of 12.7 percent, 

table 3). If it were assumed that the models should be used 

only to predict the percentage of change in phosphorus 

rather than absolute concentrations, then if phosphorus 

from septic systems were removed, the average summer 

near-surface phosphorus concentration should decrease 

from the summer average during 2001 of 49 μg/L (fig. 5) 

to 43 μg/L. 

Empirical relations have also been developed that 

predict average summer chlorophyll a concentrations and 

Secchi depths from near-surface phosphorus concentrations. Relations developed for Wisconsin lakes by Lillie 

and others (1993), also contained within WiLMS, were 

applied to determine how the changes in phosphorus concentration in the lake associated with the removal of septic 

system effluent should affect chlorophyll a concentrations 

and Secchi depths. These models predicted that, with the 

measured average summer phosphorus concentration of 

49 μg/L, the lake should have a chlorophyll a concentration of 13.5 μg/L (compared to 23 μg/L measured during 

2001) and a Secchi depth of 4.3 ft (compared to 3.6 ft 

measured). The predicted average summer phosphorus 

concentration of 43 μg/L was then input into the relations. 

Precipitation

3%

Septic 

systems

16%

Southeast 

tributary

15%

Ground 

water

58%

Nearshore 

8%

S u r f a c e - w a t e r i n f l o w

Figure 11. Phosphorus budget for Muskellunge Lake, 

Wisconsin, November 1, 2000 to October 31, 2001.
16 Water Quality and the Effects of Changes in Phosphorus Loading to Muskellunge Lake, Vilas County, Wisconsin

Table 3. Estimated phosphorus concentrations, in micrograms per liter, in Muskellunge Lake from various phosphorus-loading 

models contained in the Wisconsin Lakes Modeling Suite (Panuska and Kreider, 2002)

Lake model

Measured Modeled

Prediction

season

Seasonal 

concentration

Phosphorus loading 

of 437 pounds (with 

septic-system effluent)

Phosphorus loading 

of 368 pounds (without 

septic-system effluent)

Walker, 1985 (Reservoirs) Growing Season 49 35 31

Canfield and Bachman, 1981

 (Natural lakes)

 Spring 36 35 31

Canfield and Bachman, 1981

 (Artificial lakes)

 Spring 36 30 26

Rechow, 1977 (Anoxic lakes) Growing Season 49 54 45

Rechow, 1977

 (Water load < 164 feet/year)

 Growing Season 49 27 23

Walker, 1977 (General lakes) Spring 36 38 32

Vollenweider, 1982 (General lakes) Annual 43 28 25

Vollenweider, 1982 (Shallow lakes) Annual 43 23 20

Nurnberg, 1984 (Oxic lakes) Annual 43 46 43

Average concentration 43.4 35.1 30.7

Reduction, in micrograms per liter 4.4

Reduction, in percent 12.7

The models then predicted a chlorophyll a concentration 

of 12.6 μg/L (a 7-percent reduction from 13.5 μg/L 

predicted for 49 μg/L of phosphorus) and a Secchi depth 

of 4.3 ft (no change from that predicted for 49 μg/L of 

phosphorus). If it is again assumed that the models should 

be used only to predict the per centage of change, the 

removal of septic-system effluent should result in chlorophyll a concentrations decreasing from the measured 

summer average of 23 μg/L to 21 μg/L and little change 

in the average sum mer Secchi depth.

Removal of septic-system effluent should improve 

water quality only slightly. Phosphorus concentra tions 

in the lake should decrease by almost the same percentage as the decrease in phosphorus loading rates, whereas 

chlorophyll a concentrations should decrease by a smaller 

percentage and Secchi depths should change very little. 

Similarly, phosphorus loading from septic-system effluent 

has probably slightly degraded the water quality of Muskellunge Lake. Because there are few other anthropogenic 

sources of phosphorus, the water quality simulated by the 

models when septic-system effluent is removed is what 

the water quality should have been prior to development 

in the area. 

 Bottom sediments of a lake represent a natural record 

of the lake’s history and enable much to be inferred about 

historical changes in sediment and nutrient loading to the 

lake and water chemistry within the lake. Analyses of 

sediment cores extracted from Muskellunge Lake were 

used to describe the water quality of the lake more than 

100 years ago (P. Garrison, Wisconsin Department of 

Natural Resources, written com mun., 2000). Results 

from analysis of the diatom community from deep in the 

core (historical information) and near the sediment-water 

interface (recent information) indicated that historical 

nutrient levels in Muskellunge Lake were higher than those 

in many other lakes in the region and that the lake was 

already eutrophic more than 100 years ago. Changes in the 

diatom community indicate that phosphorus concentra tions 

in the lake have increased very little over the last 100 years 

(less than 5 μg/L); however, the changes in the diatoms 

also indicate that the macrophyte community has increased 

in density over this period.

The small changes in water quality inferred from 

changes in the diatom community in the sediment records 

agree with those found when comparing early water-quality data (collected in the 1970s) with recent data (2001).
Summary

Muskellunge Lake is a productive, eutrophic lake 

with an average summer total phosphorus concentration 

of 40 to 50 μg/L, chlorophyll a concentration of 10 to 

30 μg/L, and Secchi depth of about 4 ft. Historical data for 

each of these constituents indicate that water quality has 

degraded only slightly since the early 1970s, possibly as 

a consequence of septic-system effluent discharged to the 

lake. On the basis of nitrogen-to-phosphorus ratios measured in the lake, the productivity in the lake is limited by 

the supply of phosphorus. A detailed phosphorus 

budget for the lake indicated that 58 percent of the phosphorus enters with ground water, 23 percent with surfacewater inflow, 16 percent from septic-system effluent, and 

3 percent from direct precipitation. Most of this phosphorus appears to come from natural sources—ground water 

and surface water flowing through relatively undeveloped 

areas surrounding the lake. Modeling results indicated that 

the natural input of phosphorus was sufficient to maintain 

eutrophic conditions. An analysis of sediment cores confirms that nutrient and algal concentrations have changed 

only slightly over the past 100 years. The analysis of the 

cores indicated, however, that the macrophyte community has increased in density over this period. To alleviate 

winter anoxia (a common problem associated with shallow 

productive lakes) an aeration system was installed in Muskellunge Lake. This system functions as it was designed 

and maintains adequate dissolved oxygen concentrations 

(concentrations above 2 mg/L) throughout the main body 

of the lake during winter. 

References

Black, J.J., Andrews, L.M., and Threinen, C.W., 1963, 

Surface water resources of Vilas County: Madison, Wis., 

Wis consin Conservation Department.

Canfield, D.E., and Bachmann, R.W., 1981, Prediction 

of total phosphorus concentrations, chlorophyll a, 

and Secchi depths in natural and artificial lakes: 

Canadian Journal of Fisheries Aquatic Sciences, v. 38, 

p. 414–423.

Carlson, R.E., 1977, A trophic state index for lakes: 

Limnology and Oceanography, v. 22, p. 361–369.

Correll, D.L., 1998, The role of phosphorus in the 

eutrophication of receiving waters—A review: Journal 

of Environ mental Quality, v. 27, p. 261–266.

Garn, H.S., Olson, D.L., Seidel, T.L., and Rose, W.J., 

1996, Hydrology and water quality of Lauderdale Lakes, 

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Survey Water-Resources Investigations Report 96–4235, 

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Haitjema, H.M., 1995, Analytic element modeling of 

ground-water flow: San Diego, Calif., Academic Press, 

394 p.

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Improving a complex finite-difference ground water 

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and Goldman, R., 1998, Upper Midwest GAP analysis 

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manual, Ver sion 3.3 for Windows: Wisconsin Department of Natural Resources PUBL–WR–363–94, 32 p. 

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coefficients: U.S. Environmental Protection Agency, 

EPA–440/5–80–011.

Summary 17
18 Water Quality and the Effects of Changes in Phosphorus Loading to Muskellunge Lake, Vilas County, Wisconsin

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trophic state, Balsam Lake, northwestern Wisconsin, 

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