U.S. Department of the Interior

U.S. Geological Survey

 Scientific Investigations Report 2005–5071 

 In cooperation with the Little St. Germain Lake District 

 Water Quality, Hydrology, and Phosphorus Loading 

to Little St. Germain Lake, Wisconsin, with Special 

Emphasis on the Effects of Winter Aeration and 

Ground-Water Inputs 
Scientific Investigations Report 2005–5071

U.S. Department of the Interior

U.S. Geological Survey

Water Quality, Hydrology, and Phosphorus 

Loading to Little St. Germain Lake, 

Wisconsin, with Special Emphasis 

on the Effects of Winter Aeration and 

Ground-Water Inputs

By Dale M. Robertson, William J. Rose, and David A. Saad

In cooperation with the Little St. Germain Lake District
ii

U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.
iii

Contents

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

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

Little St. Germain Lake and its Watershed ...................................................................................... 2

Methods ......................................................................................................................................................... 2

Data Collection ..................................................................................................................................... 2

Aerator Design ..................................................................................................................................... 6

Simulation of Ground-Water Flow ..................................................................................................... 6

Lake-Water Quality and the Effects of Winter Aeration ......................................................................... 8

Water Clarity ......................................................................................................................................... 8

Distribution of Water Temperature and Dissolved Oxygen, and the Effects of 

Winter Aeration ...................................................................................................................... 8

Nutrient Concentrations ................................................................................................................... 13

Chlorophyll a Concentrations .......................................................................................................... 16

Trophic State Indices ........................................................................................................................ 16

Streamflow and Stream-Water Quality ................................................................................................... 16

Water and Phosphorus Loading to Little St. Germain Lake ................................................................ 19

Water Loading .................................................................................................................................... 19

Changes in Storage .................................................................................................................. 20

Precipitation and Evaporation ................................................................................................ 20

Surface-Water Inflow .............................................................................................................. 20

Surface-Water Outflow ............................................................................................................ 20

Ground-Water Inputs and Outputs ......................................................................................... 20

Water Budget ............................................................................................................................ 26

Phosphorus Loading ......................................................................................................................... 26

Precipitation .............................................................................................................................. 27

Surface-Water Inflow .............................................................................................................. 27

Ground-Water Inputs and Outputs ......................................................................................... 27

Septic Systems .......................................................................................................................... 27

Surface-Water Outflow ............................................................................................................ 30

Phosphorus Budget .................................................................................................................. 30

Effects of Phosphorus Reductions .......................................................................................................... 30

Summary ...................................................................................................................................................... 34

References Cited ........................................................................................................................................ 35
iv

Figures

 1-3. Maps showing:

 1. Drainage basin of Little St. Germain Lake, Vilas County, Wis. ...................................... 3

 2. Morphometry of Little St. Germain Lake, Vilas County, Wis. ......................................... 4

 3. Hydrologic features simulated with GFLOW analytic elements near Little 

 St. Germain Lake, Vilas County, Wis. ................................................................................. 7

 4-11. Graphs showing:

 4. Measured Secchi depths, total phosphorus and chlorophyll a concentrations,

 and summer average values for the various basins in Little St. Germain Lake, 

 Vilas County, Wis., 1991−2003. ............................................................................................ 9

 5. Distribution of dissolved oxygen in Little St. Germain Lake, Vilas County, Wis.,

 without aeration. ................................................................................................................. 11

 6. Late winter vertical distribution of dissolved oxygen in Little St. Germain Lake, 

 Vilas County, Wis., 1992−2004. .......................................................................................... 12

 7. Distribution of dissolved oxygen in the Upper East Bay of Little St. Germain

 Lake, Vilas County, Wis., with and without aeration. .................................................... 14

 8. Distribution of dissolved oxygen in South and Second South Bays of Little 

 St. Germain Lake, Vilas County, Wis., with and without aeration. .............................. 15

 9. Trophic state indices based on total phosphorus concentrations for the

 various basins in Little St. Germain Lake, Vilas County, Wis., 1991−2003. ................ 17

 10. Flow and total phosphorus concentrations in Muskellunge Creek and Little 

 St. Germain Creek, Vilas County, Wis., 1996−2003. ....................................................... 18

 11. Average total phosphorus concentrations in Muskellunge Creek and Little 

 St. Germain Creek, Vilas County, Wis., 1999−2003. ....................................................... 19

 12. Schematic of the components of the water budget for Little St. Germain Lake,

Vilas County, Wis. ...................................................................................................................... 19

 13-14. Graphs showing:

 13. Monthly precipitation, inflow, outflow, and daily average water elevations 

 for Little St. Germain Lake, Vilas County, Wis. ............................................................... 21

 14. Measured and simulated head for the calibrated GFLOW model for the area

 near Little St. Germain Lake, Vilas County, Wis. ............................................................ 23

 15. Map showing surface-water contributing area, simulated ground-water 

contributing area and flow direction, and ground-water flux for Little St. Germain 

Lake, Vilas County, Wis. ............................................................................................................ 25

 16. Graph showing water budgets for Little St. Germain Lake, Vilas County, Wis. 

for monitoring years MY1997, MY1999, and MY 2001. ......................................................... 26

 17. Map showing phosphorus loading into and out of Little St. Germain Lake, 

Vilas County, Wis., from ground water, by shoreline segment for monitoring 

year 2001. .................................................................................................................................... 29

 18. Graph showing phosphorus budgets for Little St. Germain Lake, Vilas County, 

Wis., for monitoring years MY1997, MY1998 and MY1999. ................................................. 31
v

Tables

 1. Morphometry of Little St. Germain Lake and each of its basins, Vilas County,

Wisconsin. .................................................................................................................................... 5

 2. Monthly water budget for Little St. Germain Lake, for October 1, 1996, to

September 30, 1997; December 1, 1998, to November 30, 1999; and November

1, 2000, to October 31, 2001. ..................................................................................................... 22

 3. Measured and simulated cumulative net ground-water discharge values for

the calibrated GFLOW model. .................................................................................................. 24

 4. Monthly phosphorus budget for Little St. Germain Lake, for October 1, 1996, 

to September 30, 1997; December 1, 1998, to November 30, 1999; and 

November 1, 2000, to October 31, 2001. .................................................................................. 28

 5. Simulated changes in average June through August near-surface total

phosphorus concentrations in the East and Upper East Bays of Little St.

Germain Lake, Vilas County, Wisconsin, in response to reductions in tributary

loading. ........................................................................................................................................ 32

 6. Simulated changes in June through August near-surface chlorophyll a

concentrations and Secchi depths in the East and Upper East Bays of Little

St. Germain Lake, Vilas County, Wisconsin, in response to reductions in

tributary loading. ........................................................................................................................ 33
vi

Conversion Factors and Abbreviated Water-Quality Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

acre 0.004047 square kilometer (km2

)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

cubic foot (ft3

) 0.02832 cubic meter (m3) 

acre-foot (acre-ft) 1,233 cubic meter (m3

)

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

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

 °F = [1.8 x °C] + 32.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Sea Level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 

(NGVD of 1929) – a geodetic datum derived from a general adjustment of the first-order level of 

both the United States and Canada, formerly called Sea Level Datum of 1929.
vii

Acknowledgments

Technical Reviewers

Herbert S. Garn, Assistant Center Director, U.S. Geological Survey, Middleton, Wis.

Thomas C. Winter, Hydrologist, U.S. Geological Survey, Denver, Col.

Local Project Coordination

Stephen Sward, Ted Ritter, Ron Mackowski, and Ervin Stiemke, Little St. Germain Lake District, 

St. Germain, Wis.

Editorial and Graphics

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

Jennifer L. Bruce, Geographer, U.S. Geological Survey, Middleton, Wis.

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

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

Approving Official

James M. Gerhart, U.S. Geological Survey, Baltimore, Md.
viii
Abstract

Little St. Germain Lake is a 978-acre, multibasin lake 

in Vilas County, Wisconsin. In the interest of protecting 

and improving the water quality of the lake, the Little St. 

Germain Lake District initiated several cooperative studies 

with the U.S. Geological Survey between 1991 and 2004 

to (1) document the water quality and the extent of winter 

anoxia in the lake, (2) evaluate the success of aerators at 

eliminating winter anoxia, (3) develop water and nutrient 

budgets for the lake, and (4) assess how the water quality 

of the lake should respond to changes in phosphorus loading. This report presents the results of these cooperative 

studies with special emphasis on the water quality in the 

lake since 2000, including the effects of winter aeration 

and the importance of ground-water contributions of phosphorus to the productivity of the lake.

Measurements collected during these studies indicate that the water quality in Little St. Germain Lake 

was consistently different among basins. The West Bay 

consistently had the best water quality, the South Bay 

had intermediate water quality, and the East and Upper 

East Bays consistently had the worst water quality. The 

water quality in each of the basins was relatively stable 

from 1991 to 2000; however, since 2001, the West Bay 

has changed from oligotrophic to mesotrophic, the South 

Bay has changed from mesotrophic to eutrophic, and the 

East and Upper East Bays have changed from eutrophic to 

eutrophic/hypereutrophic.

Winter anoxia frequently occurred throughout most 

of the lake, except in the West Bay and just below the ice 

in the East Bay. To eliminate winter anoxia, coarse-bubble 

line aerators were installed and operated in the Upper East, 

East, and South Bays. The aerators in the Upper East and 

South Bays were very successful at eliminating winter 

anoxia; however, the aerator in the East Bay had little 

impact on the dissolved oxygen concentrations throughout 

its basin. 

Detailed water and phosphorus budgets computed 

for the lake indicated that inflow from Muskellunge Creek 

was the major source of phosphorus to the lake and that 

ground water was the secondary source. Results from a 

detailed ground-water-flow model indicated that ground 

water flows into the lake from all sides, except the south 

sides of the West and Second South Bays. Most of the 

phosphorus appears to come from natural sources, such as 

ground water and surface water flowing through relatively 

undeveloped areas surrounding Little St. Germain Lake 

and Muskellunge Lake.

Several empirical water-quality models were used 

to simulate how the East and Upper East Bays of the lake 

should respond to reductions in phosphorus loading from 

Muskellunge Creek. Simulation results indicated that 

reductions in tributary loading could improve the water 

quality of the East and Upper East Bays. Improving the 

water quality of these bays would also improve the water 

quality of the South and Second South Bays because of 

the flow of water through the lake. However, even with 

phosphorus loading from Muskellunge Creek completely 

eliminated, most of the lake would remain borderline 

mesotrophic/eutrophic because of the contributions of 

phosphorus from ground water. 

Introduction 

Little St. Germain Lake, located just northeast of 

St. Germain in Vilas County, Wisconsin (fig. 1), is one of 

21 impoundments operated by Wisconsin Valley Improvement Company (WVIC) to provide storage for power 

Water Quality, Hydrology, and Phosphorus Loading 

to Little St. Germain Lake, Wisconsin, with Special 

Emphasis on the Effects of Winter Aeration and 

Ground-Water Inputs

By Dale M. Robertson, William J. Rose, and David A. Saad
and for recreational use. The level of the lake, which was 

originally dammed in 1882, has been maintained by the 

WVIC at about 5 ft above its natural level since 1929, and 

it is annually drawn down about 1.5 ft from December 

through March. In the interest of protecting and improving 

the water quality of the lake, the Little St. Germain Lake 

District has collaborated with the Wisconsin Department 

of Natural Resources (WDNR) and the U.S. Geological 

Survey (USGS) to conduct several water-quality studies. 

From 1983 to 1985, the WDNR examined the water quality of the lake and various management alternatives (Wisconsin Department of Natural Resources, 1985). Results of 

the study indicated that, because of relatively high loading 

of phosphorus to the lake, most of the lake was eutrophic 

(based on relatively high chlorophyll a concentrations), 

with the possible exception of the West Bay. The results 

also indicated that monitoring of the lake should continue 

and that actions should be taken to decrease the nutrient 

loading to the lake by controlling erosion, fertilizer runoff, 

and leakage from septic systems.

The lake was monitored in detail during 1991–94 by 

the USGS as part of a cooperative study with the Little St. 

Germain Lake District. The study demonstrated variation in water quality among the various basins of the lake 

and found extensive areas of winter anoxia (absence of 

oxygen). Further in-depth studies were done by the USGS 

during 1994–2000 to refine the water and phosphorus 

budgets of the lake, quantify the effects of annual drawdowns, define the extent of winter anoxia, and provide 

information needed to develop a comprehensive lake-management plan (Robertson and Rose, 2000). Results of the 

study indicated that Muskellunge Creek was the dominant 

source of phosphorus to the lake; however, ground-water 

contributions (based on limited information) appeared to 

be important and should be better quantified. The results 

of the study also indicated extensive areas of winter anoxia 

in the Upper East, South, and Second South Bays (fig. 2). 

As a result of that study, aerators were placed in the Upper 

East Bay, the South Bay, and in the northern end of the 

East Bay and operated throughout the winter. 

The USGS, in cooperation with the Little St. Germain Lake District, continued to study Little St. Germain 

Lake from 2000 to 2004. The goals of this effort were to 

(1) continue to document the water quality in the lake, (2) 

evaluate the success of the aerators at eliminating winter 

anoxia, (3) refine the nutrient budget of the lake by better quantifying flow and phosphorus contributions from 

ground water directly entering the lake and entering by 

way of Muskellunge Creek during November 1, 2000, to 

October 31, 2001, and (4) assess how the water quality of 

the lake should respond to changes in phosphorus loading. 

To better define the contributions from ground water (locations and amounts), a ground-water-flow model was developed for the Little St. Germain area. This report presents 

the results of the studies since 1991, with special emphasis 

on the water quality in the lake since 2000–including the 

effects of winter aeration and the importance of groundwater contributions of phosphorus to the productivity of 

the lake.

Little St. Germain Lake and its Watershed

Little St. Germain Lake (fig. 1) is a multibasin lake 

with a total surface area of 978 acres and volume of 11,498 

acre-ft (table 1). In this report, the lake is discussed in 

terms of six basins or “bays” (fig. 2 and table 1): Upper 

East Bay (119 acres), East Bay (336 acres), No Fish Bay 

(69 acres), West Bay (213 acres), South Bay (122 acres), 

and Second South Bay (119 acres). The major tributary 

to the lake is Muskellunge Creek, which flows about 3 

mi from shallow, eutrophic Muskellunge Lake into the 

north end of the East Bay (fig. 2). Muskellunge Lake was 

extensively studied by the USGS during 2000–2001 as part 

of a cooperative study with the Muskellunge Lake Association (Robertson and others, 2003); results from that 

study are used to further describe the importance of ground 

water and contributions from natural sources to Little 

St. Germain Lake. Outflow from the lake is to Little St. 

Germain Creek, which leaves the south side of the Second 

South Bay and flows about 1 mi before draining into the 

Wisconsin River. 

The total watershed area of Little St. Germain Lake 

is 10 mi2 (not including the lake itself). The watershed is 

predominantly forest (68 percent), wetland (17 percent), 

and water (8 percent), although areas of low-density residential development are increasing, especially immediately 

around the lakes (fig. 1). The soils in the watershed consist 

mainly of well-drained sand and sandy loams. These soils 

are thought to be naturally high in phosphorus content 

(Wisconsin Department of Natural Resources, 1985). 

Methods

Data Collection

Data used to describe the water quality of the lake 

were collected from April 1991 to March 2004; however, 

no data were collected from September 1994 to July 1996 

and September 1997 to February 1999. Lake water-quality 

2 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
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Figure 2. Morphometry of Little St. Germain Lake, Vilas County, Wis.

4 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
properties were generally measured five times per year 

(late winter, May, June, July, and August) at three sites: 

the centers of the East, West, and South Bays (fig. 2). In 

addition, water-quality data were collected at the center of 

the Upper East Bay from 2000 to 2004. At all sites, depth 

profiles of water temperature, dissolved oxygen, specific 

conductance, and pH were measured during each visit with 

a multiparameter instrument. Water samples were collected 

at these sites at either or both near surface (1.5 ft below 

the surface during open water or just below ice during ice 

cover) or near bottom (1.5 ft above bottom) with a Van 

Dorn sampler. Near-surface water samples were analyzed 

for concentrations of total phosphorus (an indicator of 

the amount of available nutrients) and chlorophyll a (an 

indicator of the algal population). During ice-free periods, 

Secchi depths (an indicator of water clarity) also were 

measured. During spring overturns after ice out in 1991 

through 1994, in 2002, and in 2003, surface-water samples 

were analyzed for a full suite of water-quality constituents 

in the South and West Bays. 

Additional depth-profile measurements of temperature and oxygen were made at various locations during 

the winters of 1997–2004 to assess the extent and timing 

of anoxia before and during operation of the aerators. The 

most extensive set of profiles was collected in the basins 

in which aerators were placed: Upper East Bay, East, and 

South Bays (fig. 2). 

Inflow and phosphorus loading to the lake were 

determined from flow measurements and water samples 

collected approximately monthly in Muskellunge Creek 

at Birchwood Drive (fig. 1) during October 1996–September 1997, December 1998–January 2000, and November 

2000–October 2003. During 1996–97 and 1998–2000, 

flows between measurements were estimated by linearly 

interpolating between monthly measurements. Continuous 

(15-minute intervals) water-elevation measurements were 

collected at Birchwood Drive and the outlet of Muskellunge Lake from March 2001 through October 2001 to 

better describe the flow throughout Muskellunge Creek 

and better estimate the total phosphorus contributions from 

Muskellunge Creek. The monthly streamflow measurements were used to develop stage-discharge relations, 

which were then used in conjunction with water-elevation 

data to compute flow at these locations. During 1996–97, 

water samples collected at Birchwood Drive were analyzed 

for total phosphorus concentration. During 1998–99, water 

temperature and dissolved oxygen also were measured 

at Birchwood Drive, and the samples also were analyzed 

for dissolved phosphorus. During 1999–2003, water 

temperature and dissolved oxygen measurements were 

measured, and water-quality samples (analyzed for total 

and dissolved phosphorus) were collected approximately 

monthly at the Muskellunge Lake outlet (Robertson and 

others, 2003) and at the snowmobile crossing between the 

two lakes (fig. 1) to help define the sources of water and 

phosphorus to Muskellunge Creek. 

Surface-water outflow from Little St. Germain Lake 

was estimated from water-elevation measurements made at 

the dam at Highway 70 by WVIC. Water levels at the dam 

on Little St. Germain Creek were monitored almost daily 

from 1991 to 2003 by the WVIC (U.S. Geological Survey, 

Wisconsin District Lake-Studies Team, 2004). Additional 

flow measurements and water samples were collected 

monthly just below the dam from December 1998 through 

September 2003 to better describe the outflow from the 

lake. Water samples were analyzed for total phosphorus. 

The flow measurements at the dam indicated that low 

flows were underestimated; therefore, those flows were 

adjusted accordingly. 

Fifteen small, shallow piezometers (small-diameter 

observation wells) were installed approximately 1 to 3 ft 

below the water table around the lake (fig. 2) to help define 

Table 1. Morphometry of Little St. Germain Lake and each of its basins, Vilas County, Wisconsin. 

Basin/Total

Maximum Depth

(feet)

Mean Depth

(feet)

Area

(acres)

Volume

(acre-feet)

Upper East 15 7.5 119 893

East 12 8.9 336 2,982

No Fish 6 4.4 69 305

West 53 24.1 213 5,140

South 21 11.6 122 1,417

Second South 9 6.4 119 761

Entire lake 53 11.8 978 11,498

Methods 5
areas contributing ground water to the lake, determine the 

phosphorus concentrations in the ground water entering 

the lake, and quantify the phosphorus loading from ground 

water. Ground-water gradient (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 two times: May 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). 

Data collected during this study were published in 

two annual USGS data report series. Streamflow data were 

published in “Water Resources Data, Wisconsin–Water 

Year 2001” (Waschbusch and others, 2002) and lake data 

in the series “Water Quality and Lake-Stage Data for 

Wisconsin Lakes, Water Years 1994–2003” (the most 

recent Little St. Germain Lake data were published in U.S. 

Geological Survey, Wisconsin District Lake-Studies Team, 

2004).

Aerator Design

Coarse-bubble line aerators were installed in the 

Upper East and East Bays in fall 2001 (fig. 2) and operated 

during the winters of 2002, 2003, and 2004. (All winters 

are described in terms of the year in which the ice left the 

lake.) Each aerator is supplied by 5-horsepower blowers on 

the nearby shoreline. The air travels through 4-in. pipe out 

to the aerators, which were constructed of 2-in. pipe with 

1/8-in. holes placed about 4 ft apart. The aerator in the 

East Bay is about 600 ft off the northwest shore at a 

depth of about 4 ft. This aerator was designed to release 

42 ft3

/min of air at 6 psi through two parallel 67-ft lines 

placed about 100 ft apart; each line has 16 holes. The 

aerator in Upper East Bay is near the center of the bay at 

a depth of about 10 to 12 ft. This aerator was designed to 

release 117 ft3/min of air at 10 psi through five parallel 

75-ft lines about 100 ft apart; each line has 18 holes. A 

coarse-bubble line aerator was installed into the South Bay 

in fall 2002 (fig. 2) and operated during the winters 

of 2003 and 2004. The aerator in the South Bay is about 

600 ft off the northwest shore at a depth of about 20 ft. 

This aerator was designed to release 117 ft3

/min of air at 

10 psi through five parallel 75-ft lines placed about 100 ft 

apart; each line has 18 holes. Each of the aerators is operated from shortly after ice formation until shortly before 

ice out.

Simulation of Ground-Water Flow

The two-dimensional, analytic-element, steady-state, 

ground-water-flow model, GFLOW (Haitjema, 1995) was 

used to define ground-water-source areas around Little 

St. Germain Lake (and Muskellunge Lake) and to allocate 

ground-water discharge to shoreline segments represented 

by the piezometers installed around the lake. A complete 

description of analytic-element modeling is beyond the 

scope of this report; however, a brief description is given 

below. A complete discussion of the methods for applying the model can be found in Strack (1989) and Haitjema 

(1995). 

An infinite aquifer is assumed in analytic-element 

modeling. In GFLOW, the study area (domain) does not 

require a grid or involve interpolation between cells. To 

construct an analytic-element model, features that affect 

ground-water flow (such as surface-water bodies, aquifer 

characteristics, and recharge) are entered as mathematical 

elements or strings of elements (described below). The 

amount of detail specified for the features depends on the 

distance from the area of interest. Each element is represented by an analytic solution. The effects of these individual solutions are added together to arrive at a solution 

for the ground-water-flow system. Because the solution 

is not confined to a grid, ground-water levels (heads) and 

flows can be computed anywhere in the study area without 

averaging values at specific locations in the model (nodal 

averaging). In the GFLOW model, the analytic elements 

are two dimensional and are used to simulate only steadystate conditions. Ground-water-flow systems are three 

dimensional; however, two-dimensional models can provide reasonable approximations of ground-water flowlines 

when the lengths of the flowlines are long compared to 

the aquifer thickness (Haitjema, 1995, p. 23). In the study 

area, most ground water was assumed to move through 

unconsolidated deposits that have a maximum saturated 

thickness of 200 ft or less. The lengths of flowlines from 

recharge areas to discharge areas are typically several 

thousand feet or more in length. 

The GFLOW model for the study area covers an 

area extending approximately 4 to 7 mi around Little St. 

Germain Lake (fig. 3). The geometry of the single-layer 

model includes a bottom elevation set at 1,500 ft and an 

average aquifer thickness of 200 ft, which were based 

on well logs for the study area and additional information from Patterson (1989). Recharge was applied to the 

model area using an inhomogeneity element (an element 

with different aquifer properties from the rest of the study 

area). Surface-water features were simulated using several 

6 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
Lake-Water Quality and the Effects of Winter Aeration 7

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types of linesink elements. The model includes “far-field” 

and “near-field” sources and sinks of water (collectively 

referred to as linesinks; fig. 3). The far-field area surrounds 

the near-field area of interest. In the far field, streams are 

simulated as coarse linesinks having little or no resistance 

between the surface-water feature and the ground-waterflow system. The purpose of simulating the far field is 

to have the model explicitly define the regional groundwater-flow field near the area of interest. The near field 

represents the area of interest and includes Little St. Germain and Muskellunge Lakes and other adjacent lakes and 

streams (fig. 3). Base flows in near-field streams (Muskellunge Creek and Little St. Germain Creek near the lake 

outlet) were used to calibrate the model (flux calibration); 

flows in far-field streams were not. Near-field streams 

and lakes, in good connection with the ground-water-flow 

system, were included as linesinks with resistance.

A constant value for stage was used for Little St. 

Germain Lake even though there was an annual stage 

fluctuation of about 1.5 ft. Cyclic fluctuations of this 

magnitude should have minimal effect on regional groundwater flow, and the net gains and losses to the groundwater-flow system immediately adjacent to the lake should 

be approximated by steady-state conditions on an annual 

basis. Muskellunge Lake was simulated with a special type 

of linesink with resistance, called a lake element, by use of 

the methodology described by Hunt and others (2003). The 

lake element solves for lake stage on the basis of simulated 

surface-water inflows and estimated surface-water outflows from the lake. 

In analytic-element modeling, resistance is computed 

by dividing the bed-sediment thickness by the vertical 

hydraulic conductivity. For this model, resistance was set 

equal to 0.3 day for near-field streams, and 1.0 day for 

near-field lakes. These values are similar to sediment-resistance values used in other nearby GFLOW models (Hunt 

and others, 1998; Graczyk and others, 2003). Assigned 

stream widths ranged from 10 to 40 ft. The leakage length 

for near-field lakes was set equal to 64 ft and calculated 

by use of methods described by Hunt and others (2003). 

Perched or mounded lakes (such as Pincherry and Honeysuckle Lakes) were included as discharge-specified linesinks. The value assigned to discharge-specified linesinks 

limits the amount of water that can be added to the groundwater-flow system from perched lakes. The amount of 

discharge assigned to this type of linesink (8.9 in/yr) was 

computed from the long-term precipitation for the area 

(Hunt and others, 1998) minus evaporation (estimated in 

the section entitled “Water and Phosphorus Loading to 

Little St. Germain Lake”, page 19).

Lake-Water Quality and the Effects 

of Winter Aeration

Water quality in Little St. Germain Lake was consistently different among basins, except for a few water-quality characteristics (specific conductance and pH) that were 

similar throughout the lake but varied seasonally. Specific 

conductance ranged from about 75 μS/cm in summer to 

about 90 μS/cm in winter, and pH ranged from about 7 (in 

standard units) in winter to about 8 in summer. 

Water Clarity

Water clarity, based on Secchi-depth readings, was 

highly variable and ranged from about 1.5 ft in the East 

and Upper East Bays to more than 15 ft in the West Bay. 

The West Bay consistently had the greatest water clarity, which ranged from 5 to 15 ft (average summer, June 

through August, clarity ranged from 8.6 to 13.1 ft). The 

South Bay had intermediate clarity that ranged from 1.6 to 

14 ft (average summer clarity from 3.9 to 9.6 ft). The East 

and Upper East Bays consistently had the least water clarity, which ranged from 1.6 to 8 ft (average summer clarity 

from 2.8 to 5.5 ft) (fig. 4). In the West Bay, water clarity 

was usually greatest in late summer; however, in the East 

and South Bays, it was usually greatest in early summer. 

The summer average water clarities in East, Upper East, 

and South Bays were less in 2002 and 2003 than in any 

other year measured. Water clarity in these basins in summer 2004 was comparable to 2002 and 2003 water clarity 

(Ron Mackowski, Little St. Germain Lake District, written 

commun., 2004).

Distribution of Water Temperature and 

Dissolved Oxygen, and the Effects of Winter 

Aeration

The extent of thermal stratification differed among 

basins because of differences in the morphometries of the 

basins and limited circulation between basins. The West 

Bay, being relatively deep, consistently became strongly 

stratified during summer, with bottom temperatures 

remaining around 7–9 °C. The South Bay, being moderately deep, became stratified during summer; however, in 

some years, stratification was frequently broken down by 

wind mixing. Because of the frequent deep mixing, bottom 

temperatures in the South Bay usually increased as summer progressed. Most of the rest of the lake is relatively 

shallow, which resulted in stratification being very weak in 

8 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin














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Figure 4. Measured Secchi depths, total phosphorus and chlorophyll a concentrations, and summer average (June through 

August) values for the various basins in Little St. Germain Lake, Vilas County, Wis., 1991−2003.

Lake-Water Quality and the Effects of Winter Aeration 9
these areas, with seldom more than 2 or 3°C of stratification. During winter and without aeration, thermal stratification was weak but present throughout the entire water 

column in each of the basins. With aeration in the Upper 

East Bay and South Bay, little if any thermal stratification 

existed above the depth of the aerators; however, limited 

thermal stratification occurred below the depths of the 

aerators. The aerator in the East Bay, owing to its shallow 

depth, appeared to have little effect on the winter stratification in the center of the basin.

Thermal stratification during summer, which primarily occurred in the West and South Bays, isolated 

the deepest water from surface interactions. Thus, as 

summer progressed, dissolved oxygen concentrations in 

water below the thermocline decreased as a consequence 

of decomposition of dead algae and other organic matter 

that settled from the surface and the biochemical oxygen 

demand of the sediment. Water below about 25–30 ft in the 

West Bay usually became anoxic in late June and stayed 

anoxic throughout summer. In the South Bay, the variable 

strength of thermal stratification resulted in anoxia in the 

deepest water during summers when stratification was 

extensive and low oxygen concentrations during summers 

when mixing was more frequent. Anoxia seldom occurred 

in the other bays during summer because of frequent mixing. 

Before freezing, the entire water column of most 

lakes is nearly saturated with oxygen; however, after a lake 

freezes and snow cover increases as winter progresses, 

oxygen can be quickly consumed, especially in shallow, 

productive lakes. This was the case in Little St. Germain 

Lake, especially in the shallower basins when aeration 

was not used. Oxygen depletion was much more severe 

during winter than during summer because of the lack of 

oxygen transfer through the surface as a result of the ice 

cover and because the snow cover reduced light penetration and therefore limited oxygen-producing photosynthesis. To demonstrate the spatial extent of oxygen depletion, 

transects of oxygen profiles were collected from the inlet 

to the outlet (A–B; fig. 2) and from the West Bay to the 

outlet (A'–B; fig. 2) in March 1997 (fig. 5) and March 

1999. Detailed transects were collected in March because 

this was near to when oxygen depletion was expected to be 

most severe. Anoxia occurred in each of the basins, and by 

mid-March only small areas of the lake were habitable by 

most warm-water fish (areas with dissolved oxygen concentrations greater than about 2 mg/L, Barton and Taylor, 

1996). These habitable areas include water down to depths 

of about 30 ft in the West Bay and down to about 

5 ft in the East Bay. Without aeration, in most years, oxygen was almost completely consumed by mid-February in 

the Upper East, No Fish, South, and Second South Bays. 

Water entering from Muskellunge Creek apparently 

may help alleviate the extent of late-winter anoxia in the 

East Bay. Although dissolved oxygen concentrations in 

Muskellunge Creek may be low in midwinter (less than 6 

mg/L in February 1999 and possibly much lower in other 

years), concentrations can be high later in winter when 

parts of Muskellunge Creek are free of ice (greater than 10 

mg/L in March 1999). Dissolved oxygen concentrations in 

the middle of the East Bay were lower in February 1997 

than they were later in March 1997. This increase appears 

to be associated with cold, highly oxygenated water 

originating from Muskellunge Creek propagating across 

the basin (fig. 5). Dissolved oxygen concentrations in the 

Upper East Bay, which are not influenced by Muskellunge 

Creek inflow, did not increase from February to March. A 

detailed analysis of the flow in the lake demonstrated that 

the upper 3 ft of water (just below the ice) throughout the 

East Bay could be replaced by water from Muskellunge 

Creek in about 30 days. 

Aerators were operated in the East and Upper East 

Bays in the winters of 2002, 2003, and 2004, and operated 

in the South Bay in 2003 and 2004 to eliminate winter 

anoxia and the associated fish kills and odor problems near 

the outlet. Winter fish kills (referred to as “winterkills”) 

usually result when dissolved oxygen concentrations 

throughout the lake drop below a critical concentration 

(usually considered below 2 mg/L; Wetzel, 1983). The 

success of the aerators at eliminating winter anoxia and 

the potential for fish kills can be evaluated by comparing 

oxygen profiles collected at the center of the basins (fig. 

6) and oxygen transects collected across the basins (figs. 

7 and 8) in late winter (March) in years with and without 

aeration. Each of the three years which the aerators were 

operated had distinct characteristics: 2002 had ice cover 

form later than normal, 2003 had very little snowfall, and 

2004 had a more typical freeze date and relatively heavy 

snowfall. In 2002, the late ice formation should have 

resulted in delayed dissolved oxygen depletion. In 2003, 

there was very little snowfall, which resulted in thick clear 

ice that enabled light to penetrate into the water throughout 

most of the winter. This should have resulted in oxygen 

being depleted at a much slower rate than what typically 

occurs in Little St. Germain Lake because of oxygen 

production associated with algal and macrophyte productivity. Therefore, it is difficult to evaluate the effectiveness 

of the aerators at eliminating winter anoxia in 2003. The 

earlier freeze date and more snowfall in 2004 should have 

resulted in the most severe oxygen depletion and the best 

10 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
      

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Figure 5. Distribution of dissolved oxygen in Little St. Germain Lake, Vilas County, Wis., March 18, 1997, without aeration.

Lake-Water Quality and the Effects of Winter Aeration 11














     















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Figure 6. Late winter vertical distribution of dissolved oxygen (February, 1992−94 and March, 1997−2004) in Little St. Germain 

Lake, Vilas County, Wis., 1992−2004. Years in which aeration was used are identified by diamond symbols.

12 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
evaluation of the aerators during harsh winter conditions. 

More winterkills in nearby lakes were reported in 2004 

than in most other years. 

The aerator in the East Bay appeared to have minimal 

effect on the dissolved oxygen concentrations throughout 

the basin. The dissolved oxygen profile collected in 2002 

at the center of the basin was very similar to those measured in 1994 and 2000 when aerators were not operated 

(fig. 6). The slightly higher dissolved oxygen concentrations in 2003 than in other years may have resulted from 

the very little snowfall that year. The ineffectiveness of the 

aerators may have been caused by being placed in water 

that was too shallow (approximately 4 ft) and (or) the aerator releasing too little air given the size of the East Bay.

The aerator in the Upper East Bay was very successful at eliminating winter anoxia and possible winterkills. 

In all three years of operation, dissolved oxygen concentrations remained above 3.5 mg/L throughout almost the 

entire water column, except below the depth of the aerators 

(fig. 6). Relatively high dissolved oxygen concentrations 

were found throughout the basin (fig. 7). Even during 

2004, dissolved oxygen concentrations were above 4 mg/L 

throughout most of the water column. These concentrations were below the Wisconsin state standard of 5 mg/L 

for dissolved oxygen in warm-water lakes (Shaw and others, 1993) but were above the 2-mg/L critical concentration for most fish.

The aerator in the South Bay was very successful at 

eliminating winter anoxia and possible winterkills in the 

South and Second South Bays. In both years of operation, dissolved oxygen concentrations remained above 3 

mg/L throughout almost the entire water column in the 

South Bay, except below the depth of the aerators where 

concentrations diminished sharply (fig. 6). Relatively high 

dissolved oxygen concentrations were found throughout 

most of the basin, but concentrations did diminish near the 

sediment surface (fig. 8). Even during the harsh winter of 

2004, dissolved oxygen concentrations were above 3 mg/L 

throughout most of the water column. The movement of 

water with low dissolved oxygen concentrations from No 

Fish Bay to the South Bay resulted in lower dissolved oxygen concentrations on the northwest side of the South Bay, 

especially near the bottom (fig. 8). 

The positive effects of the aerator in the South Bay 

were propagated downstream into the Second South Bay. 

The movement of water with relatively high dissolved 

oxygen concentrations from the South Bay into the Second 

South Bay increased dissolved oxygen concentrations in 

the water above the depth of the sill separating the two 

basins (fig. 8). The cooler, less dense water from the South 

Bay flowed over the warmer and more dense water below 

the sill depth in the Second South Bay. This resulted in a 

zone of oxygenated water (greater than 2 mg/L) just below 

the ice in the Second South Bay.

Nutrient Concentrations

Phosphorus and nitrogen are essential nutrients 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 accelerated eutrophication (that 

is, accelerated aging and increased productivity) of lakes. 

The ratio of near-surface concentrations of total nitrogen 

to total phosphorus is often used to indicate which of 

these nutrients should potentially 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 (Corell, 1998); however, a ratio 

greater than about 12:1, by weight, usually indicates 

that phosphorus is the potentially limiting nutrient. Total 

nitrogen was measured only during spring overturn in the 

South and West Bays and once at the four main sampling 

sites in 2002; therefore, the nitrogen to phosphorus ratio 

could only be quantified at a few times. In July 2002, the 

nitrogen-to-phosphorus ratio ranged from about 10:1 in 

the East and Upper East Bays, to about 17:1 in the South 

Bay, to about 50:1 in the West Bay. These ratios are similar 

to those measured in the South and West Bays at spring 

overturn in 2002 and other years. Therefore, phosphorus 

should usually be the nutrient limiting algal growth in most 

of the lake (possibly co-limiting in the East and Upper 

East Bays) and the nutrient to focus on when considering 

management efforts to improve water quality.

Phosphorus concentrations at the surface ranged from 

0.006 mg/L (West Bay) to 0.116 mg/L (East Bay) (fig. 4). 

The West Bay consistently had the lowest concentrations, 

which ranged from 0.006 to 0.038 mg/L (average summer concentrations from 0.009 to 0.026 mg/L; fig. 4). The 

South Bay had intermediate concentrations, which ranged 

from 0.012 to 0.094 mg/L (average summer concentrations 

from 0.018 to 0.048 mg/L). The East and Upper East Bays 

consistently had the highest concentrations, which ranged 

from 0.023 to 0.116 mg/L (average summer concentrations 

from 0.038 to 0.075 mg/L). Phosphorus concentrations 

were usually highest in late summer in the East and Upper 

East Bays; however, no consistent seasonal patterns were 

evident in the South and West Bays. The highest phosphorus concentrations were measured in 2002 and 2003 in all 

locations but the West Bay.

Lake-Water Quality and the Effects of Winter Aeration 13
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Figure 7. Distribution of dissolved oxygen in the Upper East Bay of Little St. Germain Lake, Vilas County, Wis., with and without 

aeration.

14 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
























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Figure 8. Distribution of dissolved oxygen in South and Second South Bays of Little St. Germain Lake, Vilas County, Wis., 

with and without aeration.

Lake-Water Quality and the Effects of Winter Aeration 15
Phosphorus can be released from lake sediments, 

especially during periods of anoxia. Increased phosphorus 

concentrations just above the sediments primarily were 

observed in the West Bay during late summer, when the 

deep water was anoxic. Phosphorus concentrations typically reached 0.2–0.3 mg/L just above the bottom in late 

summer in the West Bay. Because of the variable strength 

of stratification in the South Bay and variable lengths of 

time with anoxia near the bottom, the buildup of phosphorus in the deep water was variable. In 2003, phosphorus 

concentrations were greater than 0.6 mg/L in August; 

however, in most years, phosphorus concentrations were 

similar to those measured near the surface because of 

mixing. Anoxia was seldom measured in the other bays 

during summer because of frequent and extensive mixing 

events, and therefore near-bottom phosphorus concentrations were very similar to those measured at the surface. 

Although anoxia was common under the ice, near-bottom 

phosphorus concentrations were not as great as during 

summer anoxia. Near-bottom concentrations of about 0.2 

mg/L were measured in each of the basins, but only in the 

absence of aeration. 

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 of a lake. In the northern part of Wisconsin, 

concentrations greater than 15 μg/L are usually considered 

to be high and are usually associated with algal blooms. 

Chlorophyll a concentrations ranged from less than 1 μg/L 

(West Bay) to greater than 100 μg/L (East and Upper East 

Bays) (fig. 4). Differences in chlorophyll a concentrations 

among basins directly coincided with the differences in 

the phosphorus concentrations among basins. Concentrations were commonly greater than 15 μg/L in the East and 

Upper East Bays and occasionally greater than 15 μg/L in 

the South Bay, but only once was a concentration greater 

than 15 μg/L observed in the West Bay. Average summer 

concentrations were highest in the East and Upper East 

Bays (from 18 to 82 μg/L), moderate in the South Bay 

(from 6 to 39 μg/L), and lowest in the West Bay (from 2 

to 8 μg/L) (fig. 4). Chlorophyll a concentrations increased 

dramatically in all of the basins in 2002 and 2003, but 

especially in the South Bay. 

Trophic State Indices

One method of classifying the water quality or productivity of lakes is by computing Trophic State Indices 

(TSIs). These indices, based on near-surface concentrations of total phosphorus and chlorophyll a and on Secchi 

depths (Carlson, 1977), place these three characteristics on 

similar scales. Oligotrophic lakes (TSIs less than 40) typically have a limited supply of nutrients and are typically 

clear; algal populations and phosphorus concentrations 

are low, and the deepest water is likely to contain 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 waterquality problems, such as frequent seasonal algal blooms, 

oxygen depletion in lower parts of the lakes, and poor 

clarity. Lakes with TSIs greater than 60 are considered 

hypereutrophic and usually have extensive algal blooms 

throughout summer. These three indices are related to each 

other in complex ways that differ seasonally and among 

lakes. All three of the indices indicated that the East and 

Upper East Bays were eutrophic and often hypereutrophic, 

especially during 2002 and 2003 (fig. 9). For these basins, 

the average summer TSIs based on surface phosphorus 

ranged from 56 to 64, based on surface chlorophyll a

ranged from 58 to 67, and based on Secchi depth ranged 

from 53 to 63. All three indices indicated that the South 

Bay was mesotrophic to eutrophic from 1991 to 2001 and 

eutrophic to hypereutrophic in 2002 and 2003 (average 

summer TSIs based on surface phosphorus, 45 to 60; based 

on surface chlorophyll a, 47 to 61; and based on Secchi 

depth, 45 to 60). All three indices indicated that the West 

Bay was usually oligotrophic prior to 2001 and mesotrophic from 2001 to 2003 (the average summer TSIs based 

on surface phosphorus, 35 to 50; based on surface chlorophyll a, 36 to 48; and based on Secchi depth, 40 to 47). 

Based on the indices, all of the basins were more eutrophic 

in 2002 and 2003 than in any of the other years monitored.

Streamflow and Stream-Water 

Quality

Streamflows for the three sites on Muskellunge Creek 

and the outlet from Little St. Germain Lake are shown in 

figure 10. Flow increases in Muskellunge Creek from the 

Muskellunge Lake outlet, to the snowmobile crossing, to 

16 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
Birchwood Drive. During 2001, the only year with true 

daily data from March through October, average streamflow increased from 3.1 ft3

/s at the Muskellunge Lake 

outlet to 7.0 ft3/s at Birchwood Drive. There was little 

daily variability in Muskellunge Creek during 2001, which 

indicated that the interpolation between monthly measurements should be a relatively accurate means of computing 

daily flows for the other years for which daily data were 

not available. The average annual flow at Birchwood 

Drive in 2001 computed using the daily flows interpolated 

between the approximately monthly calibration data was 

7.2 ft3

/s compared to 7.0 ft3/s computed using daily measurements. Measurements throughout 1999 to 2003 indicate similar differences among sites. During 2001, the flow 

at the Little St. Germain Lake outlet was 9.7 ft3/s. Average 

annual flows into the lake (at Birchwood Drive) and out of 

the lake were highest in 1997 and were relatively similar in 

all other years. Flows were highest in Muskellunge Creek 

during spring; however, flow out of the lake was highest 

during the fall and early winter drawdown of the lake. 

Phosphorus concentrations in Muskellunge Creek 

consistently increased from the Muskellunge Lake outlet 

to Birchwood Drive (figs. 10 and 11). Concentrations at all 

of the sites were highest during summer and lowest during 

winter. Phosphorus concentrations at the Muskellunge 

Lake outlet ranged from about 0.009 mg/L in the winter 

to about 0.06 mg/L in summer. Phosphorus concentrations 

at Birchwood Drive ranged from 0.016 mg/L in winter of 

2000 to 0.14 mg/L during summer 1997. Phosphorus concentrations at the snowmobile crossing were between those 

of these two sites, but generally were more similar to those 

at the Muskellunge Lake outlet. Phosphorus concentrations 

at Birchwood Drive were much higher in 1997 than during 

any other time, especially in midsummer to late summer. 

The high concentrations in 1997 may have been due to 

effects of beaver activity on Muskellunge Creek. Ponding 

of water behind beaver dams may have resulted in a high 

release of phosphorus from the organic-rich wetland sediments that are not otherwise inundated with water. 

Concentrations at the Little St. Germain Lake outlet 

usually were lower than those entering the lake and ranged 

from 0.013 to 0.081 mg/L. The highest concentrations 

were measured in 2002 and 2003, coinciding with the high 

concentrations in the South Bay. The average total phosphorus concentration for 1999 to 2003 (fig. 11) increased 

from 0.036 mg/L at the Muskellunge Lake outlet, to 0.045 

mg/L at the snowmobile crossing, to 0.053 mg/L at Birchwood Drive. Phosphorus concentrations in Muskellunge 

Creek are high considering that most of the watershed of 

Little St. Germain Lake is relatively pristine (fig. 1). The 











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Figure 9. Trophic state indices based on total phosphorus concentrations for the various basins in Little St. Germain Lake, Vilas 

County, Wis., 1991−2003.

Streamflow and Stream-Water Quality 17
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Figure 10. Flow and total phosphorus concentrations in Muskellunge Creek and Little St. Germain Creek, Vilas County, Wis., 

1996−2003.

18 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
high concentrations may be the result of leaching from the 

soils that are thought to be rich in phosphorus in this area 

(Wisconsin Department of Natural Resources, 1985). The 

average total phosphorus concentration of the water leaving the lake during this period was 0.031 mg/L.

From 1999 through 2003, the dissolved phosphorus 

concentrations in Muskellunge Creek ranged from 0.013 to 

0.045 mg/L, representing about 55 percent of the phosphorus in the creek. The average dissolved phosphorus 

concentration at Birchwood Drive was 0.027 mg/L. The 

average dissolved phosphorus concentration at this site 

during May through August was 0.033 mg/L, representing 

about 53 percent of the phosphorus. 

Water and Phosphorus Loading to 

Little St. Germain Lake 

Because the productivity in Little St. Germain Lake 

should potentially be limited by the input of phosphorus 

(based on nitrogen-to-phosphorus ratios), reduction in 

phosphorus input to the lake would be a logical management goal. Most phosphorus that enters lakes is associated 

with the input of water. Therefore, to determine where 

the water and phosphorus originates and how changes in 

phosphorus loading should affect the trophic status of the 

lake, the water and phosphorus budgets of the lake were 

quantified. Water and phosphorus budgets were computed 

on a monthly basis for three different annual periods: 

the period from October 1, 1996, to September 30, 1997 

(referred to as monitoring year 1997; MY1997); the period 

from December 1, 1998, to November 30, 1999 (referred 

to as MY1999); and the period from November 1, 2000, 

to October 31, 2001 (referred to as MY2001). Water and 

phosphorus budgets were constructed as part of the earlier 

study on the lake (Robertson and Rose, 2000). That study 

indicated that phosphorus from Muskellunge Creek was 

the dominant source of phosphorus; however, groundwater contributions also were important and needed to 

be better quantified. Therefore, the water and phosphorus budgets for 2001 were computed with greater detail, 

especially regarding the inputs from ground water. This 

information was used to refine the budgets of the earlier 

study.

Water Loading

The hydrology of Little St. Germain Lake can be 

described in terms of components of its water budget (fig. 

12). 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 outflow (GWOut).















0(/30(/253#/.#%.42!4)/.

).-),,)'2!-30%2,)4%2

-USKELLUNGE

,AKEOUTLET

3NOWMOBILE

CROSSING

"IRCHWOOD

$RIVE

,ITTLE3T'ERMAIN

#REEK

Figure 11. Average total phosphorus concentrations in 

Muskellunge Creek and Little St. Germain Creek, Vilas County, 

Wis., 1999−2003.

'ROUNDWATER '7)N '7/UT

004

37/UT

'ROUNDWATER ROUNDWATER

%VAP

37)N 37/UT37)N 37/UT

Figure 12. Schematic of the components of the water budget 

for Little St. Germain Lake, Vilas County, Wis. (abbreviations 

are described in text).

Water and Phosphorus Loading to Little St. Germain Lake 19
Changes in Storage

Changes in lake volume were determined from water 

elevations monitored at the outlet dam near Highway 70 

(fig. 1) and the morphometry of the lake, assuming a constant surface area of the lake. Lake stage fluctuated from a 

minimum elevation of 1,611.9 ft to a maximum of 1,614.0 

ft (fig. 13). Typically, the lake stage was relatively stable 

from May through mid-November, lowered about 1.5 ft 

between mid November and early February, and remained 

relatively stable until mid-March before again filling to its 

summer level. The lake stage at the end of MY1997 was 

similar to that at the beginning of the period; therefore, the 

change in storage was only 59 acre-ft (table 2). The lake 

stage at the end of MY1999 was about 0.6 ft higher than at 

the beginning of that year and resulted in a change in storage of 529 acre-ft. The lake stage at the end of MY2001 

period was about 0.3 ft higher than at the beginning of that 

year and resulted in a change in storage of 326 acre-ft.

Precipitation and Evaporation

Precipitation was measured by a weather observer 

near St. Germain, Wis. (fig. 13). During these three 

periods, precipitation (in inches) ranged from 33.2 in 

MY2001, to 34.8 in MY1997, to 37.3 in MY1999 (table 

2). Evaporation from the lake was estimated from average 

monthly evaporation-pan data from 7 years of data collected at the Rainbow Flowage, which is about 15 mi west 

of Little St. Germain Lake. Monthly lake/pan coefficients 

computed as part of a study on Vandercook Lake, Vilas 

County (Wentz and Rose, 1991), were then 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 (1,825 acre-ft) 

was estimated to have evaporated from the lake during 

each period (table 2), most of this during summer. It was 

assumed that the monthly evaporation was the same each 

year.

Surface-Water Inflow

Surface-water inflow to the lake was measured at 

Birchwood Drive on Muskellunge Creek (fig. 13). It was 

assumed that there was no surface-water inflow to the lake 

downstream from Birchwood Drive. Any surface-water 

inflow downstream from this point was incorporated into 

ground-water inflow. Daily average flows at Birchwood 

Drive were computed by linearly interpolating between 

monthly measurements in MY1997 and MY1999. In 

MY2001, daily average flows were computed by linearly interpolating between monthly measurements from 

November through February and from the 15-minute flow 

data for March through October. Average annual inflow 

ranged from 10.6 ft3

/s (7,722 acre-ft) in MY1997, to 7.0 

ft3/s (5,084 acre-ft) in MY2001, to 6.0 ft3/s (4,383 acreft) in MY1999. Inflow to the lake in MY1997 was about 

1.8 times that in MY1999 even though there was less 

precipitation in MY1997. This demonstrates that the flow 

in Muskellunge Creek is driven by long-term changes in 

precipitation and ground-water inflow rather than shortterm fluctuations. 

Surface-Water Outflow

The average flow out of the lake was 17.3 ft3/s 

(12,552 acre-ft) in MY1997, 10.3 ft3/s (7,445 acre-ft) in 

MY1999, and 9.7 ft3/s (7,034 acre-ft) in MY2001 (table 

2). Outflow from the lake in MY1997 also was higher 

than in MY1999 even though there was less precipitation 

in MY1997, which again demonstrates that long-term 

changes in precipitation and ground-water inflow are 

important to the hydrology of Little St. Germain Lake. In 

all three years, outflow from the lake was about 1.4 to 1.7 

times greater than the input from Muskellunge Creek. 

Ground-Water Inputs and Outputs

After converting all of the hydrologic components in 

the water budget equation (eq. 1) into acre-ft and solving for net ground-water input (GWIn – GWOut), it was 

estimated that there was a net ground-water input to Little 

St. Germain Lake of about 3,900 acre-ft (5.3 ft3/s) in 

MY1997, 2,400 acre-ft (3.3 ft3

/s) in MY1999, and 1,400 

acre-ft (1.9 ft3/s) in MY2001 (table 2). In order to better 

define the contributions from ground water (locations and 

input and output amounts) a ground-water-flow model was 

developed for the Little St. Germain Lake watershed based 

on the flow conditions in MY2001.

Ground-water gradients from the 15 piezometers 

installed around the lake (fig. 2), water levels in wells and 

lakes in the area, flows at the Muskellunge Lake outlet, 

Birchwood Drive, and the Little St. Germain Lake outlet were used to calibrate the ground-water-flow model, 

GFLOW (Haitjema, 1995). The model was then used 

to define ground-water-source areas around Little St. 

Germain Lake (and Muskellunge Lake) and to allocate 

ground-water discharge to the shoreline segments represented by the piezometers. 

20 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin












-/.4(,902%#)0)4!4)/.).).#(%3















!6%2!'%-/.4(,9342%!-&,/7

).#5")#&%%40%23%#/.$

)NFLOW

/UTFLOW











7!4%2%,%6!4)/.&%%4!"/6%-%!.3%!,%6%,

      

Figure 13. Monthly precipitation, inflow, outflow, and daily average water elevations for Little St. Germain Lake, Vilas 

County, Wis.

Water and Phosphorus Loading to Little St. Germain Lake 21
Table 2. Monthly water budget for Little St. Germain Lake, for October 1, 1996, to September 30, 1997; December 1, 1998, to 

November 30, 1999; and November 1, 2000, to October 31, 2001.

[ft3/s, cubic feet per second. Numbers may not add exactly to totals due to rounding. Totals for rates in ft3/s are means.]

Month Precipitation Evaporation

Muskellunge 

Creek input

Change 

in 

storage

Net groundwater input

Groundwater 

input

Groundwater 

output Outlet

inches

acrefeet inchesacrefeet ft3

/s

acrefeetacrefeet ft3

/s

acrefeetacrefeetacrefeet ft3

/s

acrefeet

October 1, 1996, to September 30, 1997 (MY1997)

Oct. 3.7 305 1.9 153 7.4 456 39 6.1 375 425 49 15.3 944

Nov. 3.5 287 0.4 33 9.0 539 -431 7.5 443 502 58 28.0 1,668

Dec. 2.3 189 0.0 0 10.5 647 -343 5.7 348 393 46 24.8 1,527

Jan. 3.1 255 0.0 0 12.8 789 -500 2.0 123 139 16 27.0 1,666

Feb. 1.0 79 0.0 0 11.0 613 -176 6.9 382 433 50 22.5 1,250

Mar. 3.1 252 0.0 0 10.0 614 305 4.2 260 294 34 13.3 821

Apr. 0.7 56 1.0 83 13.7 816 949 9.6 570 645 75 6.9 410

May 3.0 243 3.8 310 12.3 756 196 8.5 521 589 68 16.5 1,015

June 3.7 300 4.3 350 11.4 679 20 6.3 372 421 49 16.5 982

July 4.6 376 4.4 361 10.6 654 -39 4.2 256 290 34 15.7 965

Aug. 3.7 302 3.7 305 10.4 640 59 2.0 126 142 17 11.4 704

Sept. 2.4 199 2.8 230 8.7 519 -20 1.6 92 104 12 10.1 600

Total 34.8 2,844 22.4 1,825 10.6 7,722 59 5.3 3,869 4,377 508 17.3 12,552

December 1, 1998, to November 30, 1999 (MY1999)

Dec. 1.2 100 0.0 0 5.8 358 -372 3.3 203 230 27 16.8 1,033

Jan. 2.7 224 0.0 0 5.7 352 -176 -0.4 -27 0 27 11.8 725

Feb. 1.7 140 0.0 0 4.6 285 284 6.3 348 394 46 7.9 489

Mar. 0.3 26 0.0 0 6.5 400 127 1.9 118 134 16 6.8 418

Apr. 1.7 139 1.0 83 6.9 426 313 3.7 219 248 29 6.3 388

May 9.7 790 3.8 310 8.5 526 862 6.7 413 467 54 9.3 573

June 3.8 314 4.3 350 6.5 401 -157 3.4 205 232 27 11.5 711

July 7.2 585 4.4 361 6.4 393 274 5.2 319 361 42 10.7 661

Aug. 3.4 274 3.7 305 5.9 361 -196 1.6 98 111 13 8.9 547

Sept. 2.5 202 2.8 230 5.0 307 -78 1.7 101 115 13 8.7 536

Oct. 1.9 153 1.9 153 4.7 290 20 3.3 201 227 26 7.7 472

Nov. 1.2 98 0.4 33 4.6 285 -372 2.9 172 194 23 14.5 893

Total 37.3 3,045 22.4 1,825 6.0 4,383 529 3.3 2,371 2,712 341 10.3 7,445

November 1, 2000, to October 31, 2001 (MY2001)

Nov. 1.9 155 0.4 33 6.3 389 -202 2.5 148 167 19 14.0 861

Dec. 1.3 104 0.0 0 6.2 382 -846 0.3 21 24 3 22.0 1,353

Jan. 1.2 100 0.0 0 5.7 352 49 7.2 441 499 58 13.7 843

Feb. 1.4 117 0.0 0 5.5 337 -215 0.7 40 45 5 11.5 709

Mar. 0.6 53 0.0 0 6.2 381 411 5.8 358 405 47 6.2 381

Apr. 5.7 469 1.0 83 12.9 795 1,136 2.6 157 178 21 3.3 202

May 4.8 394 3.8 310 9.2 570 215 -1.3 -79 0 79 5.8 359

June 3.4 281 4.3 350 8.1 499 -78 3.6 215 243 28 11.7 722

July 3.3 270 4.4 361 5.6 343 -176 -0.3 -19 0 19 6.6 409

Aug. 1.9 156 3.7 305 5.2 318 -215 0.3 21 24 3 6.6 405

Sept. 4.9 398 2.8 230 5.0 311 0 -1.5 -92 0 92 6.3 387

Oct. 2.6 216 1.9 153 6.6 409 248 2.9 178 201 23 6.5 402

Total 33.2 2,713 22.4 1,825 7.0 5,084 326 1.9 1,389 1,786 397 9.7 7,034

22 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
The GFLOW 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 simulated ground-water 

levels and base flows for streams in the near field. Areal 

recharge to the ground-water-flow system was set equal 

to 8.9 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 

46 water-level targets (fig. 3), the calibrated model had a 

mean difference between measured and simulated water 

levels of 0.1 ft and mean absolute difference of 3.6 ft (fig. 

14).

Simulated cumulative net ground-water discharges 

were within 0.4 ft3

/s of independently determined cumulative net ground-water discharges at the Muskellunge Lake 

outlet, at Muskellunge Creek at Birchwood Drive, and at 

Little St. Germain Creek at Highway 70 in MY2001 (table 

3). Net ground-water discharge to Muskellunge Lake was 

determined as the residual in the water-budget equation 

(2.46 ft3

/s; 1.87 ft3/s from eq. 1 plus 0.59 ft3/s, the estimated ground-water discharge to the southeast tributary 

of Muskellunge Lake that was included in the discharge 

simulated by the ground-water-flow model; Robertson 

and others, 2003). The total net ground-water discharge at 

Birchwood Drive (5.69 ft3/s) was estimated by adding the 

ground-water discharge to Muskellunge Creek (3.23 ft3/s, 

estimated by subtracting the daily outflow from Muskellunge Lake from the total flow measured at Birchwood 

Drive and by use of a graphical base-flow separation analysis on the difference between these two sites to determine 

surface runoff) to that estimated at the Muskellunge Lake 

outlet. The total net ground-water discharge at Highway 

70 was estimated by adding the net ground-water input to 

Little St. Germain Lake from table 2 (1.9 ft3

/s) to the net 

ground-water discharge at Birchwood Drive. Simulated 

net ground-water discharges in table 3 were obtained by 

removing precipitation minus evaporation on the surfaces 

of Muskellunge Lake (0.28 ft3

/s) and Little St. Germain 

Lake (1.004 ft3/s) from the model estimates. 











    

3)-5,!4%$(%!$).&%%4!"/6%3%!,%6%,

-%!352%$(%!$).&%%4!"/6%3%!,%6%,

Figure 14. Measured and simulated head for the calibrated GFLOW model for the area near Little St. Germain Lake, 

Vilas County, Wis.

Water and Phosphorus Loading to Little St. Germain Lake 23
The ground-water contributing area for Little St. 

Germain Lake was delineated using particle tracking techniques in GFLOW. The area contributing ground water to 

the lake (14.1 mi2

) is different and slightly larger than the 

area contributing surface water (10 mi2; 11.5 mi2 if the lake 

surface is included)(fig. 15). The model results indicated 

that ground water discharges into the lake from all sides 

except for two small areas at the south ends of the Second 

South and West Bays. Water levels (heads) measured in 

piezometers indicated that some parts of the lake alternate 

between contributing and losing ground water at different 

times of the year. During May and June 2001, water levels 

measured at 13 piezometers indicated that ground water 

flowed into the lake on all sides except the south side of 

West Bay, where it was leaving the lake. During August 

and September 2001, water-level measurements at 15 

piezometers indicated that ground-water discharged into 

all sides of the lake except the south side of the West Bay, 

the northeast side of the South Bay, and the south side of 

the East Bay. No piezometers were installed near the outlet 

of the lake in an area simulated as losing water. Most of 

the urban development around the southwest part of the 

West Bay and along Highway 70 is in areas that do not 

contribute ground water to the lake.

The simulated ground-water fluxes into and out of 

the lake were calculated for 16 shoreline segments. The 

segment lengths correspond to the length of shoreline 

halfway between adjacent piezometers installed around 

the lake (fig. 2), except for the segment representing the 

south end of the South Bay. No piezometers were installed 

in this area; therefore, the segment between piezometers 

in the South and Second South Bays was divided into two 

segments, one of which represents the shoreline near the 

lake outlet that was simulated to lose water to the groundwater-flow system. The amount of ground water entering 

the lake through each shoreline segment is shown in figure 

15 by the thickness of the segment lines around 

the lake. The ground-water flux into the lake ranged from 

0.4 ft3

/day per foot of shoreline (West Bay near piezometer 3, close to where water is lost to the ground-waterflow system) to 12.6 ft3

 per day per foot of shoreline 

(near piezometer 13, along the north shore of East Bay). 

Ground-water fluxes out of the lake ranged from 2.5 to 

7.4 ft3

 per day per foot of shoreline, in the West and Second South Bays, respectively.

The total annual simulated ground-water flow into 

Little St. Germain Lake (GWIn) was 2.65 ft3/s (based on 

summing contributions of shoreline segments contributing ground water to the lake), and the total annual simulated loss from the lake to the ground-water-flow system 

(GWOut) was 0.30 ft3/s (based on summing shoreline 

segments gaining water from the lake). The net annual 

ground-water flow to Little St. Germain Lake was 2.35 

ft3/s, which is similar to the total annual net ground-water 

input estimated from the MY2001 water budget (1.9 ft3/s, 

table 2). This results in the total ground-water inflow to the 

lake being about 13 percent higher than net inflow. In the 

earlier study (Robertson and Rose, 2000), total groundwater inflow was assumed to be 50 percent higher than net 

inflow. Therefore, less ground water enters the lake than 

assumed in the earlier study.

Based on the similarity between the model output and 

the base flows in Muskellunge Creek and Little St. Germain Creek (table 3) and the annual net ground-water input 

from the water budget in MY2001 (table 2), the model 

appears to accurately simulate the ground-water flow near 

Little St. Germain Lake. In the model, ground-water flow 

to the lake is assumed to be uniform throughout the year, 

although in reality there is seasonal variation caused by 

variable recharge and seasonal regulation of stage in Little 

St. Germain Lake. The monthly distribution of net groundwater input to the lake was obtained directly from the 

Table 3. Measured and simulated cumulative net ground-water discharge values for the calibrated GFLOW model.

[ft3/s, cubic feet per second; in/yr, inches per year; MY, monitoring year]

Comparison site

Cumulative net ground-water discharge (ft3/s)

Independently determined 

values for MY2001 Model-simulated values*

Muskellunge Lake at the outlet 2.46 2.20

Muskellunge Creek at Birchwood Drive 5.69 5.32

Little St. Germain Lake outlet at Highway 70 7.79 8.12

* Simulated net ground-water discharge values from the lakes were modified from the actual model output by subtracting precipitation minus evaporation (8.9 in/yr) applied to the lake surfaces. For Muskellunge Lake and Muskellunge Creek, 0.28 ft3

/s was subtracted, and for the Little St. Germain 

Lake outlet at Highway 70, 1.284 ft3/s was subtracted. 

24 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
%80,!.!4)/.

'ROUND
WATERFLOWPATHS

'ROUND
WATERCONTRIBUTINGAREA

3URFACE
WATERCONTRIBUTINGAREA

'ROUND
WATERFLUX

INCUBICFEETPERDAYPERFOOTOFSHORELINE

POSITIVENUMBERSAREFLUXESINTOTHELAKE	

 -),%

 +),/-%4%2









gg

g

g


 


Figure 15. Surface-water contributing area, simulated ground-water contributing area and flow direction, and ground-water flux 

for Little St. Germain Lake, Vilas County, Wis.

Water and Phosphorus Loading to Little St. Germain Lake 25
water budget (table 2), and the monthly inputs and outputs 

to the lake were obtained based on the assumption that 

the ratio of inputs to outputs was similar to that estimated 

by the ground-water-flow model for the entire year. The 

annual net ground-water inputs for MY1997 and MY1999 

were estimated directly from the water budgets for the 

respective years, and the monthly distribution of inputs and 

outputs for those years (table 2) was again obtained based 

on the assumption that the ratio of inputs to outputs was 

similar to that found with the ground-water-flow model. 

The ground-water inputs to the lake were estimated to be 

about 4,380 acre-ft in MY1997, 2,710 acre-ft in MY1999, 

and 1,790 acre-ft in MY2001.

Water Budget

The complete water budget (table 2 and fig. 16) 

indicated that the major source of water to the lake is from 

surface-water inflow from Muskellunge Creek, which 

represented about 43 percent (MY1999) to 53 percent 

(MY2001) of the total water input to the lake. Ground 

water and precipitation each contributed between 19 and 

30 percent of the total water input. During a dry year after 

an extended dry period (such as prior to MY1999), direct 

precipitation and ground water contribute almost as much 

water as Muskellunge Creek (about 40 percent from Muskellunge Creek and about 30 percent from both precipitation and ground water). The major loss of water from the 

lake is through the outlet (ranging from 76 percent of the 

total water loss in MY2001 to 84 percent MY1997). About 

12 to 20 percent of the water lost from the lake is through 

evaporation, and about 3 to 4 percent is lost to the groundwater-flow system. 

Phosphorus Loading 

To help define where the phosphorus in Little St. Germain Lake originates and how much may be controllable, a 

detailed phosphorus budget was computed for each of the 

three monitoring years. Sources of phosphorus to the lake 

include precipitation, surface- and ground-water inflow, 

and contributions from septic systems. Phosphorus is 

removed from the lake through surface- and ground-water 

outflow and deposition to the lake sediments.






























0RECIPITATION %VAPORATION -USKELLUNGE

#REEK

'ROUNDWATER

IN

'ROUNDWATER

OUT

#HANGEIN

STORAGE

/UTLET

7!4%2).0543!.$,/33%3).!#2%
&%%4

-9

-9

-9

Figure 16. Water budgets for Little St. Germain Lake, Vilas County, Wis. for monitoring years MY1997, MY1999, and MY 2001.

26 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
Precipitation

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 between about 52 lb (MY2001) 

and 58 lb (MY1999) of phosphorus to the lake during each 

study year (table 4). 

Surface-Water Inflow

During each monitoring year, phosphorus concentrations were measured approximately monthly in Muskellunge Creek at Birchwood Drive (fig. 10). Daily concentrations for each monitoring year were obtained by linearly 

interpolating between each measurement. 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 

Muskellunge Creek was estimated to be about 1,470 lb 

(MY1997), 690 lb (1999), and 710 lb (2001) (table 4). 

About 50 percent of the phosphorus loading from surface-water inflow occurred between May and August. 

Because dissolved phosphorus represents about 50 percent 

of the total phosphorus at this site (throughout the year 

and during May through August), the total annual load of 

dissolved phosphorus ranged from about 350 lb (MY1999 

and MY2001) to 750 lb (MY1997), of which about 175 lb 

(MY1999 and MY2001) to about 375 lb (MY1997) were 

delivered from May through August. 

Ground-Water Inputs and Outputs

Dissolved phosphorus concentrations were measured twice in 13 of the 15 piezometers installed around 

the lake (fig. 2). Concentrations were not measured if the 

ground-water head gradient at a location was away from 

the lake (such as at piezometers 3 and 14). Ground-water 

simulations indicated that the gradient along most of the 

shoreline near piezometer 3 should be towards the lake; 

therefore, the concentration in piezometer 3 was assumed 

to be the average of that measured in piezometers 1 and 

2. Ground water was leaving the lake at piezometer 14; 

therefore, a concentration was not measured. Phosphorus 

concentrations varied only slightly between measurements 

at each location. The median overall phosphorus concentration from all piezometers was 0.019 mg/L. All average concentrations were less than 0.035 mg/L, except in 

piezometers 4 (0.076 mg/L), 6 (0.303 mg/L), and 7 (0.386 

mg/L) near No Fish Bay and the South Bay (fig. 2). Phosphorus input was then computed by multiplying the average phosphorus concentration in each piezometer by the 

monthly ground-water flux computed for each shoreline 

segment. The input of phosphorus from each shoreline segment is shown in figure 17 (for MY2001) by the thickness 

of the segment line. Most phosphorus enters the middle 

part of the lake represented by piezometers 6 and 7, where 

the phosphorus concentrations were highest. Ground water 

was estimated to contribute about 740 lb (MY1997), 460 

lb (MY1999), and 300 lb (MY2001) of phosphorus during 

the various monitoring years (table 4). 

The amount of phosphorus leaving the lake with 

ground-water outflow was estimated by multiplying the 

monthly ground-water outflow (table 2; proportioned 

between piezometers 5 and 14 based on the model output 

for MY1999) by the monthly average in-lake, near-surface phosphorus concentration estimated for the South 

and West Bays, respectively. Ground-water outflow was 

estimated to have removed about 46 lb (MY1997), 20 lb 

(MY1999), and 23 lb (MY2001) of phosphorus during the 

various monitoring years (table 4). About 80 percent of the 

phosphorus removed with ground-water outflow was from 

the South and Second South Bays.

Septic Systems

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 and the most likely value 

for SR was 0.85. 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 total number of capita years was 

estimated to be 330. This value was obtained on the basis 

of the assumptions that 300 of the 415 lake-front dwellings 

were in areas where ground water flowed toward the lake, 

40 percent of these dwellings had 2 permanent residents 

(240 capita years), 30 percent of these dwellings had 3 

residents living on the lake for 3 months (68 capita years), 

and 30 percent of these dwellings had 3 residents living on 

the lake for 1 month (22 capita years) (Ted Ritter, Little 

St. Germain Lake District, oral commun., 2004). This 

estimate includes all residents on the lake except those on 

the west part of the West Bay and the southern part of the 

Water and Phosphorus Loading to Little St. Germain Lake 27
Table 4. Monthly phosphorus budget, in pounds, for Little St. Germain Lake, for October 1, 1996, to September 30, 1997; 

December 1, 1998, to November 30, 1999; and November 1, 2000, to October 31, 2001.

[MY, monitoring year. Numbers may not add exactly to totals due to rounding.]

Month Precipitation

Muskellunge 

Creek

Groundwater 

input

Groundwater 

output

Septic 

systems Outlet

Total 

input

Total 

output

Gain in 

lake

October 1, 1996, to September 30, 1997 (MY1997)

Oct. 5.8 50.1 72.0 2.7 7.4 65.0 135.3 67.7 67.6

Nov. 5.5 63.3 85.1 3.1 7.4 140.2 161.2 143.3 18.0

Dec. 3.6 75.2 66.7 3.0 7.4 149.7 152.9 152.8 0.2

Jan. 4.8 85.6 23.5 1.3 7.4 194.4 121.5 195.6 -74.2

Feb. 1.5 69.6 73.4 4.6 7.4 162.8 151.9 167.5 -15.5

Mar. 4.8 70.4 49.8 3.5 7.4 120.2 132.5 123.8 8.7

Apr. 1.1 108.4 109.4 8.3 7.4 60.6 226.3 68.9 157.4

May 4.6 181.8 99.9 8.1 7.4 146.0 293.8 154.1 139.7

June 5.7 211.9 71.4 6.1 7.4 127.9 296.4 134.0 162.4

July 7.2 221.6 49.2 2.6 7.4 82.1 285.3 84.6 200.7

Aug. 5.7 184.7 24.1 1.2 7.4 64.4 222.0 65.6 156.4

Sept. 3.8 151.3 17.7 0.9 7.4 55.3 180.2 56.3 123.9

Total 54.1 1,473.9 742.3 45.5 89.1 1,368.6 2,359.3 1,414.1 945.2

December 1, 1998, to November 30, 1999 (MY 1999)

Dec. 1.9 42.9 38.9 1.7 7.4 53.5 91.1 55.1 36.0

Jan. 4.2 44.7 0.0 1.7 7.4 68.0 56.4 69.7 -13.2

Feb. 2.7 38.4 66.8 2.9 7.4 47.0 115.3 49.9 65.4

Mar. 0.5 60.2 22.7 1.0 7.4 23.9 90.9 24.9 66.0

Apr. 2.6 80.0 42.1 1.8 7.4 26.0 132.2 27.8 104.3

May 15.0 130.3 79.2 3.3 7.4 33.2 232.0 36.5 195.4

June 6.0 79.4 39.3 1.7 7.4 36.4 132.1 38.1 94.0

July 11.1 75.3 61.2 1.9 7.4 30.9 155.1 32.8 122.3

Aug. 5.2 56.7 18.8 0.6 7.4 23.5 88.1 24.2 63.9

Sept. 3.8 33.0 19.4 0.7 7.4 22.2 63.7 22.9 40.7

Oct. 2.9 23.8 38.5 1.7 7.4 30.8 72.7 32.4 40.2

Nov. 1.9 29.6 33.0 1.4 7.4 47.2 71.9 48.6 23.3

Total 57.9 694.4 459.9 20.3 89.1 442.7 1,301.3 463.0 838.3

November 1, 2000, to October 31, 2001 (MY 2001)

Nov. 2.9 23.9 28.4 1.2 7.4 77.7 62.6 78.9 -16.3

Dec. 2.0 30.2 4.1 0.2 7.4 104.3 43.7 104.5 -60.7

Jan. 1.9 36.4 84.6 3.6 7.4 52.0 130.2 55.7 74.6

Feb. 2.2 38.9 7.7 0.3 7.4 49.7 56.2 50.0 6.2

Mar. 1.0 47.4 68.7 3.0 7.4 31.7 124.6 34.6 90.0

Apr. 8.9 102.5 30.1 1.3 7.4 21.1 149.0 22.4 126.6

May 7.5 96.2 0.0 4.9 7.4 29.3 111.1 34.2 76.9

June 5.3 95.9 41.2 1.8 7.4 46.2 149.8 47.9 101.9

July 5.1 69.1 0.0 0.8 7.4 29.7 81.7 30.6 51.1

Aug. 3.0 62.0 4.0 0.1 7.4 28.1 76.4 28.3 48.1

Sept. 7.6 52.4 0.0 4.7 7.4 23.2 67.4 27.9 39.5

Oct. 4.1 50.8 34.2 1.5 7.4 24.0 96.5 25.5 71.0

Total 51.6 705.7 302.9 23.4 89.1 517.0 1,149.2 540.5 608.8

28 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
%80,!.!4)/.

0HOSPHORUSFLUX

INPOUNDSPERFOOTOFSHORELINEPERYEAR

POSITIVENUMBERSAREFLUXESINTOTHELAKE	

 -),%

 +),/-%4%2









'ROUND
WATERFLOWPATHS

0IEZOMETERANDIDENTIFICATIONNUMBER

'ROUND
WATERCONTRIBUTINGAREA


 


gg

g

g













 

















Figure 17. Phosphorus loading into and out of Little St. Germain Lake, Vilas County, Wis., from ground water, by shoreline 

segment for monitoring year 2001.

Water and Phosphorus Loading to Little St. Germain Lake 29
Second South Bay where ground water was flowing away 

from the lake. The total input from septic systems was then 

computed to be 89 lb per year and was aassumed to be 

uniformly distributed throughout each year. This estimate 

is twice that estimated by Robertson and Rose (2000) 

because the ground-water-flow model indicated that much 

more of the lake had areas contributing ground water than 

previously estimated. By applying low and high estimates 

for ES

 (1.1 and 2.2 lb of phosphorus per capita per year, 

respectively) and SR (0.9 and 0.5, respectively), low and 

high estimates of phosphorus from septic systems were 

computed to be 36 and 363 lb, respectively.

Surface-Water Outflow

The phosphorus leaving the lake’s outlet was computed for each monitoring year by multiplying the daily 

outflow volumes by the measured or estimated daily 

concentrations. Estimated concentrations were obtained 

by linearly interpolating between the measurements in 

figure 10. The total amount of phosphorus in surface-water 

outflow was estimated to be about 1,370 lb (MY1997), 

440 lb (MY1999), and 520 lb (MY2001) (table 4). The 

larger load in MY1997 than those estimated for the other 

two years was due to a combination of higher concentrations and higher flows than in the other two years. About 

35 to 45 percent of the phosphorus load leaving the lake 

occurred between November and January when the lake 

was being drawn down.

Phosphorus Budget

The total phosphorus input to the lake was about 

2,360 lb in MY1997, 1,300 lb in MY1999, and 1,150 lb in 

MY2001 (table 4). The phosphorus budget (fig. 18) indicates that inflow from Muskellunge Creek was the major 

source of phosphorus to the lake (53 to 62 percent) and 

ground water was the secondary source (26 to 35 percent). 

Phosphorus from precipitation and septic systems each 

accounted for less than 8 percent of the total input. With 

the worst assumptions regarding input from septic systems, 

they would contribute between 14 percent (MY1997) to 26 

percent (MY2001) of the total phosphorus input. Approximately 34 to 58 percent of the total phosphorus input to 

the lake (463 lb in MY1999 and 1,414 lb in MY1997) 

was exported out of the lake through Little St. Germain 

Creek. The remaining 32 to 66 percent of the phosphorus 

input (about 610 lb in MY2001 to 950 lb in MY1997) 

was deposited in the bed sediment of the lake. The overall 

phosphorus loading from MY1997 and MY1999 is very 

similar to that estimated by Robertson and Rose (2000). 

In the earlier study, the total phosphorus input in MY1997 

was estimated to be 2,410 lb (compared to 2,360 lb in 

this study) and in MY1999 was estimated to be 1,310 lb 

(compared to 1,300 lb in this study). The primary differences in the phosphorus budgets were that phosphorus 

input from septic systems increased by about 45 lb and 

phosphorus input from ground water decreased by 93 and 

52 lb, in MY1997 and MY1999, respectively, compared to 

the earlier study. 

Effects of Phosphorus Reductions 

The total phosphorus input to the lake was estimated 

to be about 2,360 lb in MY1997, 1,300 lb in MY1999, and 

1,150 lb in MY2001. Most of this phosphorus is input into 

the East and Upper East Bays and results in the water quality in these basins being poorer than other parts of the lake. 

One way to determine how much of the phosphorus loading to these basins would need to be reduced to improve 

their water quality 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). 

Several empirical models within the Wisconsin Lakes 

Modeling Suite (WiLMS; Panuska and Kreider, 2002) 

relate the hydrology and phosphorus loading to in-lake 

phosphorus concentrations. Four of these models were 

applicable to the East and Upper East Bay of Little St. 

Germain Lake (table 5). These two basins were modeled as 

a single lake because of the mixing between the two basins 

and their similar water quality. Therefore, the morphometry (based on data from table 1), hydrology (based on data 

from table 2), and phosphorus loading (based on data from 

table 4) to these basins for the three monitoring years and 

various phosphorus-reduction scenarios were input into 

these models within WiLMS and near-surface phosphorus 

concentrations and changes in their phosphorus concentrations were simulated. In computing the total phosphorus 

load to these two basins, loading from precipitation was 

computed for only the surface area of these two basins (46 

percent of the total), loading from ground water was only 

the portion contributed to these basins, and the loading 

from septic systems were based on only 150 of the 300 

dwellings contributing phosphorus to the lake. Therefore, 

the total loading for MY1997 was 2,031 lb, for MY1999 

was 1,068 lb, and for MY2001 was 973 lb.

The models within WiLMS that were applied to the 

East and Upper East Bays use annual hydrologic and 

30 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
phosphorus loadings but simulate the water quality for 

the growing season (June through September); therefore, 

output from the model simulations was compared with the 

summer-average data for MY1997, MY1999, and MY2001 

(table 5). Data were available for only the East Bay for 

MY1997 and MY1999; therefore, all comparisons including MY2001 were done with measured East Bay data only. 

There are no calibration factors in WiLMS; however, the 

output can be adjusted to account for model biases by only 

interpreting the results as a percentage change from some 

base condition.

The average phosphorus concentration predicted by 

the models for the three years was 0.039 mg/L, which is 

26 percent lower than that measured in the East Bay (0.053 

mg/L) (table 5). For MY1997, the average simulated 

concentration was 0.047 mg/L compared to 0.061 mg/L 

measured in the East Bay. For MY1999, the average simulated concentration was 0.037 mg/L compared to 0.041 

mg/L measured in the East Bay. For MY2001, the average simulated concentration was 0.032 mg/L compared to 

0.056 mg/L measured in the East Bay. The models consistently underestimated the phosphorus concentrations in the 

East Bay. This difference between simulated and measured 

concentrations may be the result of the release of phosphorus from the sediments of Little St. Germain Lake being 

higher than the sediment releases in the lakes that were 

used to develop the models used in WiLMS. The models 

were then applied with the data for various phosphorusreduction scenarios: 25-, 50-, 75-, and 100-percent reduction in tributary loading from Muskellunge Creek; phosphorus contributions from all of the other sources were 

maintained at their present levels. The models predicted 

that these reductions in tributary loading would cause the 

average phosphorus concentration in these two basins to 

decrease 14 percent in response to a 25-percent reduction, 

by 29 percent in response to a 50-percent reduction, by 45 

percent in response to a 75-percent reduction, and by 62 

percent in response to a 100-percent reduction in tributary 

loading. If it is assumed that these percentages of change 

apply to the summer-average concentration measured in 

these bays, then a 25-percent reduction in tributary loading 

would result in a summer average concentration of 0.045 

mg/L, a 50-percent decrease would result in 0.037 mg/L, 

a 75-percent reduction would result in 0.029 mg/L, and a 

100-percent decrease in loading would result in 0.020 

mg/L. Therefore, a 75- to 100-percent reduction in tributary loading would be needed for these parts of the lake to 

have a summer average phosphorus concentration of 

0.024 mg/L and be classified as borderline mesotrophic/

eutrophic.


















0RECIPITATION -USKELLUNGE

#REEK

'ROUNDWATER

IN

3EPTIC

SYSTEMS

'ROUNDWATER

OUT

$EPOSITIONIN

LAKE

/UTLET

0(/30(/253).0543!.$,/33%3).0/5.$3

-9

-9

-9

Figure 18. Phosphorus budgets for Little St. Germain Lake, Vilas County, Wis., for monitoring years MY1997, MY1998 and MY1999.

Effects of Phosphorus Reductions 31
Table 5. Simulated changes in average June through August near-surface total phosphorus concentrations in the East and 

Upper East Bays of Little St. Germain Lake, Vilas County, Wisconsin, in response to reductions in tributary loading. 

[All concentrations are in milligrams per liter; %, percent; MY, monitoring year; ft/yr, feet per year]

MY1997 

Measured

Simulated tributary-load reductions

Base (0%) 25% 50% 75% 100%

Phosphorus loading (pounds) 2,031 1,662 1,294 925 557

Model

 Walker, 1987 0.061 0.045 0.039 0.033 0.026 0.018

 Canfield and Bachman, 1981 Natural Lakes 0.061 0.049 0.042 0.034 0.026 0.017

 Canfield and Bachman, 1981 Artificial Lakes 0.061 0.042 0.036 0.030 0.024 0.016

 Reckhow, 1977, Water load less than 164 ft/yr 0.061 0.051 0.042 0.032 0.023 0.014

Average of models 0.061 0.047 0.040 0.032 0.025 0.016

MY1999

Measured

Simulated tributary-load reductions

Base (0%) 25% 50% 75% 100%

Phosphorus loading (pounds) 1,068 894 720 547 373

Model

 Walker, 1987 0.041 0.039 0.035 0.030 0.025 0.019

 Canfield and Bachman, 1981 Natural Lakes 0.041 0.039 0.034 0.029 0.023 0.017

 Canfield and Bachman, 1981 Artificial Lakes 0.041 0.034 0.030 0.026 0.021 0.016

 Reckhow, 1977, Water load less than 164 ft/yr 0.041 0.036 0.030 0.024 0.018 0.013

Average of models 0.041 0.037 0.032 0.027 0.022 0.016

MY2001

Measured

Simulated tributary-load reductions

Base (0%) 25% 50% 75% 100%

Phosphorus loading (pounds) 973 797 620 444 267

Model

 Walker, 1987 0.056 0.034 0.030 0.025 0.020 0.013

 Canfield and Bachman, 1981 Natural Lakes 0.056 0.034 0.029 0.024 0.018 0.012

 Canfield and Bachman, 1981 Artificial Lakes 0.056 0.030 0.026 0.022 0.017 0.012

 Reckhow, 1977, Water load less than 164 ft/yr 0.056 0.031 0.025 0.020 0.014 0.008

Average of models 0.056 0.032 0.028 0.023 0.017 0.011

Average of three years

Measured

Simulated tributary-load reductions

Year Base (0%) 25% 50% 75% 100%

 1997 0.061 0.047 0.040 0.032 0.025 0.016

 1999 0.041 0.037 0.032 0.027 0.022 0.016

 2001 0.056 0.032 0.028 0.023 0.017 0.011

Average of years 0.053 0.039 0.033 0.027 0.021 0.015

Percent reduction 14 29 45 62

Predicted concentration with load reduction 0.053 0.045 0.037 0.029 0.020

32 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
Empirical relations have also been developed that 

predict summer average chlorophyll a concentrations and 

Secchi depths from near-surface phosphorus concentrations. Empirical relations developed for Wisconsin lakes 

by Lillie and others (1993) for nonstratified impoundments, also contained within WiLMS, were applied to 

determine how the various reductions in phosphorus 

loading from Muskellunge Creek (tributary loading) 

should affect near-surface, summer average chlorophyll 

a concentrations and Secchi depths in the East and Upper 

East Bays (table 6). These models predicted that with the 

measured summer average phosphorus concentration of 

0.053 mg/L, the East and Upper East Bays should have 

a chlorophyll a concentration of 18.5 μg/L (compared to 

25.8 μg/L measured in the East Bay for these three years) 

and a Secchi depth of 3.3 ft (compared to 4.3 ft measured). 

The predicted average summer phosphorus concentrations 

from table 5 were then input into the empirical relations. 

With a near-surface phosphorus concentration of 0.045 

μg/L (based on a 25-percent reduction in tributary loading), the models predicted a chlorophyll a concentration 

of 16.5 μg/L (an 11-percent reduction from 18.5 μg/L predicted for 0.053 mg/L of phosphorus) and a Secchi depth 

of 3.28 ft (no change from that predicted for 0.053 mg/L 

of phosphorus)(table 6). With a near-surface phosphorus 

concentration of 0.020 mg/L (based on a 100-percent 

decrease in tributary loading), the models predicted a chlorophyll a concentration of 9.4 μg/L (a 49-percent decrease 

from that predicted for 0.053 mg/L of phosphorus) and 

a Secchi depth of 4.3 ft (a 30-percent increase from that 

predicted for 0.053 mg/L of phosphorus). If it is assumed 

that these percentages of change again apply to the average summer conditions measured in the East Bay, then a 

25-percent reduction in tributary loading would result in a 

summer average chlorophyll a concentration of 23.0 μg/L 

and a summer average Secchi depth of 4.3 ft, a 50-percent 

reduction would result in a summer average chlorophyll a

concentration of 20.1 μg/L and a summer average Secchi depth of 4.7 ft, a 75-percent reduction would result 

in a summer average chlorophyll a concentration of 16.9 

μg/L and a summer average Secchi depth of 4.7 ft, and a 

100-percent reduction would result in a summer average 

chlorophyll a concentration of 13.1 μg/L and a summer-average Secchi depth of 5.6 ft. Therefore, although 

phosphorus-load reductions would reduce phosphorus and 

chlorophyll a concentrations and increase water clarity, 

even with a 100-percent removal of phosphorus from Muskellunge Creek, the East and Upper East Bays would still 

be classified as eutrophic.

Large reductions in phosphorus loading from Muskellunge Creek could have a significant effect on the water 

quality of not only the East and Upper East Bays, but 

Table 6. Simulated changes in June through August near-surface chlorophyll a concentrations and Secchi depths in the East 

and Upper East Bays of Little St. Germain Lake, Vilas County, Wisconsin, in response to reductions in tributary loading.

[%, percent; mg/L, milligrams per liter; μg/L, micrograms per liter]

Tributary-load reduction

Constituent – summer average Measured Base (0%) 25% 50% 75% 100%

Simulated values

Total phosphorus (mg/L) 0.053 0.053 0.045 0.037 0.029 0.020

Chlorophyll a (μg/L) 25.8 18.5 16.5 14.4 12.1 9.4

Secchi depth (feet) 4.3 3.3 3.3 3.6 3.6 4.3

Percent reduction

Chlorophyll a 0% 11% 22% 35% 49%

Secchi depth 0% 0% -10% -10% -30%

Predicted response to tributary-load reduction 

(based on simulated percent reductions)

Total phosphorus (mg/L) 0.053 0.053 0.045 0.037 0.029 0.020

Chlorophyll a (μg/L) 25.8 25.8 23.0 20.1 16.9 13.1

Secchi depth (feet) 4.3 4.3 4.3 4.7 4.7 5.6

Effects of Phosphorus Reductions 33
also the South and Second South Bays because of the net 

flow of water through the lake. A reduction in phosphorus 

concentrations in these basins would also protect the water 

quality of the West Bay. However, because of the substantial contributions of phosphorus to the lake estimated from 

ground water, even with phosphorus loading from Muskellunge Creek completely eliminated, most of the lake would 

still be classified as borderline mesotrophic/eutrophic. 

Summary

In the interest of protecting and improving the water 

quality of Little St. Germain Lake, the Little St. Germain 

Lake District initiated several cooperative studies with 

the U.S. Geological Survey between 1991 and 2004 to (1) 

document the water quality and the extent of winter anoxia 

in the lake, (2) evaluate the success of aerators at eliminating winter anoxia, (3) develop water and nutrient budgets 

for the lake, and (4) assess how the water quality of the 

lake should respond to changes in phosphorus loading. 

Water quality in Little St. Germain Lake consistently 

varied among basins. The West Bay consistently had the 

greatest water clarity and lowest total phosphorus and 

chlorophyll a concentrations, the South Bay had intermediate clarity and intermediate phosphorus and chlorophyll a

concentrations, and the East and Upper East Bays had the 

least water clarity and highest phosphorus and chlorophyll 

a concentrations. The summer average water quality in 

the lake was relatively stable from 1991 to 2000; however, 

water quality appears to have degraded since 2001. The 

West Bay has changed from oligotrophic to mesotrophic, 

the South Bay has changed from mesotrophic to eutrophic, 

and the East and Upper East Bays have changed from 

eutrophic to eutrophic/hypereutrophic.

Before aerators were installed and operated in the 

lake, winter anoxia frequently occurred throughout most 

of the lake, except in the West Bay and just below the ice 

in the East Bay. To eliminate winter anoxia, coarse-bubble 

line aerators were installed and operated in the Upper 

East, East, and South Bays. The aerator in the East Bay 

appeared to have little effect on dissolved oxygen concentrations throughout its basin. The ineffectiveness of 

the East Bay aerator may have been caused by the aerator 

being placed in water that was too shallow and (or) the 

aerator releasing too little air given the size of the East 

Bay. The aerators in the Upper East and South Bays were 

very successful at eliminating winter anoxia, and they 

maintained dissolved oxygen concentrations above 3.5 

mg/L throughout almost the entire water column, except 

below the depth of the aerators. 

Based on nitrogen-to-phosphorus ratios measured in 

the lake, the productivity in the lake is potentially limited 

by the supply of phosphorus. Detailed water and phosphorus budgets constructed for the lake indicated that the 

total phosphorus input to the lake was about 2,360 lb in 

monitoring year (MY)1997, 1,300 lb in MY1999, and 

1,150 lb in MY2001. They also indicated that inflow from 

Muskellunge Creek was the major source of phosphorus 

to the lake (53 to 62 percent) and that ground water was 

the secondary source (26 to 35 percent). Precipitation and 

septic systems each accounted for less than 8 percent of 

the total phosphorus input. Results from a detailed groundwater-flow model indicated that ground water flows into 

the lake from all sides, except the south sides of West and 

Second South Bays. Simulated total ground-water inflow 

to the lake is about 13 percent higher than net inflow 

rather than 50 percent higher than net inflow as assumed 

in an earlier study by Robertson and Rose (2000). Therefore, compared to what was estimated in the earlier study, 

there was less ground-water input into the lake (and less 

phosphorus), but more input from septic systems. The net 

result was very little change in the phosphorus budget from 

the earlier study. Most of this phosphorus appears to come 

from natural sources—ground water and surface water 

flowing through relatively undeveloped areas surrounding 

Little St. Germain Lake and Muskellunge Lake.

Several empirical models within the Wisconsin Lakes 

Modeling Suite (WiLMS) were used to simulate how the 

water quality of East and Upper East Bays of the lake 

should respond to specific reductions in phosphorus loads 

from Muskellunge Creek. Simulation results indicated that 

large reductions in tributary loads could improve the water 

quality of the East and Upper East Bays. A 75- to 100-

percent reduction in tributary loading would be needed for 

these parts of the lake to be classified as borderline mesotrophic/eutrophic. Improving the water quality of these 

bays would also improve the water quality of the South 

and Second South Bays because of the net flow of water 

through the lake. However, even with phosphorus loading 

from Muskellunge Creek completely eliminated, most of 

the lake would still be classified as borderline mesotrophic/eutrophic because of the contributions of phosphorus 

from ground water. 

34 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
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36 Water Quality, Hydrology, and Phosphorus Loading to Little St. Germain Lake, Wisconsin
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