1

Hydrology, Water Quality, and Phosphorus Loading of

Little St. Germain Lake, Vilas County, Wisconsin

U.S. Department of the Interior

U.S. Geological Survey

USGS Water-Resources Investigations Report 00–4209

October 2000

Figure 2. Morphometry of Little St. Germain Lake, Wis.,

and locations of lake-sampling sites.

Introduction

Little St. Germain Lake, which is in

Vilas County, Wisconsin, just northeast

of St. Germain (fig. 1), is one of 21

impoundments operated by Wisconsin

Valley Improvement Company (WVIC)

to provide storage for power and recreational use. The level of the lake, which

was originally dammed in 1882, has

been maintained by the WVIC at about

5 feet above its natural level since 1929,

and it is annually drawn down about 1.5

feet from December through March. In

the interest of protecting and improving

the water quality of the lake, the Little

St. Germain Lake Improvement Association was established in 1959. Later,

the Little St. Germain Lake District was

formed. The Wisconsin Department of

Natural Resources (WDNR), in collaboration with the Lake District, did a study

during 1983–85 to document the water

quality of the lake and examine management alternatives (Wisconsin Department of Natural Resources, 1985). Results of the study indicated that, because

of relatively high phosphorus loading to the lake, most of the lake

was eutrophic (relatively productive), with the possible exception

of the West Bay. The results also indicated monitoring of the lake

should continue, and that actions should be taken to decrease

nutrient loading to the lake by controlling erosion, fertilizer runoff,

and leakage from septic systems.

The lake was monitored in detail again during 1991–94 by the

U.S. Geological Survey (USGS) as part of a cooperative study with

the Lake District. This study demonstrated water-quality variation

among the basins of Little St. Germain Lake and extensive areas of

winter anoxia (absence of oxygen). Further in-depth studies were

then conducted during 1994–2000 to define the extent of winter

anoxia, refine the hydrologic and phosphorus budgets of the lake,

quantify the effects of annual drawdowns, and provide information

needed to develop a comprehensive lake-management plan. This

report presents the results of the studies since 1991.

The Lake and its Watershed

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

surface area of 977 acres and volume of 11,500 acre-feet. In this

report, the lake is discussed in terms of six basins (fig. 2): Upper East

Bay (119 acres, maximum depth—16 feet), East Bay (336 acres, 16

feet), No Fish Bay (69 acres, 10 feet), West Bay (213 acres, 53 feet),

Figure 1. Location of Little St. Germain Lake, watershed

characteristics, and location of stream-sampling sites.

St. Germain

70

155

Birchwood 

Drive

Muskellunge

Creek

Muskellunge

Lake

Little

St. Germain Lake

Snipe Lake

Little

St. Germain Lake Snipe Lake

WISCONSIN

Study area

45°57'30"

45°55'

89°27'30" 89°22'30'

0 1 2 MILES

0 1 2 KILOMETERS

Forest

Wetland

Cranberry bog

Urban

Agriculture

Grassland

Water

Barren and shrubland

EXPLANATION

Land use or land cover

Watershed boundary

Stream-sampling site

Little St. Germain Creek

A'

A

B

Primary lake-sampling site

Auxiliary lake-sampling site

EXPLANATION

Little St. Germain Lake

0 2000 4000 FEET West Bay

No Fish 

Bay

South Bay

East

Bay

Inlet

Upper East Bay

Second 

South Bay

Outlet

Dam

Depth, in feet 0

20

40

55
2

Figure 3. Schematic of the hydrologic budget of Little St. Germain Lake,

Wis. Abbreviations are defined in the text.

South Bay (122 acres, 22 feet), and Second South Bay (119 acres,

10 feet). The major tributary to the lake is Muskellunge Creek,

which flows about 3 miles from shallow, eutrophic Muskellunge

Lake into the north end of the East Bay. Outflow from the lake is to

Little St. Germain Creek, which leaves the south side of the Second

South Bay and flows about 1 mile before draining into the Wisconsin River.

The total watershed area of Little St. Germain Lake is 10 mi2

. The

watershed is predominantly forest (68 percent), wetland (17 percent), and water (8 percent), although areas of low-density residential development are increasing (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).

Data Collection—sites and techniques

Data used to describe the water quality of the lake were collected

from April 1991 to January 2000; however, no data were collected

from September 1994 to July 1996 and September 1997 to February

1999. Lake water-quality 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). 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 foot below the surface during

open water or just below ice during ice cover) or near bottom (1 foot

above bottom). Near-surface water samples were analyzed for

concentrations of total phosphorus (an indicator of nutrient availability) and chlorophyll a (an indicator of the algal population).

During ice-free periods, Secchi depths (an indicator of water

clarity) also were measured. All water samples were analyzed by

the Wisconsin State Laboratory of Hygiene.

Additional depth-profile measurements of temperature and oxygen were made at seven locations (the main sampling sites, the

center of each of the other bays, and the western end of the East Bay;

fig. 2) throughout the winter of 1996–97 to assess the extent and

timing of anoxia. Profiles also were collected between these sites in

March 1997 and 1999 to describe the spatial extent of anoxia

(transects A–B and A'–B ; fig. 2).

Data collected during this study were published in two annual

USGS data report series, the most recent of each being “Water

Resources Data, Wisconsin—Water Year 1999” (Holmstrom and

others, 2000) and “Water Quality and Lake-Stage Data for Wisconsin Lakes, Water Year 1999” (U.S. Geological Survey, Wisconsin

District Lake-Studies Team, 2000). Water levels at the dam on

Little St. Germain Creek were monitored almost daily from 1991–

99 by the WVIC (U.S. Geological Survey, Wisconsin District

Lake-Studies Team, 2000).

Inflow to the lake was determined from measurements and water

samples collected monthly in Muskellunge Creek at Birchwood

Drive (fig. 1) during October 1996–September 1997 and December

1998–January 2000. During 1996–97, water samples were analyzed for total phosphorus concentration. During 1998–99, water

temperature and dissolved oxygen also were measured, and the

samples also were analyzed for dissolved phosphorus.

Surface-water outflow from the lake was estimated from waterelevation measurements made at the dam by WVIC. To better

describe the outflow, additional flow measurements and water

samples were collected monthly just below the dam from December 1998 through November 1999. Water samples were analyzed

for total phosphorus. Measured flow at the dam indicated that low

flows were underestimated and therefore those flows were adjusted

accordingly.

Hydrology

The hydrology of Little St. Germain Lake can be described in

terms of components of its water budget (fig. 3). The water budget

for the lake may be represented by

∆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 of water

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).

Each term in the water budget was computed for two different

year-long periods: October 1996–September 1997 (1997) and

December 1998–November 1999 (1999). Changes in lake volume

were determined from water elevations monitored at the outlet dam

(fig. 2) and the morphometry of the lake. Precipitation was measured by a weather observer in St. Germain. Surface-water inflow

was estimated to equal the flow in Muskellunge Creek at Birchwood

Drive. Flows were expected to change rather slowly and therefore

daily inflows were estimated by linearly interpolating between

monthly measurements. Evaporation from the lake was estimated

on the basis of average monthly evaporation-pan data collected at

Rainbow Flowage (about 10 miles southwest of the lake). Surfacewater outflow consisted of flow past the dam into Little St. Germain

Creek. Ground water seeps into and out of the bottom of Little St.

Germain Lake. The monthly net ground-water flow (GWIn - GWOut)

was computed as the residual in the budget equation (eq. 1). These

data did not allow ground-water inflow and ouflow to be computed

independently; therefore, to estimate these components, groundwater inflow was assumed to be 50 percent more than net groundwater flow and ground-water outflow was assumed to be 50 percent

less than net ground-water flow.

Ground water GWIn GWOut

PPT

SWOut

Ground water

Evap

SWIn SWOut

SWIn
3

Total monthly precipitation at St. Germain, monthly average

surface-water inflow to and outflow from the lake, and water level

of the lake are shown in figure 4. Total precipitation during 1997

(34.8 inches) was 4.4 inches less than in 1999 (39.2 inches). The

average flow into the lake through Muskellunge Creek was 10.6

ft3/s (cubic feet per second) in 1997 and 6.0 ft3/s in 1999. The

average flow out of the lake was 17.3 ft3/s in 1997 and 10.6 ft3/s in

1999. Inflow to the lake throughout 1997 was about 1.7 times that

throughout 1999, even though there was less precipitation in 1997.

This demonstrates that the flow in Muskellunge Creek is driven by

long-term changes in precipitation rather than short-term fluctuations. Outflow from the lake in 1997 also was about 1.7 times that

in 1999. In both years, outflow from the lake was about 1.7 times

greater than that which came in from Muskellunge Creek. Evaporation from the lake was estimated to be 22.4 inches in both years.

Lake stage fluctuated from a minimum of 12.05 feet (relative to

an arbitrary datum) to a maximum of 13.95 feet (fig. 4). The lake

stage was relatively stable from May through mid November,

lowered about 1.5 feet 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 1997 was similar to

Figure 4. Monthly precipitation, inflow, outflow, and water elevation,

Little St. Germain Lake, Wis.

that at the beginning of the period; however, the lake stage was

about 0.65 foot higher at the end of 1999 than at the beginning of that

study year.

After converting all of the hydrologic components in the budget

equation (eq. 1) into acre-feet, there was a net ground-water input

to Little St. Germain Lake of about 3,900 acre-feet in 1997 and

2,400 acre-feet in 1999 (fig. 5). After assuming the total groundwater input was 50 percent more than net ground-water flow (an

assumption that needs further evaluation), the total ground-water

input was estimated to be 5,800 acre-feet in 1997 and 3,500 acrefeet in 1999. Ground-water studies conducted by the WDNR

indicate that most, if not all, of the ground water is expected to enter

into the East Bay (Wisconsin Department of Natural Resources,

1985).

The complete hydrologic budget (fig. 5) indicated that the major

source of water to the lake is from surface-water inflow from

Muskellunge Creek; however, during years following extended dry

periods (such as prior to 1999), direct precipitation and ground

water can be nearly as important. The major loss of water from the

lake is through the outlet.

Phosphorus Budget

Previous studies indicated that most of Little St. Germain Lake

was eutrophic because of relatively high phosphorus loading to the

lake (Wisconsin Department of Natural Resources, 1985). Therefore, to help define where the phosphorus originated, a detailed

phosphorus budget was computed. Sources of phosphorus to the

lake include precipitation, the inflowing stream, ground water, and

Figure 5. Hydrologic and phosphorus budgets of Little St. Germain Lake,

Wis.

12.0

12.5

13.0

13.5

14.0

0

5

10

15

20

25

30

Oct. Jan. Apr. July Oct. Jan. Apr. July Oct. Jan. Apr. July Oct.

1996 1997 1998 1999

TOTAL PRECIPITATION, IN INCHES

0

2

4

6

8

10

AVERAGE STREAMFLOW,

IN CUBIC FEET PER SECOND

WATER ELEVATION, IN FEET ABOVE 

AN ARBITRARY DATUM

Inflow

Outflow

Oct. Jan. Apr. July Oct. Jan. Apr. July Oct. Jan. Apr. July Oct.

1996 1997 1998 1999

Oct. Jan. Apr. July Oct. Jan. Apr. July Oct. Jan. Apr. July Oct.

1996 1997 1998 1999

Hydrologic budget

Precipitation Evaporation Surface-water

inflow

Surface-water

inflow

Surface-water

outflow

Surface-water

outflow

Net

ground water

Ground water

Change in

storage

Phosphorus budget

-2,000

-1,500

-1,000

-500

0

500

1,000

1,500

2,000

Precipitation Septic 

systems

Deposition

in lake

WATER INPUTS AND LOSSES, IN ACRE-FEET PHOSPHORUS INPUTS AND LOSSES, IN POUNDS

-15,000

-12,000

-9,000

-6,000

-3,000

0

3,000

6,000

9,000

1997

1999

1997

1999
4

contributions from septic systems. Phosphorus concentration in

precipitation was assumed to be 0.007 mg/L, a value found by Rose

(1993) for northern Wisconsin. Therefore, direct precipitation

contributes about 55 lbs of phosphorus per year to the lake (fig. 5).

Phosphorus concentrations in Muskellunge Creek inflow ranged

from about 0.04 mg/L in winter to about 0.12 mg/L in July 1997 and

about 0.09 mg/L in May 1999 (fig. 6). In 1999, about 30 percent of

the phosphorus was in dissolved forms; however, the percentage in

dissolved forms was not measured in 1997. Phosphorus concentrations were much higher in 1997 than in 1999, especially in mid to

late summer. The high concentrations in 1997 may have been due

to effects of beaver activity on Muskellunge Creek downstream

from Muskellunge Lake. It is thought that ponding of water behind

beaver dams resulted in a high release of phosphorus from the

organic-rich wetland sediments that are not otherwise inundated

with water. With this increased release of phosphorus from the

sediments, a higher percentage of phosphorus would probably be in

dissolved forms than was measured in 1999. Phosphorus concentrations in Muskellunge Creek, in both years, were high considering most of the watershed of Little St. Germain Lake is relatively

pristine. The high concentrations are thought to be the result of

leaching from the soils that are rich in phosphorus (Wisconsin

Department of Natural Resources, 1985). Daily phosphorus concentrations were estimated by linearly interpolating between monthly

measurements. The amount of phosphorus delivered to the lake was

then computed by multiplying the daily phosphorus concentrations

by the daily runoff volumes. The total input of phosphorus from

stream inflow was estimated to be 1,500 and 700 pounds in 1997

and 1999, respectively (fig. 5). The difference between years was

primarily due to the reduced flows in 1999, but decreased concentrations also contributed to the decreased loads in 1999.

Phosphorus concentrations in ground water were not measured

as part of this study, and those measured as part of other studies were

quite variable. Therefore, a phosphorus concentration for ground

water was estimated by use of equation 2:

[TP]GW = (QBW*[TP]BW – QMLO*[TP]MLO) . (2)

 (QBW – QMLO)

This equation is based on two assumptions: (1) during winter,

biological and chemical processes have minimal effect on the water

quality of Muskellunge Creek, and so changes in the concentration

of phosphorus in Muskellunge Creek as it flows from Muskellunge

Lake outlet (MLO) to Birchwood Drive (BW) are caused only by

the addition of ground water, and (2) ground water entering Little

St. Germain Lake has the same concentration as that entering

Muskellunge Creek. Therefore, an estimate of the phosphorus

concentration in ground water ([TP]GW) can be obtained by the

change in the phosphorus load (Q*[TP]) from MLO to BW divided

by the increase in the flow of the creek (QBW – QMLO). Average

phosphorus concentrations (from December 1999 and January

2000) increased from 0.035 mg/L at Muskellunge Lake Outlet to

0.045 mg/L at Birchwood Drive, while average streamflow increased by 2.1 ft3

/s. Therefore, an average phosphorus concentration of 0.053 mg/L was obtained for ground water after applying

these values to equation 2 and resulted in an estimated total input of

phosphorus from ground water of 835 and 512 pounds in 1997 and

1999, respectively (fig. 5). Most phosphorus contributed by ground

water is expected to enter into the East Bay of the lake.

The input of phosphorus from septic systems (M) was estimated

by use of equation 3 (Reckhow and others, 1980):

M = ES

 * (Number of Capita Years) * (1 – SR), (3)

where M is a function of an export coefficient, ES

, and a soil

retention coefficient, SR. In applying equation 3, it was assumed that

the most likely value for ES was 1.8 pounds of phosphorus per capita

per year. The number of capita years was estimated to be 165 (only

residents on the East and Upper East Bays were included: 90 fullyear residents, 270 three-month residents, and 90 one-month residents), and the most likely value of SR was 0.85. Only residents on

these bays were included because past studies indicated that most

of the ground water entered the lake through these areas (Wisconsin

Department of Natural Resources, 1985). The total input from

septic tanks was then computed to be 44 pounds per year. By

applying low and high estimates for ES (1.1 and 2.2 pounds of

phosphorus per capita per year) and SR (0.9 and 0.5), low and high

estimates of phosphorus from septic systems were 18 and 182

pounds, respectively.

Phosphorus concentrations leaving the lake ranged from about

0.02 to 0.05 mg/L (fig. 6). Concentrations in 1997 were higher than

in 1999, especially from March through June. The higher concentrations reflect higher phosphorus concentrations in the lake in

1997 than in 1999. Daily phosphorus concentrations were estimated by linearly interpolating between monthly measurements,

and the amount of phosphorus removed from the lake was then

computed by multiplying the daily phosphorus concentrations by

the daily outflows. The total amount of phosphorus in stream

outflow was estimated to be 1,370 and 440 pounds in 1997 and

Figure 6. Phosphorus concentrations and loads in the inflow and outflow

from Little St. Germain Lake, Wis., and phosphorus loads to the lake from

Muskellunge Creek.

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

0

50

100

150

200

250

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

1997

1999

TOTAL PHOSPHORUS CONCENTRATION,

IN MILLIGRAMS PER LITER

PHOSPHORUS LOAD FROM MUSKELLUNGE 

CREEK, IN POUNDS

Phosphorus loads

Phosphorus concentrations Inflow - 1997

Inflow - 1999

Outflow - 1997

Outflow - 1999
5

1999, respectively (fig. 5). The greater load in 1997 was due to a

combination of higher concentrations and flows in 1997 than in

1999.

The phosphorus budget (fig. 5) indicates that inflow from Muskellunge Creek was the major source of phosphorus to the lake (53–61

percent) and ground water was the secondary source (35–39 percent). The concentrations and volumes of ground water entering the

lake, however, are based on several untested assumptions. Approximately 57 and 33 percent (1997 and 1999, respectively) of the total

phosphorus input to the lake (2,410–1,310 pounds in 1997 and

1999, respectively) was exported through the outlet. The remaining

43 to 67 percent of the phosphorus input (1,400 and 870 pounds in

1997 and 1999, respectively) was deposited in the bed sediment of

the lake or discharged with ground-water outflow.

Lake-Water Quality

Water quality in Little St. Germain Lake varied consistently

among basins, except for a few water-quality characteristics that

were similar throughout the lake but varied seasonally: specific

conductance, which ranged from about 75 microsiemens per centimeter (μs/cm) in summer to about 90 μs/cm in winter; and pH,

which ranged from about 7 in winter to about 8 in summer.

Water Clarity

Water clarity, the distribution of temperature and dissolved

oxygen, and the concentrations of nutrients, were all consistently

different among basins. The differences indicated that the West Bay

generally had the best water quality and the East Bay had the poorest

quality. Water clarity, based on Secchi depth readings, ranged from

7–15 feet in the West Bay (average summer clarities of 9–13 feet)

to 4–14 feet in the South Bay (average summer clarities of 7–10

feet) to 2–8 feet in the East Bay (average summer clarities of 3–6

feet) (fig. 7). Clarity was usually the best in late summer in the West

Bay; however, it was usually best in early summer in the East Bay.

Water Temperature and Dissolved Oxygen

Thermal stratification also differed among basins because of

differences in their morphometries and limited circulation between

basins. The West Bay, being relatively deep and having a relatively

short length, became strongly stratified during summer, with bottom temperatures remaining around 8–9o

C. The South Bay, being

moderately deep, became only weakly stratified during summer,

and stratification was frequently broken down by wind mixing.

Bottom temperatures in the South Bay gradually increased throughout the summer. Thermal stratification throughout the rest of the

lake was very weak, with seldom more than 2 or 3o

C of stratification. During the winter, weak thermal stratification was also present

throughout the lake.

Thermal stratification during summer, primarily in the West

Bay, 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 that settled from the surface and the biochemical

oxygen demand of the sediment. Water below about 30 feet in the

West Bay usually became anoxic in late June and stayed anoxic

throughout summer. In the South Bay, the weak stratification

resulted in only the deepest water becoming nearly, but almost

never completely, anoxic.

Before freezing, most of the lake was nearly saturated with

oxygen; however, after the lake froze and winter progressed,

oxygen was quickly consumed, especially in the shallower basins.

Although oxygen is consumed slowly during periods of low temperatures, extensive oxygen depletion occurred in every basin of

the lake. 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 ice cover. Changes in oxygen concentrations for the East and Upper East Bays of the lake are shown in

figure 8. Other than the shallowest areas of the West and East Bays,

the remaining parts of the lake can become almost completely

depleted of oxygen by mid-February. To demonstrate the spatial

extent of oxygen depletion, transects of temperature and 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 and

March 1999 (fig. 9). Detailed transects were collected in March

because this was near to when oxygen depletion was expected to be

most severe. As figure 9 shows, anoxia occurred throughout each

of the basins; and by mid-March only small areas of the lake would

be habitable by most fish (areas with dissolved oxygen concentrations greater than about 2 mg/L). These habitable areas include

water down to about 30 feet in the West Bay and down to about 5

feet in the East Bay.

Figure 7. Average summer Secchi depth, and surface concentrations of

phosphorus and chlorophyll a in the three main basins of Little St.

Germain Lake, Wis., by year.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

0

10

20

30

40

50

60

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

CHLOROPHYLL 

A CONCENTRATION, 

IN MICROGRAMS PER LITER

0

3

6

9

12

15

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

SECCHI DEPTH, IN FEET

TOTAL PHOSPHORUS CONCENTRATION,

IN MILLIGRAMS PER LITER

Phosphorus

Chlorophyll a

Secchi depth

West Bay

East Bay

South Bay
6

Water entering from Muskellunge Creek can alleviate the extent

of 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 (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 (fig. 8). This increase appears to be associated

with cold, highly oxygenated water originating from Muskellunge

Creek propagating across the basin (fig. 9). 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 feet of water (just below the ice) throughout the East Bay

could be replaced by water from Muskellunge Creek in about 30

days.

Phosphorus Concentration

Phosphorus is one of the essential nutrients for plant and algal

growth and is often the nutrient that limits this growth in midwestern

lakes. High concentrations of phosphorus can cause high algal

populations (blooms) and can therefore be a major cause of eutrophication (that is, accelerated aging and increased productivity) of

lakes. Phosphorus concentrations were consistently highest in the

East Bay (average summer concentrations of 0.031–0.061 mg/L),

moderate in the South Bay (0.018–0.038 mg/L), and lowest in the

West Bay (0.009–0.026 mg/L). These differences among basins

appear to be directly related to the input of nutrients from both

Muskellunge Creek and ground water and to differences in basin

morphometry.

Phosphorus can be released from lake sediments, especially

during periods of anoxia. Increased phosphorus concentrations just

above the sediments were observed primarily in the West Bay

during late summer, when the deep water

was anoxic. Phosphorus concentrations

reached 0.2–0.3 mg/L in late summer in

the West Bay, but only 0.08–0.09 mg/L

just above the sediments in the South Bay.

The extensive anoxic area during winter,

especially during 1997, resulted in phosphorus concentrations reaching 0.17

mg/L in the West Bay, but only 0.08

mg/L in the South Bay and 0.10 mg/L in

the East Bay.

Chlorophyll a Concentration

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. Concentrations

greater thatn 15 μg/L are considered to be

very high and usually associated with

algal blooms. Differences in chlorophyll

Figure 8. Oxygen distributions in the Upper East and East Bays of Little St. Germain Lake, a concentrations among basins directly

Wis., during winter 1996–97.

Figure 9. Distributions of dissolved oxygen in Little St. Germain Lake,

Wis., March 18, 1997. (Trace of sections are shown in figure 2.)

0 14 2 4 6 8 10 12

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

0

1

2

3

4

5

6

7

8

9

10

11

12

13

DEPTH BELOW SURFACE, IN FEET

Dec. 17, 1996

Jan. 23, 1997

Feb. 19, 1997

Mar. 18, 1997

Nov. 1996, just before freezing (estimated)

Upper East Bay

0 14 2 4 6 8 10 12

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

0

1

2

3

4

5

6

7

8

9

10

11

12

13

DEPTH BELOW SURFACE, IN FEET

Nov. 1996, just before freezing (estimated)

Dec. 17, 1996

Jan. 23, 1997

Feb. 19, 1997

Mar. 18, 1997

East Bay, main site

0 3,000 6,000 9,000 12,000 15,000 18,000

A B 0

5

10

15

20

25

East Bay

South Bay

No Fish Bay

Second

South Bay

Ice Inlet Outlet

0

1

2

3

4

5

6

7

8

9

10

11

A' B 0

10

20

30

40

50

West Bay

South Bay

Second

South Bay

West End Ice Outlet

DEPTH, IN FEET DEPTH, IN FEET

DISTANCE FROM A', IN FEET

DISTANCE FROM A, IN FEET

0 5,000 10,000 15,000

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

1

1

3

1

3 1 5

5

7

9
7

coincided with the differences in the phosphorus concentrations

among basins. Concentrations were highest in the East Bay (average summer concentrations ranged from 20–48 μg/L), moderate in

the South Bay (7–11 μg/L) and lowest in the West Bay (2–4 μg/L)

(fig. 7). Concentrations were commonly greater than 15 μg/L in the

East Bay and occasionally above 15 μg/L in the South Bay, but

never observed above 15 μg/L in the West Bay.

Trophic State Indices

One method of classifying water quality or productivity of lakes

is by computing water-quality indices (Trophic State Indices, or

TSI’s). These indices, based on near-surface concentrations of total

phosphorus and chlorophyll a and on Secchi depths, were developed by Carlson (1977) and modified for Wisconsin lakes by Lillie

and others (1993). Oligotrophic lakes (TSI’s 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 (TSI’s 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 (TSI’s

greater than 50) are nutrient rich with correspondingly severe

water-quality problems, such as frequent seasonal algal blooms,

oxygen depletion in lower parts of the lakes, and poor clarity. Lakes

with TSI’s 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 Bay was eutrophic and often hypereutrophic during summer (average summer TSI based on surface phosphorus was 58,

based on surface chlorophyll a was 60, and based on Secchi depth

was 58). All three of the indices indicated that the South Bay was

mesotrophic to eutrophic (average summer TSI based on surface

phosphorus was 53, based on surface chlorophyll a was 51, and

based on Secchi depth was 48). All three of the indices indicated

that the West Bay was mesotrophic (average summer TSI based on

surface phosphorus was 47, based on surface chlorophyll a was 43,

and based on Secchi depth was 42).

Effects of Winter Drawdown

As mentioned previously, the WVIC controls the water level of

the lake in accordance with their Federal Energy and Regulatory

Commission license. Each winter the lake is drawn down about 1.5

feet. The drawdown is begun in November and completed in early

February (fig. 4). In 1997, outflows from the lake were highest

during November through February. Refilling then begins in early

March and typically by May the water level is back to its normal

summer elevation. Outflow from the lake in 1997 was lowest

during March and April.

Effects on Nutrient Loading

Total phosphorus concentrations in the outflow generally increase from November through April (fig. 6). The average concentration increased 0.015 mg/L from November–February to March–

April in 1997; however, there was no increase in 1999. Therefore,

increased early-winter water removal associated with the drawdown may decrease the amount of nutrients that would be removed

from the lake. If it is assumed that the drawdown resulted in 1,500

acre-feet of water (a 1.5-foot drawdown) being released in early

winter instead of late winter, this would equate to about 65 pounds

of phosphorus being retained in the lake in 1997 and no change in

1999. This amount represents about 0–3 percent of the total input

of phosphorus. Therefore, the drawdown has only a small effect on

the phosphorus budget for the lake as a whole.

Winter drawdown may, however, increase the phosphorus loading to the West Bay. During the drawdown period, water with a

relatively low concentration of phosphorus flows from West Bay

into No Fish Bay, whereas during refilling, water with a relatively

high concentration of phosphorus flows from No Fish Bay into

West Bay. To determine the effects of this process, the average

drawdown for the 1991–99 period was examined.

During 1991–99, average drawdown was 1.57 feet, average time

to achieve drawdown was 106 days, average precipitation during

drawdown was 0.42 foot, and evaporation was considered to be

negligible. Therefore, there was a net release of 1.99 feet of water

from West Bay. If the average concentration of phosphorus in the

water was 0.014 mg/L (the average near-surface concentration

measured in the West Bay), there would be a net removal of 14.6

pounds of phosphorus from West Bay. During 1991–99, the

average time to achieve refilling of the lake was 81 days, average

precipitation during refilling was 0.46 foot, and average evaporation was estimated to be 0.18 foot. Therefore, there was a net inflow

of 1.29 feet of water to West Bay. If the average concentration of

phosphorus was 0.045 mg/L (the average near-surface concentration measured in the East Bay), there would be a net increase of 31.2

pounds of phosphorus to West Bay. Hence the net effect, on

average, of the drawdown and refilling of the lake is a16.6-pound

increase in phosphorus loading to West Bay. This amount is

slightly more than that contributed by precipitation for the year

(12.2 pounds). Therefore, although the drawdown contributes only

a small amount of phosphorus to the West Bay, it may be a major

source given the few other sources to this basin.

Effects on Dissolved Oxygen

The drawdown may also affect dissolved oxygen concentrations

in the lake because oxygen concentrations decrease dramatically

Figure 10. Trophic state indices based on surface total phosphorus

concentrations in the West, East, and South Bays of Little St. Germain

Lake, Wis., by year.

30

40

50

60

70

West Bay

East Bay

South Bay

1991 1992 1993 1994 1995 1996 1997 1998 1999

TROPHIC STATE INDEX VALUES

Eutrophic

Mesotrophic

Oligotrophic
8

Information

For information on this study or on other USGS programs in

Wisconsin, contact:

District Chief

U.S. Geological Survey

8505 Research Way

Middleton, WI 53562

(608) 828-9901

http://wi.water.usgs.gov/

Authors: Dale M. Robertson and William J. Rose

Layout and illustrations: Michelle Greenwood, Aaron Konkol,

and David Saad

Printed on recycled paper

from November through April (fig. 8); therefore, more oxygen

would be removed if more water was taken out earlier in the winter.

The average concentration of dissolved oxygen in the South Bay

decreased 7.2 mg/L from November–February (8.8 mg/L) to

March–April (1.7 mg/L) in 1997. If it is assumed that the drawdown

resulted in 1,500 acre-feet of water being released in early winter

instead of late winter, this would equate to about 30,000 pounds of

oxygen being released. This amount represents about 8 percent of

the total dissolved oxygen in the entire lake when it freezes, or

about 18 percent of the dissolved oxygen in East, No Fish, and

South Bays combined, or about 44 percent of the dissolved oxygen

in just the South Bays when the lake freezes. The smaller the

amount of oxygen available for consumption by biochemical

reactions, the sooner the concentrations will decrease below critical levels. Therefore, the drawdown can significantly decrease the

length of time certain areas of the lake are habitable by fish.

Effects of Phosphorus Reductions

The total phosphorus input to the lake was estimated to be 2,410

and 1,310 pounds in 1997 and 1999, respectively. Most of this

phosphorus is input into the East Bay and results in the water

quality in this basin being significantly poorer than in other parts of

the lake. One way to determine how much phosphorus loading

would need to be reduced to improve the water quality of this basin

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; J. Panuska, Wisconsin Department of Natural

Resources, written commun., 1999) relate hydrologic and phosphorus loading to in-lake phosphorus concentrations. Six of these

models were applicable to the East Bay of Little St. Germain Lake.

Therefore, the recent hydrologic and phosphorus loading to the

lake (1997 and 1999) and various phosphorus-reduction scenarios

were input into these models to predict phosphorus concentrations.

The average phosphorus concentration predicted by the models for

1997 and 1999 was 0.051 mg/L, which is comparable to the

measured lake concentration of about 0.046 mg/L. The models

were then applied to various phosphorus-reduction scenarios: 50,

75, and 100 percent reduction in tributary loading, with all other

sources maintained at their present levels. The models predicted

that these reductions in tributary loading would cause the average

phosphorus concentration in the East Bay to decrease by 0.012,

0.019, and 0.021 mg/L, respectively. Another empirical model,

developed by Lillie and others (1993) and contained in WiLMS,

relates in-lake phosphorus concentration to average summer Secchi

depth. This model predicted that reductions in phosphorus concentrations of 0.012, 0.019, and 0.021 mg/L would be expected to

increase the average summer Secchi depth by 0.7, 1.0, and 2.0 feet,

respectively. Therefore, a total elimination of the phosphorus

loading from Muskellunge Creek is predicted to increase the

summer Secchi depth from 3.8 feet to about 5.8 feet. In addition to

improving water clarity, the reduction in total phosphorus would be

expected to decrease the frequency of blue-green algal blooms.

Because of the significant contributions of phosphorus to the

lake estimated from ground water, even with tributary loading

eliminated, the predicted phosphorus concentrations and Secchi

depths still resulted in the East Bay being classified as a eutrophic

system. As mentioned previously, however, estimates of groundwater inflow are considerably uncertain, and further studies would

be needed to better quantify the importance of ground water to the

lake.

References

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

Oceanography, v. 22, p. 361–369.

Holmstrom, B.K., Olson, D.L., and Ellefson, B.R., 2000, Water

resources data-Wisconsin, water year 1999: U.S. Geological

Survey Water-Data Report WI-99-1, 578 p.

Lillie, R.A., Graham, Susan, and Rasmussen, Paul, 1993, Trophic

state index equations and regional predictive equations for Wisconsin Lakes: Wisconsin Department of Natural Resources

Management Findings, no. 35, 4 p.

Reckhow, K.H., Beaulac, M.N., and Simpson, J.T., 1980. Modeling phosphorus loading in lake response under uncertainty: A

manual and compilation of export coefficients: U.S. Environmental Protection Agency. EPA-440/5-80-011.

Rose, W.J., 1993, Water and phosphorus budgets and trophic state,

Balsam Lake, northwestern Wisconsin, 1987–89: U.S. Geological Survey Water-Resources Investigations Report 91-4125, 28

p.

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

2000, Water-quality and lake-stage data for Wisconsin lakes,

water year 1999: U.S. Geological Survey Open-File Report 00-

89, 140 p.

Wisconsin Department of Natural Resources, 1985, Little St.

Germain, Vilas County, feasibility study results and management alternatives: Bureau of Water Resources Management,

Lake Management Program, 22 p.