U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 03–4238

In cooperation with the Red Cedar Lakes Association

Water Quality and the Effects of Changes in 

Phosphorus Loading, Red Cedar Lakes, Barron 

and Washburn Counties, Wisconsin 

EXPLANATION

Land use/land cover

Stream

Basin boundary

Road

Urban

Agriculture

Forest

Grassland/shrubland

Wetland

Water

Birchwood irchwood

Mikana

48

R e d C e d a r L a k e

Hemlock Lake

Balsam Lake

Birch Lake

L a k e C h e t a c

Red Cedar River

Stout Island

WISCONSIN I S C O N S I N

Location of

study area
Water-Resources Investigations Report 03–4238

U.S. Department of the Interior

U.S. Geological Survey

Water Quality and the Effects of Changes 

in Phosphorus Loading, Red Cedar Lakes, 

Barron and Washburn Counties, 

Wisconsin 

By Dale M. Robertson, William J. Rose, and Herbert S. Garn

In cooperation with the Red Cedar Lakes Association
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: 2003

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

Balsam, Hemlock, and Red Cedar Lakes and Their Watershed ........................................................... 3

Sampling Methods........................................................................................................................................ 4

Lake Sampling ...................................................................................................................................... 4

Tributary Sampling............................................................................................................................... 6

Lake Stage and Precipitation Sampling........................................................................................... 7

Lake-Water Quality....................................................................................................................................... 7

Balsam Lake ........................................................................................................................................ 7

Hemlock Lake ...................................................................................................................................... 9

Red Cedar Lake ................................................................................................................................. 10

Lake Comparison with Trophic State Indices................................................................................ 13

Sources of Water and Phosphorus.......................................................................................................... 13

Water Budget ..................................................................................................................................... 13

Phosphorus Budget .......................................................................................................................... 17

Precipitation .............................................................................................................................. 17

Gaged Surface-water Inflow .................................................................................................. 18

Ungaged Surface-water Inflow.............................................................................................. 23

Inflow from Upstream Lakes/Basins ..................................................................................... 23

Ground Water .......................................................................................................................... 23

Septic Systems.......................................................................................................................... 23

Lake Outflow.............................................................................................................................. 24

Phosphorus Budget Summary................................................................................................ 24

Relative Importance of Internal Loading of Phosphorus ............................................................ 25

Modeling the Response in Water Quality to Changes in Phosphorus Loading................................ 26

Approach............................................................................................................................................. 26

Data Requirements............................................................................................................................ 26

Algorithms and Calibration............................................................................................................... 27

Water-Quality Response to Changes in Phosphorus Loading on the 

Basis of BATHTUB Simulations.......................................................................................... 27

Changes in Phosphorus Concentrations .............................................................................. 28

Changes in Chlorophyll a Concentrations. ........................................................................... 28

Changes in Water Clarity (Secchi Depths) ........................................................................... 28

Changes in Trophic Status....................................................................................................... 28

Changes in Algal Bloom Response........................................................................................ 34

Response in Water Quality to Changes in Phosphorus Loading on the 

Basis of WiLMS Simulations. ............................................................................................ 34

Balsam Lake .............................................................................................................................. 38

Red Cedar Lake ......................................................................................................................... 38

Hemlock Lake ............................................................................................................................ 39

Comparison of Results from BATHTUB and WiLMS.................................................................... 39

Summary and Conclusions........................................................................................................................ 39

References................................................................................................................................................... 41
Figures

 1-2. Maps showing:

 1. Drainage basin of the Red Cedar Lakes, Wisconsin ....................................................... 2

 2. Direct drainage basin of the Red Cedar Lakes, Wisconsin, with 

 data-collection sites identified............................................................................................ 5

 3-6. Graphs showing:

 3. Secchi depth, and near-surface concentrations of total phosphorus 

 and chlorophyll a in Balsam Lake, Wisconsin, 1993-2001 .............................................. 8

 4. Secchi depth, and near-surface concentrations of total phosphorus 

 and chlorophyll a in Hemlock Lake, Wisconsin, 1991-2002 .......................................... 11

 5. Secchi depth, and near-surface concentrations of total phosphorus 

 and chlorophyll a in Red Cedar Lake (North Basin), Wisconsin, 1987-2002.............. 12

 6. Average summer Trophic State Index values based on water-quality data 

 collected from May through September in Balsam, Hemlock, and Red Cedar 

 (North Basin) Lakes, Wisconsin, 1987-2001 .................................................................... 14

 7. Schematic of the hydrology of the Red Cedar Lakes, Wisconsin ..................................... 15

 8-17. Graphs showing:

 8. Daily average lake stages in the Red Cedar Lakes, Wisconsin, 

 October 1, 2000 through September 30, 2001.................................................................. 16

 9. Water budget for the Red Cedar Lakes, Wisconsin, for October 1, 2000

 through September 30, 2001 .............................................................................................. 18

 10. Phosphorus and suspended sediment concentrations in the Red Cedar 

 River downstream from Birch Lake, Sucker Creek, Hemlock Creek, and 

 Red Cedar Lake, Wisconsin, from October, 2000 through September, 2001.............. 22

 11. Phosphorus budget for the Red Cedar Lakes, Wisconsin, for October 1, 2000

 through September 30, 2001 .............................................................................................. 25

 12. Measured and simulated (with BATHTUB) average summer near-surface 

 concentrations of total phosphorus and chlorophyll a, and Secchi depths 

 in Balsam, Red Cedar, and Hemlock Lakes, Wisconsin, 2001...................................... 29

 13. Simulated changes in near-surface total phosphorus concentrations 

 in Balsam, Red Cedar, and Hemlock Lakes, Wisconsin, with BATHTUB 

 and WiLMS in response to various phosphorus-loading scenarios .......................... 30

 14. Simulated changes in near-surface chlorophyll a concentrations 

 in Balsam, Red Cedar, and Hemlock Lakes, Wisconsin, with BATHTUB 

 and WiLMS in response to various phosphorus-loading scenarios .......................... 31

 15. Simulated changes in Secchi depth in Balsam, Red Cedar, and 

 Hemlock Lakes, Wisconsin, with BATHTUB and WiLMS in response 

 to various phosphorus-loading scenarios....................................................................... 32

 16. Simulated changes in Trophic State Index values in Balsam, 

 Red Cedar, and Hemlock Lakes, Wisconsin, with BATHTUB in response 

 to various phosphorus-loading scenarios....................................................................... 33

 17. Simulated changes in the frequency of algal blooms in Balsam, 

 Red Cedar, and Hemlock Lakes, Wisconsin, in response to various 

 phosphorus-loading scenarios on the basis of output from BATHTUB ..................... 35

 

iv
v

Tables

 1. Morphometric characteristics of the Red Cedar Lakes, Wisconsin................................... 3

 2. Land use/land cover in the Red Cedar Lakes watershed, Wisconsin ................................ 4

 3. Seasonal average water quality in the Red Cedar Lakes, Wisconsin ................................ 9

 4. Total annual hydrological inputs into the Red Cedar Lakes, Wisconsin

October 1, 2000, to September 30, 2001.................................................................................. 16

 5. Phosphorus inputs to the Red Cedar Lakes, Wisconsin, October 1, 2000, 

to September 30, 2001 ............................................................................................................... 19

 6. Number of capita years for septic systems use for homes within 

500 feet of Red Cedar Lakes, Wisconsin................................................................................ 24

 7. Algorithms used within BATHTUB to simulate water quality in 

Red Cedar Lakes, Wisconsin ................................................................................................... 27

 8. Calibration coefficients used in BATHTUB simulations of the 

Red Cedar Lakes, Wisconsin ................................................................................................... 27

 9. Simulated algal bloom frequency in the Red Cedar Lakes, Wisconsin, in 

response to various phosphorus-loading scenarios, on the basis of output 

from BATHTUB ........................................................................................................................... 36

 10. Measured and simulated phosphorus concentrations in Red Cedar Lakes, 

Wisconsin, from hydrologic and phosphorus-loading models contained 

in WiLMS..................................................................................................................................... 37
vi

Conversion Factors and Abbreviated Water-Quality Units

 Multiply By To Obtain

 inch (in.) 25.4 millimeter

 foot (ft) .3048 meters

 mile (mi) 1.609 kilometer

 acre .004047 kilometer squared

 square mile (mi2

) 2.590 kilometer squared

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

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

 pound (lb) 0.4536 kilogram

 pound per acre (lb/acre) 1.121 kilogram per hectare 

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

 °F = [1.8 x °C] + 32.

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

designated by the calendar year in which it ends.

Abbreviated water-quality units used in this report: Chemical concentrations and water temperature are given in metric units. Chemical concentration is given in milligrams per liter (mg/L) 

or micrograms per liter (μg/L). Milligrams per liter is a unit expressing the concentra tion of 

chemical constituents in solution as weight (milligrams) of solute per unit volume (liter) of water. 

One thousand micrograms per liter is equivalent to one milligram per liter. For concentrations 

less than 7,000 mg/L, the numerical value is the same as for concentrations in parts per million.
vii

Acknowledgments

Technical Reviewers

W. Reed Green, Hydrologist, U.S. Geological Survey, Little Rock, Ark.

Patrick W. (Buzz) Sorge, Lake Management Planner, Wisconsin Department of Natural 

Resources, Eau Claire, Wis.

Mary H. Hele, Red Cedar Lakes Association, Past President and Board Member, Birchwood, Wis.

Project Coordinator

Mary H. Hele, Red Cedar Lakes Association, Past President and Board Member, Birchwood, Wis.

Editorial and Graphics

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

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

James L. Kennedy, Geographer, U.S. Geological Survey, Middleton, Wis.

Jack H. Green, Hydrologist, U.S. Geological Survey, Middleton, Wis.

Data Collection

Josef G. Schuler, Hydrologic Technician, U.S. Geological Survey, Rice Lake, Wis.

Bernard N. Lenz, Civil Engineer, U.S. Geological Survey, Rice Lake, Wis.

Approving Official

Angel Martin, Jr., Acting Reports Improvement Advisor, U.S. Geological Survey, Urbana, Ill.
viii
Abstract

The Red Cedar Lakes consist of three mainstem lakes 

(Balsam, Hemlock and Red Cedar) on the Red Cedar River 

in Barron and Washburn Counties, Wisconsin. These lakes 

are productive because of high phosphorus loading, and 

classified as mesotrophic to eutrophic. Because of concerns 

that the water quality of these lakes was degrading, three 

cooperative studies were conducted by the U.S. Geological 

Survey between 1993 and 2003. As part of these studies, 

water quality in the lakes was documented in 1993–94, 

1996–97, and 2000–01, and water and phosphorus budgets 

were constructed for water year 2001. Historical waterquality data indicated that the lakes have changed little 

since the late 1980s. A detailed phosphorus budget indicated that most of the 14,100 pounds of phosphorus input 

to the lakes during 2001 came from the upstream lakes, 

streams draining relatively undeveloped land upstream 

of Hemlock Lake, and ground water. Simulation results 

from two water-quality models (BATHTUB and WiLMS) 

indicated that about a 50-percent reduction in phosphorus 

loading from that measured in 2001 is required for all three 

lakes to be classified as mesotrophic; therefore, appreciable 

improvements in the water quality would require improvements in the water quality of the upstream lakes. Although 

the water quality of the lakes has not changed appreciably 

in recent years and major improvements in water quality are unlikely without major improvements to upstream 

lakes, continued efforts to protect the susceptible watershed 

are necessary to maintain the current level of water quality. 

Introduction

The Red Cedar Lakes consist of three mainstem lakes 

(Balsam, Hemlock and Red Cedar) on the Red Cedar River 

in Barron and Washburn Counties, Wisconsin (fig. 1). 

Headwaters of the Red Cedar River originate as outflow 

from Lake Chetac (a large, shallow, productive lake), which 

then flow through Birch Lake (a small, deep, productive 

lake), Balsam Lake, and into Red Cedar Lake. Additional 

headwaters originate from Hemlock Creek, which flows 

through Bolger Flowage, through Hemlock Lake, and into 

Red Cedar Lake (fig. 1). The narrows separating Balsam 

and Hemlock Lakes from Red Cedar Lake are sufficiently 

large that all three lakes have the same water-surface elevation. The Red Cedar River then flows out of Red Cedar 

Lake and joins with the Brill River prior to flowing into 

Rice Lake. Approximately 70 mi downstream, the Red 

Cedar River joins with the Chippewa River; their confluence is in Dunn County, in west-central Wisconsin. 

The Red Cedar Lakes have long been considered 

a highly desirable destination for residents and visitors; 

however, development within the watershed led to concerns 

over a possible decline in the water quality of the lakes. As 

a result of these concerns, the Red Cedar Lakes Association 

originally was formed around 1980, but disbanded shortly 

thereafter. The Association was brought back into action 

in 1990 when further development was proposed in the 

watershed (M. Hele, Red Cedar Lakes Association, written 

commun., 2002). The development eventually occurred, 

and state-of-the-art septic systems were installed to minimize the influence on lake-water quality. However, septic 

systems generally are not designed to remove nutrients. 

Once the soil-retention capacity of the soil is exceeded for 

phosphorus, septic systems can and often do discharge high 

concentrations of phosphorus to the ground water. By 1992, 

two issues were identified as primary to the mission of the 

Association: assessment of the water quality of the lakes 

and development of a management plan for the Red Cedar 

Lakes. These issues led to the first cooperative study with 

the U.S. Geological Survey (USGS) during which the three 

lakes were monitored in 1993–94. During this time, 

members of the Association also began collecting waterWater Quality and the Effects of Changes in 

Phosphorus Loading, Red Cedar Lakes, Barron 

and Washburn Counties, Wisconsin

By Dale M. Robertson, William J. Rose, and Herbert S. Garn
2 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

EXPLANATION

Land use/land cover

Stream

Basin boundary

Road

County boundary

Urban

Agriculture

Forest

Grassland/shrubland

Wetland

Water

Base map digitized from U.S. Geological Survey 

7.5-minute quadrangle maps (1:24,000)

0 2 4 MILES

0 2 4 KILOMETERS

WASHBURN COUNTY

BARRON COUNTY

SAWYER COUNTY

RUSK COUNTY

Birchwood

Mikana

48

R e d C e d a r L a k e

Hemlock Lake

Hemlock Creek

Bolger Flowage

Hemlock Creek

Pigeon Creek

Sucker Creek

Pigeon Creek

Sucker Creek

Balsam Lake

Birch Lake

L a k e

C h e t a c

Bolger Flowage

Red Cedar River

Stout Island

D

F

F

WISCONSIN

Location of

study area

91 ̊33'

45 ̊43'30"

45 ̊39'

45 ̊34'30"

91 ̊27'

Figure 1. Drainage basin of the Red Cedar Lakes, Wisconsin. Land use/land cover from WISCLAND (Lillesand and others, 1998).
quality data as part of the Wisconsin Department of Natural Resource’s (WDNR’s) Self-Help Citizen Lake Monitoring Program (Wisconsin Department of Natural Resources, 

2003). After the study with the USGS was completed, it 

was suggested that 2 additional years of water-quality data 

would be beneficial to developing a management plan. 

This suggestion led to the second cooperative study with 

the USGS, during which the three lakes were monitored 

in 1996–97. 

During this same period, concern over the water quality in lakes throughout the area led to the WDNR’s Red 

Cedar River Basin pilot project (Wisconsin Department 

of Natural Resources, 1997). The goal of this project was 

for the WDNR to work with stakeholders throughout the 

basin (down to the confluence with the Chippewa River) 

on an innovative approach to watershed management with 

an initial emphasis on phosphorus control. The first step in 

setting phosphorus goals for the entire basin was to focus 

on establishing lake-specific productivity goals that could 

be achieved by reducing phosphorus loading. It was recognized that management actions for specific mainstem lakes 

might not be sufficient to achieve their productivity goals 

unless other upstream lakes also implement management 

actions and achieve similar productivity goals. Therefore, 

the WDNR recommended that upstream mainstem lakes, 

like the Red Cedar Lakes, have initial priority in receiving 

funds to develop management plans. 

As a result of the continuing concern over water quality of the Red Cedar Lakes and lakes throughout the basin, 

the Association with support of the Red Cedar River Basin 

pilot project obtained a WDNR Lake Protection grant to 

fund a third cooperative project with the USGS, during 

which data were collected in 2000–01. The goals of this 

project were to evaluate the water quality of the lakes in 

terms of long-term trends (1980s to the present), to quantify the sources of phosphorus to the lakes, and to determine how the lakes would respond to changes in phosphorus loading. To determine the sensitivity of each of the 

Red Cedar Lakes to decreases and increases in phosphorus 

loading, the water-quality model BATHTUB (Walker, 

1996) and water-quality models within the Wisconsin Lake 

Modeling Suite (WiLMS; Panuska and Kreider, 2002) first 

were calibrated using lake and tributary data collected in 

2000–01. The models then were used to simulate water 

quality with various phosphorus-loading scenarios. This 

report presents the results of this effort. The ultimate use 

of the results of this investigation is for the development of 

comprehensive lake-management plans for the Red Cedar 

Lakes, which would benefit the water quality in lakes 

throughout the entire Red Cedar River Basin.

Balsam, Hemlock, and Red Cedar 

Lakes and Their Watershed

Balsam Lake has a surface area of 295 acres and a 

volume of 7,400 acre-ft (table 1). Its maximum depth is 

49 ft and average depth is 25.1 ft. Most of the water entering the lake is from the outflow from Birch Lake (fig. 1). 

Water leaves Birch Lake through a bottom withdrawal of 

a 28-ft high dam. Outflow from Balsam Lake is through a 

connecting channel to the North Basin of Red Cedar Lake.

Hemlock Lake has a surface area of 357 acres and a 

volume of 2,990 acre-ft (table 1). Its maximum depth is 

21 ft and average depth is 8.4 ft. Most of the water entering the lake is from Hemlock Creek after flowing through 

Bolger Flowage (fig. 1). Outflow from Hemlock Lake 

is through a connecting channel or narrows to the South 

Basin of Red Cedar Lake.

Red Cedar Lake has a total surface area of 1,840 acres 

and a volume of 47,300 acre-ft (table 1). Its maximum 

depth is 53 ft and average depth is 25.7 ft. For modelsimulation purposes, the lake was subdivided into two 

basins (North and South Basins) near Stout Island (fig. 1). 

The North Basin has a total surface area of 1,130 acres, 

Table 1. Morphometric characteristics of the Red Cedar Lakes, Wisconsin

Lake/Basin

Area

(acres)

Length

(miles)

Maximum 

depth

(feet)

Mean 

depth

(feet)

Volume

(acre-feet)

Drainage 

area

(acres)

Residence 

time

(days)

Phosphorus 

turnover ratio

(for annual 

loading)

Balsam Lake 295 1.99 49 25.1 7,400 146,500 46 13.0

Hemlock Lake 357 1.95 21 8.4 2,990 17,400 31 16.0

Red Cedar Lake 1,840 4.52 53 25.7 47,300 18,000 76 7.1

 Red Cedar - North Basin 1,130 2.34 53 27.6 31,300 15,200 126 4.7

 Red Cedar - South Basin 708 2.18 42 22.6 16,000 2,760 42 12.1

1Drainage area of Balsam Lake downstream of gage on County Trunk Highway D is 1,160 acres.

Introduction 3
4 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

a volume of 31,300 acre-ft, a maximum depth of 53 ft, 

and an average depth of 27.6 ft. In addition to flow from 

Balsam Lake, there are two main tributaries to the North 

Basin: Sucker Creek and Pigeon Creek. The South Basin 

has a total surface area of 708 acres, a volume of 16,000 

acre-ft, a maximum depth of about 42 ft, and average 

depth of 22.6 ft. The maximum depth connecting these 

two basins is about 30 ft. Outflow from Red Cedar Lake is 

through the dam at Mikana into the Red Cedar River from 

the South Basin of the lake.

The total drainage area or watershed of the Red Cedar 

Lakes is about 147 mi2 (fig. 1 and table 2). The area is 

predominantly forest and wetlands (78 percent). Lakes and 

streams represent 6.4 percent of the basin. Agriculture and 

grassland represent about 5.3 and 5.0 percent, respectively. 

Urban areas, mostly around Birch Lake (Birchwood) and 

near the outlet (Mikana), represent only about 0.1 percent 

of the basin. Much of the watershed drains into Lake Chetac 

and Birch Lake. The area draining directly into the Red 

Cedar Lakes (fig. 2 and table 2) is about 61 mi2

. The area is 

predominantly forest and wetlands (83 percent). Agriculture and grassland represent about 6.0 and 3.6 percent of 

the basin, respectively. Most of the agriculture in the basin 

drains into Sucker Creek. Urban areas, near the outlet 

(Mikana), represent less than 0.1 percent of the basin. A 

few parts of the basin, mostly north and southeast of Hemlock Lake, do not drain directly into the lakes, but drain 

internally to closed depressions. The soils in the area consist mainly of well-drained sandy deposits. Ground water 

in various locations in the Red Cedar River Basin has been 

found to have naturally high background concentrations 

of phosphorus (exceeding 200 μg/L); however, most of 

the highest concentrations were found south of the Red 

Cedar Lakes watershed (Wisconsin Department of Natural 

Resources, 1997). 

Sampling Methods

Lake Sampling

Water-quality sampling in each of the three Red 

Cedar Lakes (two locations in Red Cedar Lake: North and 

South Basin) was conducted by the USGS in 1993–94, 

1996–97, and 2001. During each year, the lakes/basins 

were sampled near their deepest location (fig. 2) in late 

winter, spring (April or May), June, July, and August. In 

addition, in 2001, the four sites were sampled in September. During each sampling, profiles of water temperature, 

dissolved oxygen, specific conductance, and pH were 

measured with a multiparameter meter, and Secchi depths 

were measured with a standard black-and-white Secchi 

disk during open-water periods. At all locations, water 

samples were collected near the surface (1.5 ft) and bottom of the water column (approximately 3 ft above the 

bottom); however, in 2001, no near-bottom samples were 

collected in the South Basin of Red Cedar Lake in March, 

May, June, and July. Water samples were collected at 

additional depths near the top and middle of the hypolimnion (the deep colder layer of water in the lake) at each 

site in August and September 2001 to better describe the 

distribution of phosphorus and enable phosphorus release 

from the sediments to be quantified. All water samples 

were collected with a Van Dorn sampler and analyzed for 

total phosphorus. During all open-water samplings, surface 

samples also were analyzed for chlorophyll a. During 

spring overturn, the surface and bottom water samples 

also were analyzed for color, turbidity, hardness, alkalinity, solids, calcium, chloride, iron, magnesium, manganese, potassium, silica, sodium, and sulfate; only surface 

samples were analyzed for these constituents in 2001. 

These data were published in annual USGS data reports 

Table 2. Land use/land cover in the Red Cedar Lake 

watershed, Wisconsin (total drainage area and direct 

drainage area) from WISCLAND

[Lillesand and others, 1998]

Land use Square miles Percent 

of total

Total drainage area

Urban 0.18 0.1

Agriculture 7.83 5.3

Forest and forested wetland 115.07 78.1

Shrubland 7.06 4.8

Grassland 7.41 5.0

Barren .46 .3

Water 9.37 6.4

Total 147.38 100.0

Direct drainage area

Urban 0.04 0.1

Agriculture 3.68 6.0

Forest and forested wetland 50.78 83.4

Shrubland 3.11 5.1

Grassland 2.17 3.6

Barren .18 .3

Water .95 1.6

Total 60.91 100.0
0 2 4 MILES

0 2 4 KILOMETERS

91 ̊33'

91 ̊ 27'

45 ̊34'30"

45 ̊37'30"

EXPLANATION

Land use/land cover

Stream

Basin boundary

Subbasin boundary

Road

Water level and outlet site

Inflow site

Lake site

Urban

Agriculture

Forest

Grassland/shrubland

Wetland

Water

Internal drainage

Sucker Creek Subbasin

Balsam Lake

Red Cedar Lake

Hemlock Lake Hemlock Creek

Sucker Creek

Pigeon Creek

North

Basin

South

Basin

Hemlock

Creek

Subbasin

To Birch Lake

Red Cedar River

Stout Island

Bolger

Flowage

Base map digitized from U.S. Geological Survey 

7.5-minute quadrangle maps (1:24,000)

Figure 2. Direct drainage basin of the Red Cedar Lakes, Wisconsin, with data-collection sites identified. Land use/land cover 

from WISCLAND (Lillesand and others, 1998).

Sampling Methods 5
6 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

(the most recent report being U.S. Geological Survey, 

Wisconsin District Lake-Studies Team, 2002). Additional 

water-quality data have been collected from each of the 

lakes as part of the Wisconsin Self-Help Citizen Lake 

Monitoring Program (Wisconsin Department of Natural 

Resources, 2003). Self-help data included Secchi depth, 

surface total phosphorus and chlorophyll a concentrations, 

and perceived water-quality conditions. These data are 

archived in the Wisconsin Self-Help database that is available on the World Wide Web (Wisconsin Department of 

Natural Resources, 2002). 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). 

One method of classifying the water quality of a lake 

is by computing water-quality or trophic state index (TSI) 

values as developed by Carlson (1977) and modified for 

Wisconsin lakes by Lillie and others (1993). The indices, 

based on near-surface concentrations of phosphorus and 

chlorophyll a, and on Secchi depths, were developed to 

place these three characteristics on similar scales to allow 

comparison of different lakes. TSI values based on phosphorus concentrations (TSIP

), chlorophyll a concentrations 

(TSIC), and Secchi depths (TSISD) were computed for each 

sampling with equations 1–3 and were used to compute 

summer (May through September) average TSI values:

 TSIP = 28.24 + 17.81 [log10P 

 (in micrograms per liter)] (1)

 TSIC = 34.82 + 17.41 [log 10 chlorophyll a

 (in micrograms per liter)] (2)

 TSISD = 77.13 – 33.2 [log 10 Secchi depth

 (in feet)] (3)

Oligotrophic lakes (TSI values less than 40) have a 

limited supply of nutrients, typically are clear with low 

algal populations and low phosphorus concentrations, and 

typically contain oxygen throughout the year in their deepest zones (Wisconsin Department of Natural Resources, 

1992). Mesotrophic lakes (TSI values between 40 and 50) 

have a moderate supply of nutrients, moderate clarity, and 

are prone to moderate algal blooms; occasional oxygen 

depletions in the deepest zones of the lake are possible. 

Eutrophic lakes (TSI values greater than 50) are nutrient 

rich with correspondingly severe water-quality problems, 

such as frequent seasonal algal blooms; oxygen depletion 

is common in the deeper zones of the lake, and clarity 

typically is poor.

Tributary Sampling

To describe the flow, water quality, and phosphorus 

and sediment loading from the watershed and upstream 

lakes, three inflow sites were sampled (fig. 2). A site 

downstream of Birch Lake (at County Trunk D) was 

used to monitor all of the inputs to the Red Cedar Lakes 

entering from the upstream lakes. Sites on Sucker Creek 

(at Loch Lomond Boulevard near Birchwood) and Hemlock Creek (at County Trunk Highway F) were chosen 

because they are major tributaries and were thought to 

best represent conditions in the watershed. Sucker Creek 

has a 7,630-acre subwatershed with large agricultural 

areas, and Hemlock Creek has a 13,100-acre subwatershed 

dominated by forest and wetlands (fig. 2). To describe the 

flow, water quality, and nutrients leaving the lakes, the Red 

Cedar River (lake outlet) was monitored just downstream 

of the dam. Water levels at these four sites were measured 

continuously (every 15 minutes) and used to determine 

flow by use of stage-discharge relations. These data then 

were used to compute daily average flows for each site for 

water year (WY) 2001 (October 1, 2000, to September 30, 

2001). The 15-minute data are stored in the National Water 

Information System database (U.S. Geological Survey, 

1998) and daily average data were published in the annual 

USGS data report (Waschbusch and others, 2002). These 

data were used to estimate the total flow to and from the 

lakes, described in the section on “Sources of Water and 

Phosphorus.”

Water samples were collected from each of the 

three inflow sites and analyzed for total phosphorus and 

suspended sediment concentrations. Storm-runoff samples 

were collected at Sucker and Hemlock Creeks by means 

of stage-activated automatic samplers. Additional samples 

were collected manually, by use of the equal-width-increment (EWI) method described by Edwards and Glysson 

(1999), at times when automated samples also were collected, to evaluate the representativeness of the automated 

samples. EWI samples also were collected at low flow at 

these sites to estimate loading during these conditions. A 

total of 98 and 74 samples were collected and analyzed in 

Sucker and Hemlock Creeks, respectively. At the outlet 

from Birch Lake, samples were collected manually at 

monthly intervals because only gradual changes in water 

quality were expected. About 20 percent of the inflow 

samples were analyzed for dissolved ortho-phosphorus. 

At the outlet from Red Cedar Lake, samples also were 
collected monthly, but only analyzed for total phosphorus. 

All total and dissolved ortho-phosphorus analyses were 

performed by the Wisconsin State Laboratory of Hygiene 

(Wisconsin State Laboratory of Hygiene, 1993) and suspended sediment analyses were performed by the USGS 

Iowa Sediment Laboratory in accordance with standard 

analytical procedures described by Fishman, and others 

(1994). Daily phosphorus and suspended sediment loads 

then were computed for each site, except the Red Cedar 

Outlet where only phosphorus loads were computed, for 

WY2001 by use of the integration method described by 

Porterfield (1972). All water-quality data and computed 

loads were published in annual USGS data reports 

(Waschbusch and others, 2002) and stored in the National 

Water Information System database (U.S. Geological 

Survey, 1998).

Lake Stage and Precipitation Sampling

Daily lake stages were estimated from continuous 

(every 15 minutes) water-elevation measurements recorded 

at the outlet of Red Cedar Lake (fig. 2). During the snowfree period, precipitation was measured continuously at the 

stream gages near the Birch Lake outlet, Hemlock Creek, 

and near the Red Cedar Lake outlet. Precipitation on the 

lakes was assumed to be the average of that measured at 

these three gages. During the colder periods, daily precipitation was assumed to be the average of that measured by 

weather observers in Couderay and Rice Lake, Wisconsin 

(National Oceanographic and Atmospheric Administration 

Stations 1847 and 7132, respectively). 

Lake-Water Quality

Water quality varies among the Red Cedar Lakes; 

therefore, the water quality of each lake is described 

separately and then compared. The most extensive year of 

water-quality data collection was in 2001; therefore, these 

data primarily were used to describe seasonal changes in 

the water quality of the lakes.

Balsam Lake 

Balsam Lake is a dimictic lake, meaning that the lake 

thermally stratifies throughout summer and under the ice 

during winter. During summer, the thermocline (depth 

range where there is a rapid temperature change) developed in early June and stayed well established through 

September. The thermocline usually developed from 

around 13 to 16 ft from the surface. Bottom temperatures 

usually were from around 7 to 9°C, and changed little 

throughout summer. Dissolved oxygen concentrations 

were near saturation throughout the lake just after the 

ice melted, but oxygen quickly was depleted below the 

thermocline by mid June. Throughout most of summer, 

dissolved oxygen concentrations were near zero below the 

thermocline. During late fall, the lake completely mixed 

and dissolved oxygen concentrations at all depths were 

near saturation. Reverse thermal stratification developed 

under the ice, with bottom temperatures warming to near 

5°C by late winter while temperatures just beneath the ice 

remained near 0°C. Dissolved oxygen concentrations usually decreased as winter progressed but usually remained 

above 5 mg/L in the upper 10 ft of the lake; however, 

below that depth, concentrations decreased to near 0 mg/L 

by late winter. The specific conductance throughout the 

lake at spring and fall overturn and near the surface during 

other periods was about 150 μS cm-1, but increased to over 

200 μS cm-1 near the bottom of the lake during late summer and late winter. The pH throughout the lake was about 

7 (all pH values are in standard units) during spring and 

winter, but photosynthesis by aquatic plants and algae 

during summer caused concentrations in the epilimnion 

(the shallower, warmer layer of water in the lake) to 

occasionally increase to just less than 9 (the State pH 

water-quality standard) in July and August.

The summer average water clarity in Balsam Lake, as 

measured with a Secchi disk, was about 8.2 ft, but each year 

had at least one monthly measurement of about 11–13 ft 

(fig. 3a). Secchi depths less than 6 ft indicate eutrophic 

conditions. The average Secchi depth during May through 

September 2001 was 7.4 ft (table 3). Little change in Secchi depths were observed from 1993 to 2001 (fig. 3).

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 eutrophication (that is, accelerated aging and increased productivity) of lakes. Phosphorus concentrations greater than about 17–20 μg/L indicate 

eutrophic conditions. Near-surface total phosphorus 

concentrations ranged from 12 to 46 μg/L (fig. 3b). The 

average concentration in Balsam Lake during May through 

September 2001 was 24.8 μg/L (table 3). Phosphorus 

concentrations in the lake appear to have increased slightly 

since 1993. Near-bottom phosphorus concentrations 

increased dramatically after the onset of anoxic conditions, 

which indicates a release of phosphorus from the lake 

Sampling Methods 7
8 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

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Figure 3. Secchi depth (A), and near-surface concentrations of total phosphorus (B) and chlorophyll a (C) in Balsam Lake, 

Wisconsin, 1993–2001.
sediments. Near-bottom total phosphorus concentrations 

typically increased to over 500 μg/L in late summer and 

winter. A near-surface dissolved ortho-phosphorus concentration of 9 μg/L was measured during July 2001. 

Near-surface total nitrogen concentration (computed 

as the sum of Kjeldahl nitrogen and dissolved nitrite plus 

nitrate) was measured at spring overturn in May 2001 and 

dissolved nitrogen was measured during July 2001. During spring overturn, the total nitrogen concentration was 

787 μg/L and in July the near-surface dissolved nitrogen 

concentration was 220 μg/L. 

The ratio of the concentrations of total nitrogen to 

total phosphorus (N:P ratio) often is used to determine 

the potential limiting nutrient in a lake. The specific value 

of this ratio that determines which nutrient potentially is 

limiting, varies under different conditions such as water 

temperature, light intensity, and nutrient deficiencies (Correll, 1998); however, a ratio greater than from about 12:1 

to about 16:1, by weight, usually indicates that phosphorus 

should be the potentially limiting nutrient. The N:P ratio in 

Balsam Lake during spring was 21:1 and in July the nearsurface dissolved N:P ratio was 24:1. Therefore, phosphorus typically should be the nutrient limiting algal growth in 

Balsam Lake and is the nutrient to focus on when considering management efforts to improve water quality.

Chlorophyll a is a photosynthetic pigment found in 

algae and other green plants. Its concentration, therefore, 

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

population of a lake. Concentrations greater than about 

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

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

algal blooms. During the study period, near-surface 

chlorophyll a concentrations ranged from 3 to 34 μg/L 

(fig. 3). The highest concentrations were measured from 

1995 through 1997. The average concentration in Balsam 

Lake during May through September 2001 was 9.8 μg/L 

(table 3). From 1993 to 2001, there was no apparent 

long-term change in chlorophyll a concentrations (fig. 3).

Hemlock Lake 

Hemlock Lake, the shallowest of the three lakes, 

also is a dimictic lake. During summer, the thermocline 

developed in early June and was present at about 13 ft in 

July and August. The temperature gradient in the thermocline was weaker in Hemlock Lake than in Balsam Lake. 

Bottom temperatures usually were from around 14 to 15°C 

and changed little throughout summer. Dissolved oxygen 

was quickly depleted below the thermocline (13 ft) by 

mid June. Thermal stratification developed under the ice, 

with bottom temperatures warming to near 4°C by late 

winter, whereas temperatures just beneath the ice remained 

near 0°C. Dissolved oxygen concentrations decreased as 

winter progressed but usually remained above 5 mg/L 

in the upper 5 ft of the lake; however, below that depth 

concentrations decreased to near 0 mg/L by late winter. 

The specific conductance in Hemlock Lake was the lowest 

of the three lakes. The specific conductance throughout 

the lake at spring and fall overturn and near the surface 

during other periods was near 100 μS cm-1, but increased 

to over 150 μS cm-1 near the bottom during late summer. 

The pH throughout the lake typically was about 7 to 8 during spring and winter, but occasionally increased to near 

9 during July and August in the epilimnion. Only on one 

occasion (July 2001) did pH values in the epilimnion reach 

the State pH water-quality standard of 9.

Secchi depths in Hemlock Lake ranged from 2.5 to 

11 ft (fig. 4a). The average Secchi depth during May 

through September 2001 was 5.2 ft (table 3). No long-term 

changes were found in Secchi depths from 1993 to 2001 

(fig. 4a).

Table 3. Seasonal average (May through September 2001) water quality in the Red Cedar Lakes, Wisconsin

[μg/L, micrograms per liter, c.v., coefficient of variation]

Lake/Basin Total phosphorus Secchi depth Chlorophyll a

μg/L c.v. feet c.v. μg/L c.v.

Balsam Lake 24.8 0.33 7.4 0.29 9.8 0.45

Hemlock Lake 35.8 .24 5.2 .16 13.7 .73

Red Cedar Lake1 26.2 .20 7.6 .26 9.1 .52

 Red Cedar - North Basin 26.2 .20 7.7 .24 10.5 .51

 Red Cedar - South Basin 26.2 .19 7.4 .28 7.7 .53

1

Values based on averaging water-quality data from the North and South Basins of Red Cedar Lake.

Lake-Water Quality 9
10 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

Hemlock Lake had the highest near-surface total 

phosphorus concentrations of the three lakes. Near-surface 

total phosphorus concentrations ranged from 17 to 60 μg/L 

(fig. 4b). The average concentration in Hemlock Lake during May through September 2001 was 35.8 μg/L (table 3). 

No long-term changes in phosphorus concentrations have 

occurred since 1993. A near-surface dissolved ortho-phosphorus concentration of 11 μg/L was measured during July 

2001. Near-bottom phosphorus concentrations increased 

dramatically when anoxic conditions were present. Nearbottom total phosphorus concentrations were measured as 

high as about 700 μg/L in August, but were variable. Nearbottom total phosphorus concentrations increased little 

during winter. 

In 2001, during spring overturn, the total nitrogen 

concentration was 595 μg/L and in July the near-surface 

dissolved nitrogen concentration was 280 μg/L. The N:P 

ratio in Hemlock Lake during spring was 15:1 and in July 

the near-surface N:P ratio was 25:1. Therefore, phosphorus 

typically should be the nutrient limiting algal growth.

Hemlock Lake had the highest chlorophyll a concentrations of the three lakes. Near-surface chlorophyll 

a concentrations ranged from 4 to 61 μg/L (fig. 4c), with 

an average concentration during May through September 

2001 of 13.7 μg/L (table 3). Chlorophyll a concentrations 

may have decreased slightly from 1993 to 2001.

Red Cedar Lake 

The extent of vertical mixing in the two basins in 

Red Cedar Lake is quite different. The North Basin, being 

deeper, is a dimictic lake, had strong thermal stratification 

set up in early June, and it stayed stratified through September. The South Basin, being shallower than the North 

Basin, had weak stratification set up and break down various times during summer. The thermocline in the North 

Basin developed in early June and was present at about 20 

to 26 ft in July through September. Bottom temperatures 

in the North Basin were usually from around 10 to 11°C 

and changed little throughout summer; however, those in 

the South Basin gradually increased throughout summer. 

In the North Basin, dissolved oxygen was depleted quickly 

below the thermocline by mid July; however, in the South 

Basin low dissolved oxygen concentrations only were 

occasionally found just above the bottom. 

Throughout Red Cedar Lake, thermal stratification 

developed under the ice, with bottom temperatures warming to near 4°C by late winter, whereas temperatures just 

beneath the ice remained near 0°C. Dissolved oxygen 

concentrations usually decreased as winter progressed 

but usually remained above 10 mg/L down to about 20 ft 

throughout the lake; however, below that depth concentrations quickly dropped to near 0 mg/L. Because most of the 

area near the center of the South Basin is only about 

25 ft deep, dissolved oxygen concentrations throughout the 

basin usually were about 8 mg/L even in late winter. The 

specific conductance throughout the lake during spring and 

fall overturn and near the surface during other periods was 

near 125 μS cm-1, but in the North Basin increased to about 

200 μS cm-1 near the bottom during late summer. The pH 

throughout the lake typically was about from 7 to 8 during 

spring and winter, but occasionally increased to just less 

than 9 in July and August.

Although differences are present in the near-surface 

water quality between the North and South Basins on individual dates, the summer average values for Secchi depth, 

total phosphorus, and chlorophyll a were similar when 

data only collected on the same dates were examined. 

Therefore, because the North Basin has more extensive 

long-term records because of the WDNR Citizen Lake 

Monitoring Program, only the data for the North Basin are 

presented in figure 5. 

Secchi depths in Red Cedar Lake usually ranged from 

5 to 17 ft (fig. 5a); however, one Secchi depth in the South 

Basin was as shallow as 4 ft. The average Secchi depth 

during May through September 2001 was 7.7 ft and 7.4 ft, 

in the North and South Basins, respectively (table 3). 

Although there is considerable interannual variability, no 

long-term changes were found in Secchi depths from 1987 

to 2001 (fig. 5a). The longer period of record (starting in 

1987) enables the data collected since 1993 to be placed in 

a better long-term perspective. Data from 1993 to 2001 

would suggest that there was a long-term increase in 

Secchi depths; however, the earlier data indicate that the 

mid 1990s actually were from 4 to 5 years of poor clarity 

and no long-term trend in clarity is present. 

Near-surface total phosphorus concentrations ranged 

from 11 to 60 μg/L (fig. 5b). The average concentration 

throughout Red Cedar Lake during May through September 2001 was 26.2 μg/L (table 3). A small increase in 

phosphorus concentrations may have occurred since 1993; 

however, most of the variance in phosphorus concentrations was associated with high concentrations in 1997. 

A near-surface dissolved ortho-phosphorus concentration 

of 11 μg/L was measured in the North Basin during July 

2001. Near-bottom phosphorus concentrations dramatically increased with the onset of anoxic conditions in the 

North Basin. Near-bottom total phosphorus concentrations 

in the North Basin were measured as high as 885 μg/L 

in late summer. Anoxic conditions were not found in the 
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1991 1994 1995 1996 1997 1998 1999 2000 2001 2002 1992 1993

Figure 4. Secchi depth (A), and near-surface concentrations of total phosphorus (B) and chlorophyll a (C) in Hemlock Lake, 

Wisconsin, 1991–2002.

Lake-Water Quality 11
12 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin SECCHI DEPTH, IN FEET TOTAL PHOSPHORUS CONCENTRATION, IN MICROGRAMS PER LITER CHLOROPHYLL A CONCENTRATION, IN MICROGRAMS PER LITER

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1987 1992 1994 1995 1996 1997 1998 2000 2002 1988 1989 1990 1991 1993 1999 2001

Figure 5. Secchi depth (A), and near-surface concentrations of total phosphorus (B) and chlorophyll a (C) in Red Cedar Lake 

(North Basin), Wisconsin, 1987–2002.
South Basin and, therefore, concentrations did not increase 

during summer. In late winter, phosphorus concentrations 

were measured as high as 1,500 μg/L (in 1997) in the 

North Basin, although, other near-bottom measurements 

during winter in other years were much lower than this 

concentration. 

In 2001, during spring overturn, the total nitrogen 

concentration in the North Basin was 900 μg/L and in 

July the near-surface dissolved nitrogen concentration was 

281 μg/L. The N:P ratio in Red Cedar Lake during spring 

was 24:1 and in July the near-surface N:P ratio was 26:1. 

Therefore, phosphorus typically should be the nutrient 

limiting algal growth. Nitrogen concentrations were not 

measured in the South Basin.

Near-surface chlorophyll a concentrations ranged 

from 1.6 to 43 μg/L (fig. 5c), with an average concentration during May through September 2001 of 10.5 μg/L 

and 7.7 μg/L in the North and South Basins, respectively (table 3). Chlorophyll a concentrations may have 

decreased slightly from 1993 to 2001, which would agree 

with the changes observed in Secchi depths during this 

period. However, the longer Secchi depth record would 

suggest that there probably was no long-term change in 

chlorophyll a concentrations.

Lake Comparison with Trophic State Indices

Average summer Trophic State Index values (TSIs), 

based on total phosphorus and chlorophyll a concentrations from May through September, indicate that Balsam, 

Red Cedar, and Hemlock Lakes are eutrophic (fig. 6). 

However, water clarity in all of the lakes was slightly 

better than would be expected based on total phosphorus 

and chlorophyll a concentrations. TSIs based on Secchi 

depth indicate that Balsam and Red Cedar Lakes should 

be classified as mesotrophic. When all three TSIs are 

considered, the lakes would be classified as mesotrophic 

to eutrophic. All three TSIs indicate that Hemlock Lake is 

more productive than the other two lakes. There were no 

consistent long-term trends in the TSIs for any of the lakes, 

except for the TSIs based on chlorophyll a concentrations 

in Hemlock Lake that may have improved (decreased) 

slightly. 

Sources of Water and Phosphorus

Because the productivity in each of the lakes usually 

should be limited by the input of phosphorus, reduction 

in the phosphorus input to the lake would be a logical 

management goal. Most of the phosphorus enters the lake 

associated with the input of water. Therefore, to determine where the water and phosphorus originates and how 

changes in phosphorus loading would affect the trophic 

status of the lakes, water and phosphorus budgets were 

determined for each lake.

Water Budget

The hydrology of each of the lakes can be described 

in terms of components of their water budgets (fig. 7). 

The water budget for each lake may be represented by the 

equation

 ∆S = (PPT + SWIn + GWIn) – (Evap + SWOut + GWOut), (4) 

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, 

including inflow from a connected upstream lake), and 

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

evaporation (Evap), surface-water outflow (SWOut, including that to a connected downstream lake), and groundwater outflow (GWOut).

The water budget for all three lakes treated as single 

body of water, or the collective water budget, was estimated first, and then each individual lake’s water budget 

was estimated. Each term in the collective water budget 

was computed on a daily basis from October 1, 2000, to 

September 30, 2001. Changes in the volume of the lakes 

were determined from lake stages monitored near the 

outlet dam, assuming a constant surface area. Lake stage 

ranged from a minimum of 10.14 ft (relative to an arbitrary 

datum) in late November to a maximum of 11.87 ft in mid 

April (fig. 8). The lake stage fluctuated by about ± 0.5 ft 

from October through late November, was relatively constant throughout winter, rose rapidly by about 1.5 ft during 

snow melt and rain in early April, decreased quickly 

through April, and then fluctuated only slightly until 

September. The lake stage at the end of September 2001 

was the same as at the beginning of the 1-year period; 

therefore, ∆S = 0.0 for that year.

Lake-Water Quality 13
14 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

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70

Total phosphorus

Chlorophyll a

Secchi depth

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Balsam Lake

Red Cedar Lake - North Basin

Hemlock Lake

TSI VALUES TSI VALUES TSI VALUES

Eutrophic

Mesotrophic

Oligotrophic

Eutrophic

Mesotrophic

Oligotrophic

Eutrophic

Mesotrophic

Oligotrophic

1987 1988 1989 1990 1991 1992 1993 1995 1997 1999 2001 1994 1996 1998 2000

Figure 6. Average summer Trophic State Index (TSI) values based on water-quality data collected from May through September 

in Balsam, Hemlock, and Red Cedar (North Basin) Lakes, Wisconsin, 1987–2001.
During the non-freezing period, precipitation (PPT) 

was estimated to be the average of the precipitation measured at the USGS gages at Birch Lake outlet, Hemlock 

Creek, and at the Red Cedar Lake outlet. For the freezing 

period, daily precipitation was estimated to be the average 

of precipitation measured by National Weather Service 

observers in Couderay and Rice Lake, Wisconsin. The 

total annual precipitation was 40.36 in. or 8,390 acre-ft on 

the surface of the lake (table 4), which is about 8 in. more 

than the long-term average for the area. The most unusual 

precipitation fell in April when there was about 7.8 in. of 

rain, which is over 5 in. more than normal.

Daily surface-water inflow, SWIn, was divided into 

that measured from Birch Lake, that measured at Sucker 

Creek (drainage area of 7,630 acres), that measured at 

Hemlock Creek (drainage area of 13,100 acres), and that 

estimated from the remaining ungaged areas (15,900 

acres). The total annual surface-water input to all Red 

Cedar Lakes from gaged areas was 79,300 acre-ft (table 

4), which equates to about 32 ft on the surface of the lakes. 

The total annual surface-water input from gaged areas 

into Balsam Lake (from Birch Lake) was 50,000 acre-ft, 

into Hemlock Lake (from Hemlock Creek) was 20,400 

acre-ft, and North Basin of Red Cedar Lake (from Sucker 

Creek) was 8,860 acre-ft. Daily surface-water inflows from 

all the remaining ungaged areas were estimated from the 

unit-area water yields determined for the Sucker Creek 

watershed. Flows at Sucker Creek were used to estimate 

flow from all of the remaining ungaged areas because the 

drainage area upstream of the gage on Hemlock Creek 

GWIn GWOut

PPT

SWOut

Ground water

EXPLANATION

Evap Evaporation

GWIn Ground-water inflow

GWOut Ground-water outflow

PPT Precipitation

SWIn Surface-water inflow

SWOut Surface-water outflow

NOT TO SCALE

Evap

SWIn SWOut

SWIn

Figure 7. Schematic of the hydrology of the Red Cedar Lakes, Wisconsin.

Source of Water and Phosphorus 15
16 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

9.5

10.0

10.5

11.0

11.5

12.0

12.5

13.0

Calculated stage

Measured stage

 LAKE STAGE, IN FEET ABOVE AN ARBITRARY DATUM

OCT. NOV. DEC. JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT.

2000 2001

Figure 8. Daily average lake stages (above an arbitrary datum) for the Red Cedar Lakes, Wisconsin, October 1, 2000, through 

September 30, 2001. Data collected in Red Cedar Lake. Calculated daily lake stages are based on the daily water budget constructed for the Red Cedar Lakes.

Table 4. Total annual hydrological inputs into the Red Cedar Lakes, Wisconsin, October 1, 2000, to September 30, 2001

[NA, not applicable]

Hydrologic component Balsam Lake Hemlock Lake

Red Cedar 

North Basin

Red Cedar

South Basin Red Cedar Lakes

Inputs (acre-feet)

Precipitation 993 1,200 3,810 2,380 8,390

Inflow (gaged) 50,000 20,400 8,860 NA 79,300

Ungaged inflow 1,340 5,040 8,810 3,200 18,400

Upstream lake NA NA 58,700 121,000 NA

Ground water 7,060 7,840 10,200 11,000 36,100

Total inputs 59,400 34,500 90,400 138,000 142,000

Outputs (acre-feet)

Evaporation 738 894 2,830 1,770 6,230

Outflow (or transport to 

adjacent lake/basin)

 58,700 33,600 87,500 136,000 136,000

Total outputs 59,400 34,500 90,400 138,000 142,000
had more wetlands and impounded flow than the other 

ungaged areas in the watershed. The total annual surfacewater inflow from ungaged areas to all of the Red Cedar 

Lakes was estimated to be 18,400 acre-ft (table 4), which 

equates to about 7.4 ft on the surface of the lakes. Ungaged 

inflows to the individual lakes/basins are given in table 4. 

Runoff during this period was higher than normal 

due to the higher than normal precipitation, especially in 

April. Many streams in northwestern Wisconsin had runoff 

approximately 130 to 140 percent above normal during the 

1-year study period (Waschbusch, and others, 2002). 

Total annual evaporation from the lake was estimated 

from the Evaporation Atlas for the United States (Farnsworth and others, 1982) to be 30 in/yr (6,230 acre-ft). The 

total evaporation was then partitioned to daily rates based 

on estimated monthly pan evaporation rates for LaCrosse, 

Wis. (Farnsworth and Thompson, 1982). 

On the basis of the elevation of the surface of the Red 

Cedar Lakes relative to the elevation of water in nearby 

lakes and streams, it was assumed that ground water only 

moved from the aquifer to the lakes and not from the lakes 

to the aquifer. A total ground-water input to the lakes of 

36,100 acre-ft (table 4), which equates to about a change 

in water level of about 14.5 ft, was then computed as the 

residual in the water-budget equation (Eq. 4). To determine 

the daily input of ground water, the temporal distribution 

of ground water flowing into the lakes was assumed to be 

similar to the temporal distribution of base flow in Sucker 

Creek. The total ground-water input was then partitioned 

to the individual lakes based on the relative proportion of 

shoreline. The shoreline length of Balsam Lake was about 

20 percent of the entire shoreline of the Red Cedar Lakes 

and, therefore, received 20 percent of the daily groundwater input; the North Basin of Red Cedar Lake received 

28 percent of the input; the South Basin received 30 percent of the input; and Hemlock Lake received 22 percent 

of the ground-water input.

The complete daily water budget fairly accurately 

simulated the changes in the water level of the lakes (fig. 

8). The largest error in the hydrologic balance of the lakes 

was about 0.75 ft in early April during spring runoff. This 

difference probably was caused by overestimating the rate 

of input from ungaged areas. However, by the middle of 

May, the estimated and measured water levels were similar, which indicates the overall water budget was accurate. 

A large component of the water budget of the individual lakes/basins is flow from one lake/basin to another. 

This flow was computed using an adaptation to equation 4. 

Because ∆S for the year was 0.0 and GWOut was assumed 

to be 0.0, outflow from a particular lake/basin was determined by the equation

 SWOut = (PPT + SWIn + GWIn) – Evap, (5)

or by subtracting evaporation from the total annual inputs 

of water (table 4). Flow from Balsam Lake to the North 

Basin of Red Cedar Lake was estimated to be 58,700 acreft; flow from the North Basin to the South Basin was estimated to be 87,500 acre-ft, and flow from Hemlock Lake 

to the South Basin was estimated to be 33,600 acre-ft.

In summary, the major sources of water to the Red 

Cedar Lakes are from Birch Lake (36 percent), gaged and 

ungaged surface-water inflow (33 percent), and ground 

water (25 percent) (table 4 and fig. 9). Direct precipitation 

contributed 6 percent of the total water input to the lake. 

The major output of water from the lake was through its 

outlet (96 percent) and secondarily through evaporation 

(4 percent). The major output of water from Balsam Lake, 

Hemlock Lake, and the North Basin of Red Cedar Lake is 

to the next downstream lake/basin. These outputs are the 

major sources of water to the North and South Basins of 

Red Cedar Lake.

Phosphorus Budget 

To help define where the phosphorus in the Red 

Cedar Lakes originates and how much leaves the lakes, 

a detailed phosphorus budget was computed for this 

1-year study period. Sources of phosphorus to the lakes 

include precipitation, surface-water inflow (from gaged 

and ungaged flow including that transported from adjacent lakes/basins), ground water (including seepage out of 

the Birchwood Sewage-Disposal Pond that is considered 

a point source), and contributions from septic systems. 

Because some of the phosphorus transported in the streams 

is attached to suspended sediment particles, suspended 

sediment concentrations and loads in inflowing streams 

also were examined. Almost all of the phosphorus that 

enters the lake is either exported out the outlet of the lake 

or is deposited in the lake sediments.

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 about 160 lb of phosphorus to the 

lakes during the study year (table 5). Monthly loadings to 

individual lakes are given in table 5. 

Source of Water and Phosphorus 17
18 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

Gaged Surface-water Inflow

Phosphorus concentrations were determined approximately monthly at the outlet of Birch Lake, and approximately monthly and more frequently during flow events 

in Sucker and Hemlock Creeks (fig. 10). There were no 

strong seasonal variations in phosphorus concentrations 

at any of the sites, although concentrations may have 

increased slightly from May through August at the outlet 

of Birch Lake. The average total phosphorus concentration 

leaving Birch Lake was 42 μg/L (median concentration 

of 39 μg/L), and about 55 percent of the phosphorus was 

in the dissolved form. The high percentage of phosphorus 

in the dissolved form may have resulted from water being 

released well below the surface of the lake because of the 

28-ft high dam. Suspended sediment concentrations generally were low, as was expected immediately downstream 

of a lake; however, concentrations of 7 and 16 mg/L were 

measured in June and July, respectively. The higher concentrations in June and July may have been associated with 

algal production in Birch and Chetac Lakes.

The average total phosphorus concentration in 

Sucker Creek was 56 μg/L (median concentration of 51 

μg/L). Concentrations increased during high-flow events; 

concentrations over 100 μg/L were measured in April, July 

and August (fig. 10). About 60 percent of the phosphorus 

was in the dissolved form, with dissolved phosphorus 

concentrations over 60 μg/L measured in July and August. 

Suspended sediment concentrations generally were low 

(less than 10 mg/L); concentrations exceeded 20 μg/L only 

during high-flow events. The average particulate phosphorus content (particulate phosphorus concentration divided 

by the suspended sediment concentration) was about 3,000 

micrograms per gram (μg/g). This value is high compared 

to other streams in the state (LaLiberte and Kampa, 1999). 

The average total phosphorus concentration in 

Hemlock Creek was 56 μg/L (median concentration of 

52 μg/L). Concentrations increased little during high-flow 

events, except during the early part of some events (fig. 

10). During the first part of the events in late May and 

July, concentrations were over 100 μg/L. About 45 percent 

of the phosphorus was in the dissolved form. Suspended 

sediment concentrations generally were low (less than 

10 mg/L); concentrations exceeded 20 μg/L only during a high-flow event in August. The average particulate 

phosphorus content was about 9,000 μg/g. This extremely 

high particulate phosphorus content compared to other 

streams in the state may be the result of the impounded 

water upstream of the gage trapping most of the mineralized sediment entering the stream. 

The water-quality data and 15-minute-interval flow 

values were used to compute the total loadings of phosphorus and suspended sediment from these gaged tributaries 

and estimate the total monthly surface-water phosphorus 

loading to each lake during the 1-year study period (table 

5). Approximately 5,390 lb of phosphorus were exported 

Birch Lake

36 percent

Red Cedar

River

96 percent

Evaporation

4 percent

Precipitation

6 percent

Ground water

25 percent

Ungaged

inflow

13 percent

Hemlock

Creek

14 percent

Sucker

Creek

6 percent

Water inputs Water outputs

Figure 9. Water budget for the Red Cedar Lakes, Wisconsin, for October 1, 2000 through September 30, 2001.
Balsam Lake

October 0.4 221 1.5 1

NC 29.1 17.3 2.4 272

November 2.8 721 8.7 1

NC 40.7 17.3 2.4 793

December .4 360 2.4 1

NC 44.5 17.3 2.4 427

January .6 229 2.4 1

NC 41.4 17.3 2.4 293

February .7 219 2.6 1

NC 34.6 17.3 2.4 276

March .3 203 4.0 1

NC 35.4 17.3 2.4 263

April 3.1 1,630 80.9 1

NC 39.1 17.3 2.4 1,780

May 2.0 585 17.8 1

NC 48.6 17.3 2.4 673

June 2.5 531 23.4 1

NC 42.2 17.3 2.4 618

July 2.0 148 6.3 1

NC 39.4 17.3 2.4 216

August 3.0 312 9.9 1

NC 41.4 17.3 2.4 386

September 1.0 229 2.9 1

NC 43.1 17.3 2.4 296

Total annual 18.9 5,390 163 1

NC 480 207 28.7 6,290

Hemlock Lake

October .5 27 5.8 0 32.3 0.0 1.9 67

November 3.4 76 32.5 0 45.2 .0 1.9 159

December .5 61 9.1 0 49.4 .0 1.9 122

January .8 102 9.0 0 46.0 .0 1.9 159

February .9 121 9.8 0 38.5 .0 1.9 172

March .4 156 14.9 0 39.3 .0 1.9 212

April 3.8 716 304 0 43.5 .0 1.9 1,070

May 2.4 243 67.0 0 54.1 .0 1.9 369

June 3.0 275 87.9 0 46.9 .0 1.9 414

July 2.4 170 23.8 0 43.8 .0 1.9 242

August 3.6 284 37.2 0 46.0 .0 1.9 373

September 1.2 74 10.8 0 47.9 .0 1.9 136

Total annual 22.9 2,300 612 0 533 0.0 23.1 3,490

Table 5. Phosphorus inputs, in pounds (lb), to the Red Cedar Lakes, Wisconsin, October 1, 2000 to September 30, 2001

[NC, not computed; NCM, not computed on a monthly basis]

Month Precipitation Gaged Ungaged Upstream 

lake

Ground 

water

Birchwood 

Disposal 

Pond

Septic 

systems Total

Source of Water and Phosphorus 19
20 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

Table 5. Phosphorus inputs, in pounds (lb), to the Red Cedar Lakes, Wisconsin, October 1, 2000 to September 30, 2001—Continued

[NC, not computed; NCM, not computed on a monthly basis]

Month Precipitation Gaged Ungaged Upstream 

lake

Ground 

water

Birchwood 

Disposal 

Pond

Septic 

systems Total

Red Cedar - North Basin

October 1.5 10 10.1 NCM 42.1 0.0 8.1 72

November 10.7 57 56.8 NCM 58.8 .0 8.1 191

December 1.6 16 15.9 NCM 64.3 .0 8.1 106

January 2.4 16 15.7 NCM 59.7 .0 8.1 102

February 2.8 17 17.1 NCM 50.0 .0 8.1 95

March 1.2 26 26.1 NCM 51.1 .0 8.1 113

April 12.1 534 531 NCM 56.5 .0 8.1 1,140

May 7.7 118 117 NCM 70.3 .0 8.1 321

June 9.4 154 154 NCM 60.9 .0 8.1 386

July 7.7 42 41.6 NCM 56.9 .0 8.1 156

August 11.4 65 65.1 NCM 59.8 .0 8.1 210

September 3.9 19 19.0 NCM 62.3 .0 8.1 112

Total annual 72.5 1,070 1,070 24,850 693 0.0 96.8 7,860

Red Cedar - South Basin

October 1.0 0 3.7 NCM 45.3 0.0 8.1 58

November 6.7 0 20.6 NCM 63.3 .0 8.1 99

December 1.0 0 5.8 NCM 69.2 .0 8.1 84

January 1.5 0 5.7 NCM 64.3 .0 8.1 80

February 1.8 0 6.2 NCM 53.9 .0 8.1 70

March .7 0 9.5 NCM 55.0 .0 8.1 73

April 7.5 0 193 NCM 60.9 .0 8.1 269

May 4.8 0 42.5 NCM 75.7 .0 8.1 131

June 5.9 0 55.8 NCM 65.6 .0 8.1 135

July 4.8 0 15.1 NCM 61.3 .0 8.1 89

August 7.1 0 23.6 NCM 64.4 .0 8.1 103

September 2.5 0 6.9 NCM 67.0 .0 8.1 85

Total annual 45.3 0 388 3

11,100 746 0.0 96.8 12,400
Table 5. Phosphorus inputs, in pounds (lb), to the Red Cedar Lakes, Wisconsin, October 1, 2000 to September 30, 2001—Continued

[NC, not computed; NCM, not computed on a monthly basis]

Month Precipitation Gaged Ungaged Upstream 

lake

Ground 

water

Birchwood 

Disposal 

Pond

Septic 

systems Total

All Red Cedar Lakes (total)

October 3.4 258 21.1 1

NC 149 17.3 20.4 469

November 23.5 854 119 1

NC 208 17.3 20.4 1,240

December 3.5 436 33.2 1

NC 227 17.3 20.4 738

January 5.3 346 32.7 1

NC 211 17.3 20.4 633

February 6.2 357 35.7 1

NC 177 17.3 20.4 614

March 2.6 385 54.5 1

NC 181 17.3 20.4 661

April 26.5 2,880 1,110 1

NC 200 17.3 20.4 4,250

May 17.1 946 245 1

NC 249 17.3 20.4 1,490

June 20.8 960 321 1

NC 216 17.3 20.4 1,550

July 16.9 360 86.9 1

NC 202 17.3 20.4 703

August 25.1 661 136 1

NC 212 17.3 20.4 1,070

September 8.7 322 39.6 1

NC 220 17.3 20.4 628

Total annual 160.0 8,760 2,230 1

NC 2,450 207 245 14,100

1Phosphorus inputs from upstream lakes are included in gaged inputs.

2Phosphorus input from Balsam Lake.

3Phosphorus input from the North Basin of Red Cedar Lake and from Hemlock Lake.

Source of Water and Phosphorus 21
22 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

0

10

20

30

40

50

60

70

0

2

4

6

8

10

12

14

16

18

0

20

40

60

80

100

120

140

160

180

0

10

20

30

40

50

60

Total phosphorus

Dissolved ortho-phosphorus

Suspended sediment

0

50

100

150

200

250

300

0

5

10

15

20

25

30

35

Total phosphorus

Dissolved ortho-phosphorus

Suspended sediment

0

10

20

30

40

50

Total phosphorus

Total phosphorus

Dissolved ortho-phosphorus

Suspended sediment

PHOSPHORUS CONCENTRATION,

IN MICROGRAMS PER LITER

PHOSPHORUS CONCENTRATION,

IN MICROGRAMS PER LITER

PHOSPHORUS CONCENTRATION,

IN MICROGRAMS PER LITER

PHOSPHORUS CONCENTRATION,

IN MICROGRAMS PER LITER

SUSPENDED SEDIMENT CONCENTRATION,

IN MILLIGRAMS PER LITER

SUSPENDED SEDIMENT CONCENTRATION,

IN MILLIGRAMS PER LITER

SUSPENDED SEDIMENT CONCENTRATION,

IN MILLIGRAMS PER LITER

Red Cedar River downstream from Birch Lake

Sucker Creek

Hemlock Creek

Red Cedar River downstream from Red Cedar Lake

OCT. NOV. DEC. JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT.

2000 2001

Figure 10. Phosphorus and suspended sediment concentrations in the Red Cedar River downstream from Birch Lake, 

Sucker Creek, Hemlock Creek, and Red Cedar River downstream from Red Cedar Lake, Wisconsin from October, 2000 

through September, 2001.
from Birch Lake to Balsam Lake (gaged). The highest 

monthly export (over 1,630 lb) was estimated in April. 

Approximately 1,070 lb of phosphorus were delivered 

from Sucker Creek (gaged) to the North Basin of Red 

Cedar Lake (0.141 lb/acre). Over half of the phosphorus 

was exported from Sucker Creek in April and about 75 

percent was exported between April and June. Approximately 234,000 lb of suspended sediment were delivered 

from Sucker Creek (30.7 lb/acre). Approximately 2,300 

lb of phosphorus were delivered from Hemlock Creek 

(gaged) to Hemlock Lake (0.176 lb/acre). The monthly 

exports of phosphorus from Hemlock Creek were more 

uniform than from Sucker Creek. Approximately 206,000 

lb of suspended sediment were delivered from Hemlock 

Creek (15.8 lb/acre). 

Ungaged Surface-water Inflow

Phosphorus loading associated with surface-water 

inflow from all ungaged areas was estimated using the 

unit-area phosphorus yield determined for Sucker Creek. 

The unit-area yields from Sucker Creek watershed were 

used because this watershed is more similar to the other 

ungaged areas than is the Hemlock Creek watershed. 

The Hemlock Creek watershed contains more wetlands, 

impounded flow, and less agricultural areas than the 

ungaged areas. The unit-area yield for Sucker Creek 

(0.141 lb/acre) was multiplied by the total ungaged area 

(15,857 acres) to determine the loading from all ungaged 

areas. The total annual phosphorus loading from ungaged 

surface-water inflow to the Red Cedar Lakes was approximately 2,230 lb (table 5). Phosphorus loadings from 

ungaged inflows to the individual lakes/basins are given 

in table 5.

Inflow from Upstream Lakes/Basins

A large component of the phosphorus budget of the 

individual lakes/basins is associated with flow from the 

adjacent upstream lake/basin. The total annual transfer of 

phosphorus from the adjacent upstream lake/basin was 

computed from the product of the estimated annual flow 

(described in the previous section) and the average-annual, 

near-surface, phosphorus concentration measured in the 

lake. The average-annual concentration was computed 

from the measured monthly concentrations (concentrations 

for months without measurements were obtained by 

linearly interpolating between measured concentrations). 

Approximately 4,850 lb of phosphorus was transferred 

from Balsam Lake to the North Basin of Red Cedar Lake 

(table 5), 7,000 lb of phosphorus was transferred from the 

North Basin to the South Basin of Red Cedar Lake, and 

4,090 lb was transferred from Hemlock Lake to the South 

Basin of Red Cedar Lake. Therefore, the South Basin of 

Red Cedar Lake receives a total of approximately 11,100 lb 

of phosphorus from the upstream adjacent lakes/basins 

(table 5). 

Ground Water 

Phosphorus loading from ground water to each of the 

lakes/basins was computed from the estimated daily groundwater flows described in the Water Budget section (based 

on the length of shoreline) and an estimated constant 

phosphorus concentration. The estimated constant total 

phosphorus concentration in ground water (25 μg/L) was 

based on the concentrations measured at Sucker and 

Hemlock Creeks during base-flow periods when flow 

mostly was from ground-water discharge to the streams 

(fig. 10). The daily ground-water loads of phosphorus then 

were computed as the product of daily average flow and 

25 μg/L. The annual input of phosphorus from ground 

water was 2,450 lb. The annual and monthly phosphorus 

loads into the individual lakes/basins are given in table 5.

Only one documented “point” source of phosphorus 

is in the Red Cedar Lakes drainage basin: seepage out of 

the Birchwood Sewage-Disposal Pond. Effluent from this 

site is placed into a ground-water seepage pond about 

0.5 mi east of Balsam Lake. Annual effluent to this pond is 

approximately 63.5 acre-ft with an average concentration 

of about 6 mg/L, or approximately 1000 lb/yr (P. Prusak, 

Wisconsin Department of Natural Resources Engineer, 

Cumberland, Wisconsin, oral commun., 2002). If it is 

assumed that 20 percent of this input is not retained by 

the soil, then 207 lb of phosphorus may reach Balsam 

Lake (table 5). 

Septic Systems

The input of phosphorus from septic systems (M) 

was estimated by use of equation 5 (Reckhow and others, 

1980):

 M = ES • (number of capita years) • (1 – SR), (5)

where M is a function of an export coefficient, ES

, and a 

soil retention coefficient, SR. In applying equation 5, it 

was assumed that the most likely value for ES was 1.54 lb 

(0.7 kilogram) of phosphorus per capita per year. Typical 

export coefficients range from 1.1 lb per capita per year 

Source of Water and Phosphorus 23
24 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

(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 

530 (88 full-year households assumed to have 2 people and 

354 3-month households assumed to have 4 people within 

500 ft of the Red Cedar Lakes; table 6). The estimated 

number of capita years for each lake is given in table 6. 

It was assumed based on the soil conditions that the most 

likely value of SR was 0.7 (70-percent retention of phosphorus by the soil). The total annual input from septic tanks 

then was computed to be 245 lb (table 5). Because inputs 

from septic systems were not measured directly, low and 

high estimates were obtained by applying high and low 

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

estimates of phosphorus from septic systems were 82 and 

410 lb, respectively. Monthly loadings from septic systems 

into each lake/basin were assumed to be constant (table 5).

Lake Outflow

Phosphorus concentrations in the water leaving the 

lake, which were measured approximately monthly, varied 

little seasonally. Concentrations in the outflow ranged from 

14 to 46 μg/L (fig. 10) and were similar to surface concentrations measured in the lake. The average total phosphorus concentration leaving Red Cedar Lake was 29 μg/L 

(median concentration of 28 μg/L). Daily phosphorus 

concentrations were obtained by linearly interpolating 

between measurements. The total output of phosphorus 

from Red Cedar Lake was estimated to be 10,200 lb; 

about 50 percent was exported between April and June.

Phosphorus Budget Summary

The total input of phosphorus to the Red Cedar Lakes 

during this 1-year study period was 14,100 lb (table 5). 

The major source of phosphorus was from the gaged and 

ungaged tributaries that collectively contributed almost 

80 percent of the total (table 5 and fig. 11). This load was 

almost equally divided between that coming from Birch 

Lake and that from the area directly draining into the lakes 

(Sucker Creek, Hemlock Creek, and ungaged areas). Just 

over half of this load was delivered to the lake between 

April and June. Loading was relatively uniform throughout 

the watershed; the yield from areas with minimal anthropogenic impact was similar to that from areas affected by 

agriculture. Ground water was estimated to contribute 

17 percent of the load. Precipitation, seepage from the 

Birchwood Sewage-Disposal Pond, and septic systems were 

only minor contributors of phosphorus. Nearshore septic 

systems were estimated to have contributed between 

0.6 and 2.9 percent of the total phosphorus input, depending on the assumptions on soil retention of phosphorus. Of 

the 14,100 lb of phosphorus that entered the lake during the 

1-year period, about 10,200 lb (72.4 percent) were exported 

out of the lake; the remaining almost 3,900 lb were 

deposited in the lake-bed sediment. Precipitation during 

the study period was about 25 percent higher than normal; 

therefore, runoff and the estimated load were higher than 

would be expected in a year with normal precipitation. 

The contributions of phosphorus to each of the 

individual lakes/basins are given in table 5. For each of 

the lakes/basins, the relative contributions from individual 

sources were similar to that found for the Red Cedar 

Lakes as a whole (fig. 11). The main difference was that 

for some lakes/basins, the main source was transport from 

the adjacent upstream lake/basin. For example, the North 

Basin of Red Cedar Lake received 4,850 lb of phosphorus 

from Balsam Lake, and the South Basin received 11,100 lb 

collectively from the North Basin of Red Cedar Lake and 

from Hemlock Lake. 

Table 6. Number of capita years for septic systems used for homes within 500 feet of Red Cedar Lakes, Wisconsin

[Number of homes on each lake were obtained from E. Lund, Cedar Corporation, written commun., 2002]

Lake/Basin Total 

households

Full-time 

households

People per 

full-time 

household

Quarter-time 

households

People per 

quarter-time 

household

Capita years

Balsam Lake 52 10 2 42 4 62

Hemlock Lake 42 8 2 34 4 50

Red Cedar Lake 348 70 2 278 4 418

 Red Cedar - North Basin 174 35 2 139 4 209

 Red Cedar - South Basin 174 35 2 139 4 209

Red Cedar Lakes 442 88 2 354 4 530
Relative Importance of Internal Loading of 

Phosphorus

Phosphorus that is deposited in the lake-bed sediments is not permanently removed from the lakes and 

may be released back into the water column. The rate of 

phosphorus release from sediments is increased greatly 

when the sediment-water interface becomes anoxic (Wetzel, 1983), such as in the hypolimnion of all of the Red 

Cedar Lakes except the South Basin of Red Cedar Lake. 

Phosphorus released from hypolimnetic sediments typically results in a steady increase in phosphorus concentrations above the sediment-water interface after the onset 

of anoxia. This increase consistently was observed near 

the bottom in Balsam Lake, Hemlock Lake, and the North 

Basin of Red Cedar Lake during late summer and occasionally observed in Balsam and the North Basin of Red 

Cedar Lake in winter. 

To quantify the internal loading of phosphorus in each 

basin, the accumulation of phosphorus in the hypolimnion 

was computed during summer stratification in 2001. The 

phosphorus accumulation during late winter was not 

quantified, but would be less than that estimated for summer 

stratification because near-bottom dissolved oxygen 

concentrations were higher and total phosphorus concentrations were lower than in the summer. In winter 2001, only 

Balsam Lake had a appreciable phosphorus accumulation. 

Total phosphorus accumulation in the hypolimnion was 

computed as the phosphorus mass in the hypolimnion in 

late summer (using detailed phosphorus profiles collected 

in August and September before the breakdown of 

stratification and morphometric data for each lake/basin) 

minus the phosphorus mass in the hypolimnion in May at 

the onset of stratification. It was assumed that phosphorus 

concentrations changed linearly with depth between vertical 

phosphorus measurement intervals, that the total accumulation from internal loading only occurred below the thermocline, and that phosphorus export from the hypolimnion 

to the epilimnion during summer and from bed sediments 

in oxic areas of the lake were small. The net accumulations 

of phosphorus in the hypolimnia were 1,280 lb for Balsam 

Lake, 480 lb for Hemlock Lake, 2,790 lb for the North 

Basin of Red Cedar Lake, and 118 lb for the South Basin 

of Red Cedar Lake. The total internal loading to the Red 

Cedar Lakes was 4,660 lb. If this loading were considered 

additional phosphorus added to the lake, the total annual 

loading would increase to about 18,700 lb, of which 

internal loading would represent about 25 percent. About 

8,660 lb then would be retained in the Red Cedar Lakes 

(total input minus that exported from the lake). Therefore, 

the net annual result still is a loss of about 4,000 lb to the 

sediment. The contributions from the sediments can, 

however, drive seasonal productivity, such as fall algal 

blooms that may occur shortly after thermal stratification 

breaks down and the phosphorus that was accumulating in 

the hypolimnion is mixed into the entire water column. 

Phosphorus release from the sediments is not 

expected to have a major effect on summer productivity 

because most of the internal phosphorus release is in late 

summer. Most of the loading from the watershed, however, 

is delivered when stratification is being established (during April though June when concentrations of almost all 

sources are about 30-50 μg/L). This range in concentration 

is relatively similar to that measured in the lakes during 

this period. Phosphorus release from the sediment could, 

however, have a larger effect on phosphorus concentrations 

(proportionally) in the lakes if external phosphorus loading 

is reduced. If external phosphorus loading was reduced, 

phosphorus release from the sediments would not be 

expected to decrease immediately, therefore, the internal 

loading would be higher (relative to external loading) for 

a few years and may result in higher in-lake phosphorus 

concentrations than expected given the external phosphorus loading. 

Birch Lake

39 percent

Precipitation

1 percent

Birchwood

disposal pond

1 percent

Septic

systems

2 percent

Ground

water

17 percent

Sucker

Creek

8 percent

Annual phosphorus inputs 

Ungaged

surface-water

input

16 percent

Hemlock

Creek

16 percent

Figure 11. Phosphorus budget for the Red Cedar Lakes, 

Wisconsin, for October 1, 2000, through September 30, 2001.

Relative Importance of Internal Loading of Phosphorus 25
26 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

Modeling the Response in Water 

Quality to Changes in Phosphorus 

Loading

The total external phosphorus input to the Red Cedar 

Lakes was estimated to be 14,100 lb in WY2001. One way 

to determine how much of this phosphorus would need to 

be eliminated to improve the present water quality of the 

lakes or to determine how increases in phosphorus loading 

would degrade water quality is by the use of empirical 

models that relate phosphorus loading to measured water 

quality. These models were developed on the basis of comparisons of hydrologic and phosphorus loading estimates 

determined for many different watershed/lake systems to 

specific measures describing lake-water quality, such as 

near-surface phosphorus and chlorophyll a concentrations, 

and Secchi depth. Some of these empirical models are 

contained within BATHTUB (Walker, 1996), and WiLMS 

(Panuska and Kreider, 2002). The BATHTUB model has 

hydrologic transport algorithms and is, therefore, capable 

of simulating changes in water quality in complex lakes 

(multi-basin lakes or lakes connected by short channels), 

whereas WiLMS is more suited for simulating changes 

in single-basin lakes. Therefore, to predict the expected 

changes in the water quality of the Red Cedar Lakes that 

should occur with changes in phosphorus loading, BATHTUB was applied to the entire system using the most applicable empirical model for each water-quality constituent. 

To determine if this response would vary if other models 

were used, some of the models within WiLMS also were 

applied to each of the individual lakes. When applying 

WiLMS, the two basins in Red Cedar Lake were treated 

collectively as one lake.

Approach

To predict the expected changes in water quality in 

the Red Cedar Lakes in response to changes in phosphorus 

loading, nine scenarios were simulated with BATHTUB: 

the conditions in 2001 (base case with which to compare 

other simulations), six basin-wide strategies (decreases 

in total external loading by 50, 25, and 10 percent and 

increases in total external phosphorus loading by 10, 25, 

and 50 percent, including that from ground water and 

septic input), and two additional specific scenarios with 

decreased phosphorus loads. The two specific phosphorus reduction scenarios were Scenario A—elimination 

of phosphorus inputs associated with Birchwood Sewage-Disposal Pond, private septic systems, and tributaries downstream of Birch Lake and the Hemlock Creek 

gage, and Scenario B—only the removal of inputs from 

the Birchwood Sewage-Disposal Pond and private septic 

systems. Scenario A represents the maximum reduction 

if all phosphorus that is directly input into the Red Cedar 

Lakes from septic systems and tributaries were eliminated 

(assumes phosphorus input from tributaries upstream from 

Balsam Lake and Hemlock Lake, ground water, and from 

precipitation remained unchanged). Only three scenarios 

were simulated with WiLMS: the conditions in 2001, and a 

50-percent decrease and 50-percent increase in total external loading. Because the lakes are believed to be primarily 

phosphorus limited, only alterations to phosphorus loads 

were simulated. 

Data Requirements

Four types of data are required as input to these 

models for each lake/basin: morphometric data (table 1), 

water-quality data (table 3), hydrologic data (table 4), 

and nutrient-loading data for each source (table 5) . The 

time period for the hydrologic and nutrient loading used 

in the BATHTUB model is dependent on the phosphorusturnover ratio (number of times the phosphorus mass in 

the lake is replaced in a specific time period). The model 

should be run for a period that results in a phosphorusturnover ratio greater than 2. Annual simulations resulted 

in the phosphorus turnover ratios for each lake between 

4.7 (North Basin of Red Cedar Lake) and 16.0 (Hemlock 

Lake) (table 1) and 5.0 for the four basins as a whole. 

Therefore, nutrient-loading data were summarized over 

the entire period from October 1, 2000, through September 30, 2001, and water-quality data for the basins/lakes 

were averaged from May through September. Even though 

loading data are summarized for the entire year, the model 

simulates water quality only for the growing season (May 

through September). The models within WiLMS all use 

annual hydrologic and phosphorus loadings, but simulate 

the water quality for different discrete seasons (described 

later). Phosphorus from internal loading (sediment release) 

was not included as a phosphorus source for BATHTUB 

and WiLMS because most empirical models already 

incorporate this source and only should be added when 

a lake/reservoir has abnormally high internal loading 

(W. Walker, Jr., U.S. Army Corps of Engineers, oral 

commun., 2002).
Algorithms and Calibration

In applying BATHTUB, the dispersion coefficient for 

each basin (exchange of water with the adjacent upstream 

basin/lake) and the specific algorithms to simulate each 

water-quality constituent must be defined. The algorithms 

chosen to simulate total phosphorus and chlorophyll a

concentrations are listed in table 7, all of which are default 

algorithms and applicable to most lakes and reservoirs. 

Non-algal turbidity factors (or the reduction in clarity 

caused by factors other than algae) were computed within 

BATHTUB. Calibration coefficients for each water-quality 

constituent may be applied for each lake/basin. The coefficients applied in simulating the Red Cedar Lakes are given 

in table 8. All of these coefficients are within the typical 

range applied for most lakes, except that for phosphorus 

in Balsam Lake. One possible explanation for the large 

calibration coefficient required to accurately simulate the 

phosphorus concentrations in Balsam Lake may be that 

the lake has a relatively short residence time (just over 

1 month) and had total phosphorus concentrations entering 

the lake during May through September that were lower 

than other times of the year. The model is capable of only 

simulating one time period for the entire system, which, 

in this case, was 1 year. The 1-year period was needed 

because of the long residence time in the North Basin of 

Red Cedar Lake (over 4 months; table 1). Another possibility for the large calibration coefficient may be that 

the water entering from Birch Lake (from the bottom withdrawal dam) was cooler than Balsam Lake water and continuously sank below the epilimnion in the lake. Therefore, 

this loading did not affect near-surface phosphorus concentrations as much as it would have if the water did not sink. 

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 present conditions (described in detail later).

Water-Quality Response to Changes 

in Phosphorus Loading on the Basis 

of BATHTUB Simulations

To simulate the effects of basin-wide changes in 

phosphorus loading, the average volumetrically weighted 

total and dissolved ortho-phosphorus concentrations (total 

nutrient load divided by total water load) for each of the 

sources were decreased and increased by 50, 25, and 

10 percent. To determine the response in water quality 

(total phosphorus, chlorophyll a and Secchi depth) to 

eliminating specific nutrient sources, the phosphorus 

concentrations for the eliminated sources were set to 

near 0 μg/L (0.1 μg/L). Nitrogen concentrations were 

not altered in the various phosphorus-loading scenarios. 

Because the lakes/basins are phosphorus limited, altering 

nitrogen concentrations was shown to have little effect on 

the simulated results. 

Table 7. Algorithms used within BATHTUB to simulate water quality in Red Cedar Lakes, Wisconsin

Process Algorithm 

number Algorithm description

Phosphorus balance 1 Second order, available phosphorus

Nitrogen balance 0 Not computed

Chlorophyll a concentration 2 Function of phosphorus, light, and temperature

Secchi depth 1 Function of chlorophyll a and turbidity

Dispersion 1 Fisher numeric

Phosphorus calibration 1 Decay rates

Nitrogen calibration 0 None

Table 8. Calibration coefficients used in BATHTUB simulations of the Red Cedar Lakes, Wisconsin

Lake/Basin Total phosphorus Chlorophyll a Secchi depth

Balsam Lake 3.0 0.85 1.0

Red Cedar - North Basin .6 1.00 1.0

Red Cedar - South Basin 1.0 1.00 1.0

Hemlock Lake 2.0 .85 1.0

Modeling the Response in Water Quality to Changes in Phosphorus Loading 27
28 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

With the loading data from WY2001, BATHTUB 

was used to simulate the summer average water quality 

(May through September) in each lake/basin. The simulated values are compared with the measured values in the 

lakes/basin in 2001 (table 3) in figure 12. The coefficient 

of variation was computed for each measured value (on 

the basis of the monthly data) and used to calculate an estimate of the certainty (plus and minus one standard error 

around the average). After model calibration, BATHTUB 

accurately simulated total phosphorus (within ± 5 μg/L), 

chlorophyll a (within ± 2 μ/L), and Secchi depths (within 

± 0.7 ft) in all of the lakes/basins for 2001. 

Changes in Phosphorus Concentrations

On the basis of BATHTUB model simulations, 

phosphorus concentrations in each of the lakes/basins have 

a relatively linear response to a linear change in phosphorus loading (fig. 13); however, on a percentage basis, the 

changes in phosphorus concentrations are not as large as 

the changes in phosphorus loadings. Changes in phosphorus concentrations are about 60–70 percent of the changes 

in phosphorus loading. In other words, a 50-percent 

increase in phosphorus loading causes a 30–35-percent 

increase in phosphorus concentrations. This relation results 

in a larger absolute change in phosphorus concentration 

in Hemlock Lake for a given percent change in nutrient 

loading than the other lakes/basins because Hemlock Lake 

presently has the highest phosphorus concentration.

In response to Scenario A (elimination of all phosphorus input from the Birchwood Sewage-Disposal Pond, 

private septic systems, and tributary inputs downstream 

of Birch Lake and the Hemlock Creek gage), phosphorus 

concentrations change little in Balsam Lake, decrease by 

about 15 percent in Hemlock Lake, and decrease by about 

30 percent in Red Cedar Lake. Most of the phosphorus 

reductions in Scenario A are associated with the reduction 

in nearby surface-water contributions, of which Balsam 

Lake has the least (other than that from Balsam Lake that 

was not reduced) and Red Cedar Lake has the most. Phosphorus inputs associated with the sewage-disposal pond 

and private septic systems are small compared to other 

sources; therefore, elimination of these sources (Scenario 

B) has little effect on the water quality of the lakes. 

Changes in Chlorophyll a Concentrations.

Chlorophyll a concentrations also have a relatively 

linear response to a linear change in phosphorus loading, 

except for high phosphorus-loading scenarios where there 

is a smaller percentage response (fig. 14). Changes in 

chlorophyll a concentrations are about 80–100 percent of 

the changes in phosphorus loading; however, with loadings greater than 25 percent, the increase only is between 

50 and 80 percent of the increase in phosphorus loading. 

Chlorophyll a concentrations in Hemlock Lake again are 

most responsive to changes in phosphorus loading on the 

basis of concentration; however, the changes are similar 

on a percentage basis. 

Changes in phosphorus loading with Scenario A has 

almost no effect on chlorophyll a concentrations in Balsam 

Lake, about a 15-percent decrease in chlorophyll a concentrations in Hemlock Lake, and about a 30-percent decrease 

in chlorophyll a concentrations in Red Cedar Lake. These 

percent changes are similar to those for total phosphorus 

concentrations. Elimination of phosphorus inputs associated with the sewage-disposal pond and private septic 

systems (Scenario B) has little effect on chlorophyll a

concentrations in any of the lakes.

Changes in Water Clarity (Secchi Depths)

Water clarity is more responsive to decreases in 

phosphorus loading than to increases in loading (fig. 15). 

With a decrease in phosphorus loading of 50 percent, Secchi depths increase about 2–3 ft (20–40 percent); however, 

with a 50-percent increase in loading, Secchi depths only 

decrease by about 1 ft (10–20 percent). With Scenario 

A, Secchi depths increase about 2 ft (25 percent) in Red 

Cedar Lake and increase by about 1 ft in Hemlock Lake, 

but do not change in Balsam Lake. No changes in Secchi 

depths result in any of the lakes with Scenario B. 

Changes in Trophic Status

Trophic state index values indicate that Balsam Lake 

and Red Cedar Lake presently are borderline mesotrophic 

to eutrophic (mesotrophic with respect to Secchi depth and 

eutrophic with respect to phosphorus and chlorophyll a) 

and Hemlock Lake is eutrophic with respect to all three 

characteristics (fig. 6). On the basis of simulation results 

from BATHTUB, about a 50-percent reduction in phosphorus loading from that measured in WY2001 would be 

required for all three lakes to be classified as mesotrophic 

with respect to all three characteristics (fig. 16). The 

50-percent reduction is needed for Hemlock Lake to be 

classified as mesotrophic with respect to its Secchi Depth; 

the other lakes already are classified as mesotrophic with 

respect to their Secchi depths. With Scenario A, changes 

in the water quality of Red Cedar Lake should result in it 
0

5

10

15

20

25

30

35

40

45

Measured

Simulated

0

6

12

18

24

Measured

Simulated

0

2

4

6

8

10

Balsam Red Cedar -

North Basin

Red Cedar -

South Basin

Hemlock

Measured

Simulated

SECCHI DEPTH, IN METERS

CHLOROPHYLL A CONCENTRATION,

IN MICROGRAMS PER LITER

TOTAL PHOSPHORUS CONCENTRATION,

IN MICROGRAMS PER LITER

Figure 12. Measured and simulated (with BATHTUB) average summer (May through September) near-surface concentrations of 

total phosphorus and chlorophyll a, and Secchi depths in Balsam, Red Cedar, and Hemlock Lakes, Wisconsin, 2001.

Modeling the Response in Water Quality to Changes in Phosphorus Loading 29
30 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

0

10

20

30

40

50

-50

-40

-30

-20

-10

0

10

20

30

40

50

-50 -25 0 25 50 SCENARIO

A

SCENARIO

B PERCENTAGE CHANGE IN PHOSPHORUS LOADING

-50 -25 0 25 50 SCENARIO

A

SCENARIO

B PERCENTAGE CHANGE IN PHOSPHORUS LOADING

PERCENTAGE CHANGE IN PHOSPHORUS CONCENTRATION

PHOSPHORUS CONCENTRATION,

IN MICROGRAMS PER LITER

BATHTUB simulations WiLMS simulations

Balsam

Red Cedar - North

Red Cedar - South

Hemlock

Balsam - WiLMS

Red Cedar - WiLMS

Hemlock - WiLMS

Figure 13. Simulated changes in near-surface total phosphorus concentrations in Balsam, Red Cedar, and Hemlock Lakes, 

Wisconsin, with BATHTUB and WiLMS in response to various phosphorus-loading scenarios. Scenario A represents the elimination of phosphorus inputs from the Birchwood Sewage-Disposal Pond, private septic systems, and tributaries downstream of 

Birch Lake and the Hemlock Creek gage. Scenario B represents only the removal of phosphorus inputs from the Birchwood 

Sewage-Disposal Pond and private septic systems.
0

5

10

15

20

-50

-40

-30

-20

-10

0

10

20

30

40

50

SCENARIO

A

SCENARIO

B

PERCENTAGE CHANGE IN PHOSPHORUS LOADING

PERCENTAGE CHANGE IN CHLOROPHYLL 

A CONCENTRATION

CHLOROPHYLL 

A CONCENTRATION,

IN MICROGRAMS PER LITER

-50 -25 0 25 50

SCENARIO

A

SCENARIO

B

PERCENTAGE CHANGE IN PHOSPHORUS LOADING

-50 -25 0 25 50

BATHTUB simulations WiLMS simulations

Balsam

Red Cedar - North

Red Cedar - South

Hemlock

Balsam - WiLMS

Red Cedar - WiLMS

Hemlock - WiLMS

Figure 14. Simulated changes in near-surface chlorophyll a concentrations in Balsam, Red Cedar, and Hemlock Lakes, Wisconsin, with BATHTUB and WiLMS in response to various phosphorus-loading scenarios. Scenario A represents the elimination of 

phosphorus inputs from the Birchwood Sewage-Disposal Pond, private septic systems, and tributaries downstream of Birch 

Lake and the Hemlock Creek gage. Scenario B represents only the removal of phosphorus inputs from the Birchwood SewageDisposal Pond and private septic systems.

Modeling the Response in Water Quality to Changes in Phosphorus Loading 31
32 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

0

2

4

6

8

10

12

-50

-40

-30

-20

-10

0

10

20

30

40

50

-50 -25 0 25 50 SCENARIO

A

SCENARIO

B

PERCENTAGE CHANGE IN PHOSPHORUS LOADING

-50 -25 0 25 50 SCENARIO

A

SCENARIO

B

PERCENTAGE CHANGE IN PHOSPHORUS LOADING

PERCENTAGE CHANGE IN SECCHI DEPTH SECCHI DEPTH, IN FEET

BATHTUB simulations WiLMS simulations

Balsam

Red Cedar - North

Red Cedar - South

Hemlock

Balsam - WiLMS

Red Cedar - WiLMS

Hemlock - WiLMS

Figure 15. Simulated changes in Secchi depth in Balsam, Red Cedar, and Hemlock Lakes, Wisconsin, with BATHTUB and 

WiLMS in response to various phosphorus-loading scenarios. Scenario A represents the elimination of phosphorus inputs from 

the Birchwood Sewage-Disposal Pond, private septic systems, and tributaries downstream of Birch Lake and the Hemlock Creek 

gage. Scenario B represents only the removal of phosphorus inputs from the Birchwood Sewage-Disposal Pond and private 

septic systems.
Eutrophic

Mesotrophic

Oligotrophic

Eutrophic

Mesotrophic

Oligotrophic

Eutrophic

Mesotrophic

Oligotrophic

30

40

50

60

70

Balsam

Red Cedar - North

Red Cedar - South

Hemlock

30

40

50

60

70

30

40

50

60

70

-50 -25 0 25 50

PERCENTAGE CHANGE IN PHOSPHORUS LOADING

SCENARIO

A

SCENARIO

B

TSI VALUE (SECCHI DEPTH) TSI VALUE (CHLOROPHYLL A) TSI VALUE (PHOSPHORUS)

Figure 16. Simulated changes in Trophic State Index (TSI) values in Balsam, Red Cedar, and Hemlock Lakes, Wisconsin, with 

BATHTUB in response to various phosphorus-loading scenarios. Scenario A represents the elimination of phosphorus inputs from 

the Birchwood Sewage-Disposal Pond, private septic systems, and tributaries downstream of Birch Lake and the Hemlock Creek 

gage. Scenario B represents only the removal of phosphorus inputs from the Birchwood Sewage-Disposal Pond and private 

septic systems.

Modeling the Response in Water Quality to Changes in Phosphorus Loading 33
34 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

being classified as mesotrophic; however, the other lakes 

should remain eutrophic. Removal of all phosphorus from 

the sewage-disposal pond and private septic systems 

(Scenario B) should have little effect on the trophic state 

of any of the lakes.

Changes in Algal Bloom Response

All of the above model simulations were based on 

changes in the average water quality during the growing 

season (May–September). However, often what is perceived as the biggest problem is the extremes in water 

quality, for example, algal blooms. The definition of what 

constitutes the occurrence of an algal bloom can be site 

specific and subjective. In what typically is a clear lake, it 

may be considered that the lake has an algal bloom when 

the lake has a slight green color, whereas for a typically 

productive lake, it may considered that the lake has an 

algal bloom only when it is very green. A chlorophyll a

concentration of 10 μg/L represents a small amount of 

algae in a water body, whereas a concentration of 60 μg/L 

represents an extremely severe algal bloom. A chlorophyll 

a concentration of 30 μg/L represents what many would 

consider a moderate algal bloom. Results from BATHTUB 

simulations are used to demonstrate how the occurrence of 

algal blooms in each of the lakes (percent of days from May 

through September with a chlorophyll a concentration 

exceeding a certain value) should respond to variable phosphorus loadings as a function of how an algal bloom is 

defined (chlorophyll a concentrations from 10 to 40 μg/L) 

(fig. 17). Results for Red Cedar Lake are only shown for 

the North Basin because the results for the North and 

South Basins were similar. These results are summarized 

in table 9.

On the basis of BATHTUB simulations, algal blooms 

(as defined by chlorophyll a concentrations greater than 

30 μg/L) are relatively uncommon, and only occur on 

about 6 percent of the days between May and September 

in Hemlock Lake and less than this percentage in the other 

lakes. However, if phosphorus loading increases by 

50 percent, algal blooms in Hemlock would increase to 

about 17 percent of the days and to about 5–6 percent of 

the days in the other lakes. With a 50-percent decrease in 

loading, an algal bloom of this magnitude would occur on 

less than 1 percent of the days in any of the lakes. Algal 

blooms defined as chlorophyll a concentrations greater 

than 40 μg/L rarely occur presently, but would become 

more common in Hemlock Lake (7 percent of the days) 

with increases in nutrient loading of 50 percent. The 

response in the percentage of days with algal blooms, 

based on any definition, is relatively linear with respect to 

a linear change in phosphorus loading, except with load 

reductions greater than 25 percent. Reductions in loading by more than 25 percent cause a larger change in the 

frequency of algal blooms than for any other incremental 

change in phosphorus loading. 

Algal blooms defined as chlorophyll a concentrations greater than 10 μg/L or greater than 20 μg/L have a 

much larger response to changes in phosphorus loading 

than if defined by greater than 30 and 40 μg/L. A 50-percent reduction in phosphorus loading into Red Cedar Lake 

would decrease the percentage of days with chlorophyll 

a concentrations greater 10 μg/L from about 40 percent 

of the days currently to about 13 percent of the days, and 

the percentage of days with chlorophyll a concentrations 

greater than 20 μg/L would decrease from 8 percent to 

1 percent. A 50-percent increase in loading would increase 

the occurrence of algal blooms with concentrations greater 

than 10 μg/L to about 53 percent of the days and to 15 percent of the days with concentrations greater than 20 μg/L. 

Responses in algal blooms to loading changes for Balsam 

Lake are similar to those for Red Cedar Lake; however, 

algal blooms are more responsive in Hemlock Lake than 

in Red Cedar Lake.

Changes in the occurrence of algal blooms with 

Scenario A would be similar to that for a 10-percent 

reduction phosphorus loading in Balsam Lake, a 35-percent reduction in Red Cedar Lake, and a 15- to 20-percent 

reduction in Hemlock Lake. There would be little change 

in the occurrence of algal blooms in any of the lakes with 

Scenario B.

Response in Water Quality to Changes in 

Phosphorus Loading on the Basis of WiLMS 

Simulations. 

To determine how the response in water quality (total 

phosphorus, chlorophyll a and Secchi depth) varies with 

respect to changes in phosphorus loading if other models 

were used, some of the models within WiLMS (Panuska 

and Kreider, 2002) were also applied to each of the lakes 

(the two basins in Red Cedar Lake were treated collectively as one lake), but only for a subset of the scenarios. 

Of the 13 models contained in WiLMS, 9 of the models 

were applicable to the hydrologic systems of Balsam Lake 

and Red Cedar Lake and 8 were applicable to the hydrologic system of Hemlock Lake. Therefore, the morphometry of the lakes and the hydrology and phosphorus loadings 

for WY2001 first were input into these models to simulate 

near-surface phosphorus concentrations. When simulating 
10

20

30

40

-50

-25

-10

0

10

25

50

Scenario A

Scenario B

0

15

30

45

60

75

Balsam Lake

10

20

30

40

-50

-25

-10

0

10

25

50

Scenario A

Scenario B

0

15

30

45

60

75

Red Cedar Lake - North Basin

10

20

30

40

-50

-25

-10

0

10

25

50

Scenario A

Scenario B

0

15

30

45

60

75

-50

-25

-10

0

10

25

50

Scenario A

Scenario B

BLOOM OCCURRENCE, IN PERCENT

BLOOM OCCURRENCE, IN PERCENT 

BLOOM OCCURRENCE (PERCENT)

BLOOM

DEFINITION,

IN MICROGRAMS

PER LITER

BLOOM

DEFINITION,

IN MICROGRAMS

PER LITER

BLOOM

DEFINITION,

IN MICROGRAMS

PER LITER

Hemlock Lake

PERCENTAGE CHANGE IN

PHOSPHORUS LOADING

PERCENTAGE CHANGE IN

PHOSPHORUS LOADING

PERCENTAGE CHANGE IN

PHOSPHORUS LOADING

Loading scenario -

percentage change in

phosphorus loading

EXPLANATION

Figure 17. Simulated changes in the frequency (percent of days from May through September) of algal blooms (as defined by 

different chlorophyll a concentrations) in Balsam, Red Cedar, and Hemlock Lakes, Wisconsin in response to various phosphorusloading scenarios on the basis of output from BATHTUB. Scenario A represents the elimination of phosphorus inputs from the 

Birchwood Sewage-Disposal Pond, private septic systems, and tributaries downstream of Birch Lake and the Hemlock Creek 

gage. Scenario B represents only the removal of phosphorus inputs from the Birchwood Sewage-Disposal Pond and private 

septic systems.

Response in Water Quality to Changes in Phosphorus Loading 35
36 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

Table 9. Simulated algal bloom frequency in the Red Cedar Lakes, Wisconsin, in response to various phosphorus-loading 

scenarios, on the basis of output from BATHTUB

[Algal blooms defined as chlorophyll a concentrations greater than (>) 10, 20, 30, and 40 micrograms per liter (μg/L); %, percent. Scenario A represents 

the elimination of phosphorus inputs from the Birchwood Sewage-Disposal Pond, private septic systems, and tributaries downstream of Birch Lake and 

the Hemlock Creek gage. Scenario B represents only the removal of phosphorus inputs from the Birchwood Sewage-Disposal Pond and private septic 

systems.]

Change in Phosphorus 

Loading/Scenario

Percentage of days (from May through September) 

with algal blooms in indicated lake/basin

Balsam Lake Red Cedar 

North Basin Hemlock Lake

Chlorophyll a > 10 μg/L

-50% 11.5 12.7 20.6

-25% 26.7 27.3 42.2

-10% 35.4 35.3 53.5

0% 40.8 39.1 59.6

10% 46.1 42.5 64.9

25% 51.8 46.9 71.4

50% 58.3 52.9 79.2

Scenario A 37.3 18.9 47.4

Scenario B 38.7 37.7 59.1

Chlorophyll a > 20 μg/L

-50% 1.0 1.2 2.6

-25% 4.1 4.3 9.4

-10% 6.8 6.7 15.1

0% 8.8 8.1 19

10% 11.2 9.5 23.1

25% 14.2 11.6 29

50% 18.2 14.8 38

Scenario A 7.5 2.3 11.8

Scenario B 8. 7.6 18.8

Chlorophyll a > 30 μg/L

-50% 0.1 0.2 0.5

-25% .8 .9 2.4

-10% 1.6 1.6 4.6

0% 2.3 2.0 6.3

10% 3.1 2.5 8.2

25% 4.2 3.2 11.4

50% 5.9 4.5 16.9

Scenario A 1.8 .4 3.3

Scenario B 2. 1.8 6.2

Chlorophyll a > 40 μg/L

-50% 0.0 0.0 0.1

-25% .2 .2 .7

-10% .5 .4 1.6

0% .7 .6 2.3

10% 1. .8 3.2

25% 1.4 1. 4.7

50% 2.1 1.5 7.7

Scenario A .5 .1 1.1

Scenario B .6 .5 2.2
Table 10. Measured and simulated phosphorus concentrations, in micrograms per liter (μg/L), in Red Cedar Lakes, Wisconsin, 

from hydrologic and phosphorus-loading models contained in the Wisconsin Lakes Modeling Suite (Panuska and Kreider, 2002)

[%, percent; >, greater than; <, less than; NA, model not applicable]

Measured Simulated

Lake Phosphorus Model 

(Type of lakes that the model 

was developed from)

Prediction 

season

Seasonal 

concentration 2001

50% reduction 

in phosphorus 

loading

50% increase 

in phosphorus 

loading 

Balsam Lake

Canfield and Bachman, 1981 (Natural lakes) Spring 37.0 31.0 16.0 45.0

Canfield and Bachman, 1981 (Artificial lakes) Spring 37.0 27 15 38

Rechow, 1979 (General lakes) Growing season 24.8 32 16 47

Rechow, 1977 (Anoxic lakes) Growing season 24.8 39 19 58

Rechow, 1977 (Water load >165 feet per year) Growing season 24.8 30 15 44

Vollenweider, 1982 (General lakes) Annual 30.4 27 15 38

Vollenweider, 1982 (Shallow lakes) Annual 30.4 22 12 31

Walker, 1977 (General lakes) Spring 37.0 33 17 50

Walker, 1985 (Reservoirs) Growing season 24.8 29 17 38

Average 30.1 30.0 15.8 43.2

Change, μg/L -14.2 13.2

Change, percentage from Base Scenario -47.4% 44.1%

Red Cedar Lake

Canfield and Bachman, 1981 (Natural lakes) Spring 37.0 24 13 35

Canfield and Bachman, 1981 (Artificial lakes) Spring 37.0 21 12 29

Rechow, 1979 (General lakes) Growing season 26.2 21 11 32

Rechow, 1977 (Anoxic lakes) Growing season 26.2 31 16 47

Rechow, 1977 (Water load <165 feet per year) Growing season 26.2 22 11 32

Vollenweider, 1982 (General lakes) Annual 28.0 20 12 29

Vollenweider, 1982 (Shallow lakes) Annual 28.0 16 9 23

Walker, 1977 (General lakes) Spring 37.0 24 12 37

Walker, 1985 (Reservoirs) Growing season 26.0 21 13 27

Average 30.2 22.2 12.1 32.3

Change, μg/L -10.1 10.2

Change, percentage from Base Scenario -45.5% 45.5%

Hemlock Lake

Canfield and Bachman, 1981 (Natural lakes) Spring 41.0 28 14 41

Canfield and Bachman, 1981 (Artificial lakes) Spring 41.0 25 13 35

Rechow, 1979 (General lakes) Growing season 35.8 24 12 36

Rechow, 1977 (Anoxic lakes) Growing season 35.8 34 17 51

Rechow, 1977 (Water load <165 feet per year) Growing season 35.8 24 12 36

Vollenweider, 1982 (General lakes) Annual 44.8 25 14 35

Vollenweider, 1982 (Shallow lakes) Annual 44.8 20 11 29

Walker, 1977 (General lakes) Spring 41.0 30 15 46

Walker, 1985 (Reservoirs) Growing season NA NA NA NA

Average 40.0 26.3 13.5 38.6

Change, μg/L -12.8 12.4

Change, percentage from Base Scenario -48.6% 47.1%

Modeling the Response in Water Quality to Changes in Phosphorus Loading 37
38 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

the water quality in Red Cedar Lake, the transfer of water 

and phosphorus from Balsam and Hemlock Lakes (tables 

4 and 5) were included. An increase and decrease in total 

external phosphorus loading by 50 percent then were input 

into these models to simulate the expected changes in nearsurface phosphorus concentrations. Each of the models 

within WiLMS uses annual hydrologic and phosphorus 

loads, but simulates an average phosphorus concentration 

for a specific season (table 10). For each lake, all eight or 

nine models were applied and their results were then averaged and compared with those simulated with BATHTUB 

(figs. 14–16).

Balsam Lake

The average simulated phosphorus concentration for 

Balsam Lake when all phosphorus inputs were included in 

the models was 30.0 μg/L. This value is similar to the average of the seasonally summarized data measured in 

the lake (30.1 μg/L, table 10). The models were then 

applied with the phosphorus load decreased and increased 

by 50 percent. With a 50-percent reduction in phosphorus 

loading, the average simulated phosphorus concentration 

was 15.8 μg/L (a decrease of 14.2 μg/L or 47.4 percent, 

table 10). With a 50-percent increase in phosphorus loading, the average simulated concentration was 43.2 μg/L (an 

increase of 13.2 μg/L or 44.1 percent). If it is assumed that 

these percentage changes also apply to the summer average concentration measured in Balsam Lake of 24.8 μg/L, 

then a 50-percent reduction in loading would result in a 

summer average concentration of 13.0 μg/L and a 50-percent increase in loading would result in a summer average 

concentration of 35.7 μg/L. This response is only slightly 

more than that simulated with BATHTUB (fig. 13). 

Empirical relations have been developed to predict 

average summer chlorophyll a concentrations and Secchi depths from near-surface phosphorus concentrations. 

Relations developed for Wisconsin lakes by Lillie and 

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

to determine how the simulated changes in phosphorus 

concentration in the lake associated with decreases and 

increases in phosphorus loading should affect chlorophyll 

a concentrations and Secchi depths. When the average 

measured summer phosphorus concentration of 24.8 μg/L 

was input into the empirical relations, it was predicted that 

the lake should have a chlorophyll a concentration of 

8.7 μg/L (compared to 9.8 μg/L measured during 2001) 

and a Secchi depth of 6.2 ft (compared to the measured 

7.4 ft). The simulated average summer phosphorus concentrations for the 50-percent decrease and increase in 

phosphorus loading of 13.0 μg/L and 35.7 μg/L, respectively, were then input into the relations. For 13.0 μg/L of 

phosphorus, the empirical relations predicted a chlorophyll 

a concentration of 5.2 μg/L (a 40-percent reduction from 

the predicted 8.7 μg/L) and a Secchi depth of 9.19 ft (a 

47-percent improvement from the predicted 6.2 ft). For 

35.7 μg/L of phosphorus, the empirical relations predicted 

a chlorophyll a concentration of 11.6 μg/L (a 33-percent 

increase from the predicted 8.7 μg/L) and a Secchi depth 

of 4.9 ft (a 21-percent reduction from the predicted 6.2 ft). 

If it is assumed that the empirical relations more 

accurately predict the percentage of change rather than 

absolute values, a 50-percent decrease in phosphorus 

loading results in chlorophyll a concentrations decreasing 

from the measured summer average of 9.8 μg/L to 

5.9 μg/L and Secchi depth increasing from 7.4 ft to 

11.0 ft. Applying the percentage of change to the measured 

summer average concentrations essentially results 

in site-specific calibration. A 50-percent increase in 

phosphorus loading then results in chlorophyll a concentrations increasing from 9.8 μg/L to 13.1 μg/L and Secchi 

depth decreasing from 7.4 ft to 5.9 ft. These changes were 

only slightly less than that simulated with BATHTUB for 

chlorophyll a and slightly more than that simulated for 

Secchi depth (figs. 14 and 15).

Red Cedar Lake

The average phosphorus concentration simulated for 

Red Cedar Lake with all phosphorus inputs included in the 

models was 22.2 μg/L, which is less than the average of 

the measured data (30.2 μg/L, table 10). With a 50-percent 

reduction in phosphorus loading, the average simulated 

phosphorus concentration was 12.1 μg/L (a decrease of 

10.1 μg/L or 45.5 percent, table 10). With a 50-percent 

increase in loading, the average simulated concentration 

was 32.3 μg/L (an increase of 10.2 μg/L or 45.5 percent). 

If it is assumed that these percentage changes also apply to 

the summer average concentration (site-specific calibration) 

of 26.2 μg/L, then a 50-percent reduction in loading would 

result in a summer average concentration of 14.0 μg/L and 

a 50-percent increase in loading would result in 38.0 μg/L. 

This response is only slightly more (about 15 percent) than 

that simulated with BATHTUB (fig. 13). 

The empirical models in WiLMS predicted that with 

the measured average summer phosphorus concentration 

of 26.2 μg/L, the lake should have a chlorophyll a concentration of 9.0 μg/L (compared to 9.1 μg/L, average measured in the North and South Basins, table 3) and a Secchi 

depth of 5.9 ft (compared to the measured 7.6 ft). For 
14.0 μg/L of phosphorus, the relations predicted a chlorophyll a concentration of 5.5 μg/L (a 39-percent reduction 

from the predicted 9.0 μg/L) and a Secchi depth of 8.9 ft 

(a 50-percent increase from the predicted 5.9 feet). For 

38 μg/L of phosphorus, the relations predicted a chlorophyll a concentration of 12.2 μg/L (a 36-percent increase) 

and a Secchi depth of 4.9 ft (a 17-percent reduction). If it 

again is assumed that the empirical relations are best used 

to predict the percentage of change, a 50-percent decrease 

in phosphorus loading should result in chlorophyll a concentrations decreasing from the measured summer average 

of 9.1 μg/L to 5.6 μg/L and average Secchi depth increasing from 7.6 ft to 11.4 ft. A 50-percent increase in phosphorus loading should result in the average chlorophyll 

a concentration increasing from 9.1 μg/L to 12.4 μg/L 

and Secchi depth decreasing from 7.6 ft to 6.3 ft. These 

changes are similar to those simulated with BATHTUB 

(figs. 14 and 15).

Hemlock Lake

The average phosphorus concentration simulated for 

Hemlock Lake with all phosphorus inputs included in the 

models was 26.3 μg/L, less than the 40 μg/L measured 

in the lake (table 10). With a 50-percent reduction in 

phosphorus loading, the average simulated phosphorus 

concentration was 13.5 μg/L (a decrease of 12.8 μg/L or 

48.6 percent). With a 50-percent increase in loading, the 

average simulated concentration was 38.6 μg/L (an 

increase of 12.4 μg/L or 47.1 percent). If it is assumed 

these percentage changes also apply to the summer 

average concentration of 35.8 μg/L, then a 50-percent 

reduction in loading would result in a summer average 

concentration of 18.4 μg/L and a 50-percent increase 

would result in 52.7 μg/L. This response is only slightly 

more than that simulated with BATHTUB (fig. 13). 

The empirical relations in WiLMS predicted that, 

with the measured average summer phosphorus concentration of 35.8 μg/L, the lake should have a chlorophyll a

concentration of 11.6 μg/L (compared to the measured 

13.7 μg/L) and a Secchi depth of 4.9 ft (compared to the 

measured 5.2 ft). For the simulated 18.4 μg/L of phosphorus, the relations predicted a chlorophyll a concentration 

of 6.7 μg/L (a 42-percent reduction from the predicted 

11.6 μg/L) and a Secchi depth of 7.6 ft (a 53-percent 

increase from the predicted 4.9 ft). For 52.7 μg/L of 

phosphorus, the relations predicted a chlorophyll a

concentration of 15.9 μg/L (a 37-percent increase) and a 

Secchi depth of 3.9 ft (a 20-percent reduction). If it again 

is assumed that the relations best predict the percentage of 

change, a 50-percent decrease in phosphorus loading should 

result in chlorophyll a concentrations decreasing from the 

measured summer average of 13.7 μg/L to 7.9 μg/L and 

Secchi depth increasing from 5.2 ft to 7.9 ft. A 50-percent 

increase in phosphorus loading should result in chlorophyll 

a concentrations increasing from 13.7 μg/L to 18.8 μg/L 

and Secchi depth decreasing from 5.2 ft to 4.1 ft. These 

changes are similar to those simulated with BATHTUB 

(figs. 14 and 15).

Comparison of Results from BATHTUB and 

WiLMS

Simulations with BATHTUB and WiLMS gave 

similar results for the expected response in water quality 

to changes in phosphorus loading. Total phosphorus and 

chlorophyll a, concentrations should increase almost 

linearly with a linear increase in phosphorus loading, but 

the changes in in-lake concentrations are not expected to 

be as large as the changes in phosphorus loadings. Results 

from WiLMS indicated a slightly larger response in total 

phosphorus concentrations but similar response in chlorophyll a concentrations. Results from both models indicated 

that water clarity should be more responsive to decreases 

in phosphorus loading than to increases in loading (fig. 15), 

although results from WiLMS indicated a slighter larger 

response to reductions in phosphorus loading than did 

those from BATHTUB. 

Summary and Conclusions

Balsam Lake and Red Cedar Lake presently are classified as borderline mesotrophic to eutrophic (mesotrophic 

with respect to Secchi depth and eutrophic with respect 

to phosphorus and chlorophyll a), and Hemlock Lake is 

classified as eutrophic with respect to all three characteristics. Because of concerns that the water quality of 

these lakes was degrading, three cooperative studies were 

conducted by the USGS between 1993 and 2003. As part 

of these studies, water quality in the lakes was documented 

in 1993–94, 1996–97, and 2000–01. The goals of the final 

study were to evaluate the water quality of the lakes in 

terms of long-term trends (1980s to the present), to quantify the sources of phosphorus to the lakes, and to determine how the lakes would respond to changes in phosphorus loading. Little change in water quality has been 

measured in these lakes since data collection began on the 

lakes from 1987 (Red Cedar Lake) and 1993 (Balsam and 

Hemlock Lakes). 

Modeling the Response in Water Quality to Changes in Phosphorus Loading 39
40 Water Quality and the Effects of Changes in Phosphorus Loading to the Red Cedar Lakes, Wisconsin

A water budget, constructed for WY2001, indicated 

that the major sources of water to the Red Cedar Lakes are 

from Birch Lake (36 percent), gaged and ungaged surfacewater inflows (33 percent), and ground water (25 percent). 

Precipitation on the surface of the lakes contributed about 

6 percent of the total inputs. The major output of water 

from the lakes was through the outlet of Red Cedar Lake 

(96 percent) and secondarily from evaporation (4 percent). 

Phosphorus budgets also were constructed for 

WY2001. The total annual input of phosphorus to the Red 

Cedar Lakes was 14,100 lb. The major source of phosphorus was from the tributaries that collectively contributed 

almost 80 percent of the total. This load was almost equally 

divided between that coming from Birch Lake and that from 

areas directly draining into the lakes. Yields from direct 

surface-water inputs were relatively similar between areas 

with minimal anthropogenic impact and areas with agriculture. Ground water was estimated to contribute about 

17 percent of the phosphorus load, whereas precipitation, 

loss from the Birchwood Sewage-Disposal Pond, and septic 

systems were only minor contributors. Of the 14,100 lb 

of phosphorus that entered the lake, about 10,200 lb 

(72.4 percent) were exported out of the lake; the remaining 

almost 3,900 lb were deposited in the lake sediments.

It was estimated that the sediments release about 

4,660 lb of phosphorus during summer stratification; 

however, on an annual basis, the sediments accumulate 

phosphorus. Phosphorus release from the sediments is 

not expected to have a major effect on summer productivity because most of this phosphorus is not released to the 

upper part of the lake until late summer. This release 

could drive fall algal blooms. 

Two water-quality models (BATHTUB and WiLMS) 

were used to simulate the expected changes in water quality in response to specific phosphorus-loading scenarios. 

These simulations determined how much of the phosphorus load would need to be eliminated to improve water 

quality and how increases in phosphorus loading would 

degrade the water quality of the lakes. Simulation results 

from both models indicated total phosphorus and chlorophyll a concentrations increase almost linearly with 

a linear increase in phosphorus loading, but changes in 

in-lake concentrations are not expected to be as large as 

the changes in phosphorus loadings. Simulation results 

from both models also indicated that water clarity is more 

responsive to decreases in phosphorus loading than to 

loading increases. About a 50-percent reduction in phosphorus loading is required for all three lakes to achieve a 

mesotrophic classification. Changes in phosphorus loading 

affect the percentage of days with algal blooms, especially 

the number of days with less extensive algal blooms.

Simulations of specific phosphorus-reduction strategies indicated that elimination of all tributary and septic 

inputs downstream of Birch Lake and the Hemlock Creek 

gage (Scenario A) would have little effect on the water 

quality of Balsam Lake, about a 15-percent decrease in 

phosphorus concentrations in Hemlock Lake, and about a 

30-percent decrease in phosphorus concentrations in Red 

Cedar Lake. This limited effect occurs because most phosphorus entering the lakes comes from Birch Lake, ground 

water, and the relatively undeveloped land upstream of 

Hemlock Lake. Therefore, to attain appreciable improvements in the water quality of the Red Cedar Lakes would 

require that the water quality of the upstream lakes be 

improved. This agrees with the statement made by the 

WDNR (1997) that “phosphorus loading goals set for 

mainstem lakes (such as the Red Cedar Lakes) only can 

be attained if they are considered at the same time decisions are being made in upstream lakes and their drainage 

areas.” Presently, Lake Chetac and Birch Lake are productive lakes and both are more eutrophic than the Red Cedar 

Lakes (based on Self-help monitoring data).

The effects of phosphorus-load reductions immediately after a load reduction are not expected to be as large 

as those simulated by the models because of phosphorus 

release from the sediments. Empirical models, such as 

those in BATHTUB and WiLMS, assume a typical internal 

phosphorus release for a lake with the specified external 

loading. Therefore, it is assumed in model simulations 

that the lake is in equilibrium with its external loading. 

However, if external phosphorus loading is reduced, 

internal loading would be expected to remain relatively 

high for several years. Therefore, phosphorus release from 

sediments of the Red Cedar Lakes could become relatively 

more important and possibly delay the anticipated effects 

of external phosphorus-load reductions.

Although a decrease in phosphorus loading from areas 

immediately adjacent to the Red Cedar Lakes by itself 

would only slightly improve the water quality of the lakes, 

increases in phosphorus loading from areas adjacent to 

the Red Cedar Lakes could appreciably degrade the water 

quality of the lakes. Suspended sediment in the tributaries 

was found to have a high particulate phosphorus content; 

therefore, any increase in erosion from the watershed could 

result in unusually high phosphorus loading to the lakes. 

Therefore, although the water quality of the Red Cedar 

Lakes has not changed appreciably in recent years, continuing efforts to protect the lake and its susceptible watershed are necessary to maintain the current water quality.
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