Sources and Transport of Phosphorus in the Western Lake

Michigan Drainages

by Dale M. Robertson

Fact Sheet FS-208-96

Introduction

Figure 1. Land-use categories, subunits, and reference basins in the Western

Lake Michigan Drainages study unit.

In 1991, the U.S. Geological Survey (USGS) began full implementation

of the National Water-Quality Assessment (NAWQA) Program. The goals

of the NAWQA program are to (1) provide a nationally consistent description of water-quality conditions for a large part of the Nation’s water

resources, (2) define long-term trends (or lack of trends) in water quality,

and (3) identify, describe, and explain, as possible, the major factors that

affect the observed water-quality conditions and trends (Hirsch and others,

1988). To fulfill the goals of the NAWQA program, the USGS plans to

examine 60 areas (study units) across the United States on a rotational

cycle. The first 20 of these study units began intensive investigations in

1991. One of these study units is the Western Lake Michigan Drainages

(WMIC) (fig. 1).

The WMIC study unit contains many individual drainage basins that

empty into Green Bay or directly into Lake Michigan. These basins can be

grouped into four main subunits based on land use: the Northern-Forested

subunit (88 percent forest and 8 percent agriculture), the Oconto-Peshtigo

subunit (71 percent forest and 25 percent agriculture), the Fox-WolfPensaukee subunit (35 percent forest and 53 percent agriculture), and the

Direct Lake Michigan subunit (13 percent forest and 73 percent

agriculture) (fig. 1 and table 1). Within these subunits, extensive

water-quality data have been collected at eight sites. Six sites have

large drainage basins: Escanaba (1) and Ford (2) River Basins in

Upper Michigan, Menominee River Basin (3) in Wisconsin and

Upper Michigan (primarily forest), and Fox River Basin (5) (mixed

agriculture and forest), and Manitowoc (7) and Milwaukee (8) River

Basins (primarily agriculture) in Wisconsin. Two sites in Wisconsin

have small drainage basins with one primary land use: Popple River

Basin (4, forest) and White Creek Basin (6, agriculture). These eight

rivers are referred to as “reference rivers” and their watersheds are

referred to as “reference basins”. These reference rivers are identified in figures 1, 2, and 3 and described in table 1. In this report, the inputs

of total phosphorus into the reference basins, subunits, and entire WMIC

study unit are quantified and used in conjunction with the export rates from

the reference rivers to estimate the phosphorus transported from the four

subunits and the entire WMIC study unit, as well as to determine the

importance of point sources to the overall phosphorus export.

Phosphorus is essential for algal and macrophyte growth in aquatic

environments, but in sufficiently high concentrations, it can lead to

excess algae and macrophytes. Phosphorus transport from runoff has been

identified as a primary water-quality concern throughout the WMIC study

unit, especially downstream from large agricultural areas of the study unit,

such as Lake Winnebago and Green Bay. With the belief that point sources

of phosphorus in the study unit are under control, much of the current effort

in improving water quality focuses on quantifying and reducing nonpointsource loading. Nonpoint-source loading of phosphorus into rivers is

primarily a function of land-use practices and the types of surficial deposits

(Clesceri and others, 1986; U.S. Environmental Protection Agency, 1980);

therefore, the inputs and exports of phosphorus described in this report are

discussed with respect to these characteristics.

Forested land covers more than 50 percent of the study unit (fig. 1) and

dominates the northern part, where deciduous forests grade to evergreen

forests to the northeast. Agricultural land, mostly cropland and pasture, in

the southern and central part of the study unit is the second largest land-use

category. Areas of urban or developed land are primarily found along Lake

Michigan in the southeastern part of the study unit and around Lake

Winnebago and Green Bay.

Surficial deposits range in thickness from zero to several hundred feet

and consist of glacial and recent-aged deposits. Sand and gravel deposits are

interspersed with sandy deposits in the southern and western parts of the

study unit and with loamy deposits in the central parts (fig. 2). Loamy

deposits dominate in the northeastern part of the study unit and clayey

deposits dominate in the central and eastern parts.

Sources of Phosphorus

Fertilizers and Manure

Application of phosphorus to croplands, urban lawns, and golf courses,

in the form of fertilizers, is commonplace within the WMIC study unit.

Application of manure is common to croplands associated with dairy

operations. Application rates of phosphorus from fertilizers for 1985 and

manure for 1987 were derived from county application rates estimated by

Green Bay

Lake

Michigan

89°

46°

88°

45°

44°

43°

87°

1

8

2

3

4

5

6

7 2

Subunit boundary


Reference basin


boundary


Sampling site on


reference river


(see table 1)


Land-use category


Agriculture


Forest


Agriculture/forest


Urban


Mixed


Water

EXPLANATION

0 25 50 MILES

0 25 50 KILOMETERS

WISCONSIN

MICHIGAN

Northern-


Forested Subunit

Oconto-


Peshtigo Subunit

Fox-Wolf-


Pensaukee 


Subunit

Direct Lake Michigan


Subunit

Lake


Winnebago
Figure 2. Texture of surficial deposits in the Western Lake Michigan

Drainages study unit.

Alexander and Smith (1990) and R.B. Alexander (U.S. Geological Survey,

written commun., 1993), respectively. The county rates were proportioned

to the individual reference basins on the basis of the percentage of agriculture in each reference basin compared to that in the counties included in the

reference basins (Robertson and Saad, 1996) and proportioned to the four

subunits by weighting the county application rates by the percentage of the

subunit in each county.

Annual application rates of phosphorus from fertilizers ranged from 0.08

kg/ha (kilograms per hectare) in the Popple River Watershed to 11.09

kg/ha in the White Creek Watershed (table 1). Application rate in pounds

per acre is obtained by multiplying the rate in kilograms per hectare by 0.89.

Application rates in all of the forested reference basins (Escanaba, Ford,

Menominee, and Popple River Watersheds) were less than 0.6 kg/ha and in

agricultural reference basins (Manitowoc and Milwaukee River Watersheds) were greater than 6 kg/ha. The average application rates for the

subunits ranged from 0.50 kg/ha in the Northern-Forested subunit to 6.08

kg/ha in the Direct Lake Michigan subunit (fig. 3).

Annual application rates of phosphorus from manure were generally

slightly less than those from fertilizers and ranged from 0.09 kg/ha in the

Popple River Watershed to 8.83 kg/ha in the White Creek Watershed (table

1). Application rates in all of the forested reference basins were less than 0.4

kg/ha and in agricultural reference basins were greater than 5 kg/ha. The

average application rates for the subunits ranged from 0.31 kg/ha in the

Northern-Forested subunit to 5.04 kg/ha in the Direct Lake Michigan

subunit (fig. 3).

Atmospheric Deposition

Phosphorus-deposition rates were estimated for two areas near the

WMIC study unit: annual deposition of phosphorus near Chicago was

estimated to be 0.38 kg/ha (Murphy, 1974) and almost an order of magnitude less, 0.05 kg/ha, in a remote forested area in northern Wisconsin (Rose,

1993). These two areas are expected to represent the extremes in the

phosphorus-deposition gradient across the study unit. In estimating the total

input of phosphorus into various areas, the total contribution of atmospheric

phosphorus was assumed to be 0.05 kg/ha in forested areas and 0.20 kg/ha

in agricultural areas (table 1 and fig. 3). Therefore, compared to fertilizer

and manure, atmospheric deposition of phosphorus represents only a small

source of phosphorus.

Point Sources of Phosphorus

Point-source releases of phosphorus are primarily from sewage-treatment plants, papermills, and cheese factories located mostly in the southern

half of the study unit (Robertson and Saad, 1996). The estimated annual

inputs of phosphorus from point sources during 1992 were obtained from

the Wisconsin Department of Natural Resources (G. North, Office of

Technical Services, Wisconsin Department of Natural Resources, written

commun., 1994). Input of phosphorus from point sources within the

reference basins is very small compared to that from nonpoint sources.

Input from point sources was highest in the Fox River reference basin

(0.08 kg/ha; table 1). However, point sources of phosphorus at specific

locations near major cities can be very important locally, such as from the

cities of Green Bay and Milwaukee. Both sites are just downstream from

sampling sites on reference rivers, where most is directly discharged to

Green Bay and Lake Michigan. The importance of the input from point

sources relative to that from nonpoint sources is discussed in detail later in

this report.

The total annual input of phosphorus is directly related to

the percentage of agriculture in an area; more than 20 kg/ha

are input into areas completely in agriculture (White Creek),

less than 1 kg/ha are input in forested areas, and intermediate

amounts are input in mixed agricultural and forested areas

(table 1). Almost all the phosphorus input into each area is

from agricultural applications, except in isolated forested

areas where the small rate of atmospheric deposition can

nevertheless be important (fig. 3). For the entire study unit,

greater than 96 percent of the total input of phosphorus

comes from fertilizers and manure; point sources contribute

only about 1.2 percent of the total input (fig. 4).

Transport of Phosphorus

Reference Basins

Annual exports of phosphorus from each of the eight reference rivers

were computed for 1980-90 by Robertson and Saad (1996) using constituent-transport models based on relations between the water-quality-constituent load and two variables: stream discharge and day of the year (Cohn and

others, 1989). Phosphorus export was found to be primarily related to

land use and secondarily related to the type of surficial deposits. No data

were available for completely urbanized areas for this extended period.

Export of total phosphorus ranged from 3.13 kg/ha in White Creek (small

agricultural reference basin) and 0.40 to 0.46 kg/ha in large agricultural

reference basins to less than or equal to 0.1 kg/ha in forested reference basins

of any size (table 1). Of the agricultural reference basins, the basin with the

highest percentage of fine-grained deposits (clay) had the highest phosphorus export rate (Manitowoc River had the highest percentage of clay in the

basin (fig. 2) and had the highest export rate, 0.46 kg/ha). During high runoff

years, export rates increased significantly, especially in the Manitowoc

River; whereas during low runoff years, export rates were significantly

reduced in all agricultural and small forested reference basins, but not

significantly reduced in large forested basins (Robertson and Saad, 1996).

The calculated export rates are similar to those compiled from studies done

throughout Wisconsin by Panuska and Lillie (1995). Panuska and Lillie

found average annual export rates of phosphorus for forested basins ranged

from 0.04 to 0.88 kg/ha (median value of 0.52 kg/ha) and for agricultural

basins ranged from 0.15 to 4.01 kg/ha (median value of 1.04 kg/ha).

0 25 50 MILES

0 25 50 KILOMETERS

89°

46°

88°

45°

44°

43°

87°

Direct Lake Michigan


Subunit

Green Bay

Lake

Michigan

Northern-


Forested Subunit

Oconto-


Peshtigo Subunit

Lake


Winnebago

Fox-Wolf-


Pensaukee 


Subunit

1

8

2

3

4

5

6

7

Sand/sand and gravel


Loam/sand and gravel


Loam


Clay/sand


Clay


Mixed


Water

2

Subunit boundary


Reference basin


boundary


Sampling site on reference


river (see table 1)


Texture of surficial deposit

EXPLANATION
Figure 3. Phosphorus

inputs (including input

into Lake Michigan and

Green Bay) and exports

from subunits. (In each

subunit the darker

shades represent

reference basins and

the lighter shades

represent unmonitored

areas. All inputs and

exports are in kilograms

per hectare; reference

rivers are identified in

table 1.)

Regional Transport of Phosphorus and

Importance of Point Sources

The export rates from the six large reference basins were used to estimate

the total phosphorus transported from the four subunits and the entire

WMIC study unit. This extrapolation process was based on making one of

two assumptions regarding the export of phosphorus from point sources. In

the first assumption, all phosphorus from point sources is exported out of the

basin and study unit. Therefore, export rates from nonpoint sources were

computed by subtracting point-source contributions. In the second assumption, phosphorus inputs from point sources within the study unit are treated

as other nonpoint sources and are exported at a rate proportional to its

percentage of total input. Therefore, only point sources that are discharged

directly to Lake Michigan and Green Bay are completely exported out of the

study unit. For example, in the Direct Lake Michigan subunit, the additional

phosphorus from point sources in the basin represents 0.2 percent of the total

input and therefore represents 0.2 percent of the export. These assumptions

represent the extremes in how point sources can be considered. The export

of phosphorus from unmonitored areas of each subunit was estimated by

determining how much of the unmonitored area resembled each of the

reference rivers in the subunit (in terms of similar land use and surficial

deposits) and then multiplying the export rates of each of the six large

reference basins by the total area each represents (“effective area” in table

1). Therefore, each of the six large reference rivers is used to estimate the

total export from an area larger than its own drainage area. For a more

detailed description of this extrapolation process see Robertson and Saad

(1996). An annual export rate of 0.18 kg/ha was used to estimate the

phosphorus transported from the Oconto-Peshtigo subunit. This subunit

had no reference basins within it; therefore this rate was computed by

weighting the rates of the Fox and Menominee Rivers by the percentage of

agriculture in each basin.

With the first assumption, 100 percent of the phosphorus input from

point sources is exported from the study unit. For most of the reference

basins, point-source contributions upstream from the sampling site are very

small and represent less than 15 percent of the export, except for the Fox

River where point sources within the basin may represent 27 percent of the

export (table 1). More than half of the phosphorus contributed by point

sources is input directly into Green Bay or Lake Michigan. If 100 percent

of the point sources is exported, then in an average year 1,421,000 kg of

phosphorus would be exported out of the WMIC study unit; the overall

annual export rate would be 0.28 kg/ha (0.20 kg/ha from nonpoint sources

plus 0.08 kg/ha from point sources) (table 1). With this assumption, point

sources contribute 27.5 percent of the total export (fig. 4). If point sources

directly input into Lake Michigan and Green Bay are neglected, then the

overall export rate would be 0.24 kg/ha, and point sources would contribute

15.0 percent of the total export.

If point sources (except those directly discharged to Lake Michigan and

Green Bay) are treated as nonpoint sources and are incorporated into the

export rates computed for each reference basin, then point sources generally

represent less than 2 percent of the total input into any reference basin or

subunit, and therefore, contribute less than 2 percent of that exported. By

this method, in an average runoff year, the total phosphorus exported from

the WMIC study unit again would be 1,421,000 kg (fig. 4). The overall

annual export rate would be 0.28 kg/ha (0.23 kg/ha contributed by nonpoint

sources plus 0.05 kg/ha contributed by point sources). Of the 0.05 kg/ha

contributed by point sources, 0.04 kg/ha is directly input by point sources

to Lake Michigan and Green Bay. Point sources within the basins are very

small and would contribute approximately 0.01 kg/ha. With this assumption, point sources contribute about 15.3 percent of the total export, almost

all of which is directly input into Lake Michigan and Green Bay (fig. 4).

In an average runoff year, the total export of phosphorus

from the WMIC study unit is estimated to be about 1,400,000

kg. Point sources contribute between about 15 percent (if it is

assumed that only point sources input directly into Green Bay

and Lake Michigan are 100 percent exported) and 28 percent

(if it is assumed that 100 percent of all point sources are

exported from the study unit) of the total phosphorus exported.

Green Bay

Lake

Michigan

89°

46°

88°

45°

44°

43°

87°

0 25 50 MILES

0 25 50 KILOMETERS

Fox-Wolf-Pensaukee Subunit

0

1

2

3

4

5

6

7

0.0

0.1

0.2

0.3

0.4

0.5

INPUTS

EXPORT

Oconto-Peshtigo Subunit

0

1

2

3

4

5

6

7

0.0

0.1

0.2

0.3

0.4

0.5

Fertilizers Manure Atmosphere Point

Sources

Export

INPUTS

EXPORT

Northern-Forested Subunit

0

1

2

3

4

5

6

7

0.0

0.1

0.2

0.3

0.4

0.5

INPUTS

EXPORT

Direct Lake Michigan Subunit

0

1

2

3

4

5

6

7

0.0

0.1

0.2

0.3

0.4

0.5

INPUTS

EXPORT

Fertilizers Manure Atmosphere Point

Sources Export

Fertilizers Manure Atmosphere Point

Sources Export

Fertilizers Manure Atmosphere Point

Sources

Export

Lake


Winnebago

1

8

2

3

4

5

6

7

2

Subunit boundary


Reference basin boundary


River


Sampling site on reference river

EXPLANATION
Figure 4. Total annual inputs to (including that input into Lake Michigan and Green

Bay) and export of phosphorus from the Western Lake Michigan Drainages and

allocation of export to that attributed to point and nonpoint sources.

a 100 percent of all point sources are exported out of study unit.

b All point sources within study unit exported at a rate proportional to its percentage of total inputs,

except that released into Lake Michigan and Green Bay which are all exported.

c Number in parentheses does not include that phosphorus released directly into Green Bay and

(or) Lake Michigan.

Table 1. Average annual inputs and export of phosphorus and the importance of point source contributions from reference basins, subunits, and the

Western Lake Michigan Drainages Study Unit [locations of rivers are shown in figures 1, 2, and 3; kg/ha, kilograms per hectare; km2, square kilometers; Agric.,

agriculture; Fert., fertilizers; Atm. Dep., atmospheric deposition; %, percent; LM/GB, Lake Michigan/Green Bay; NA, not applicable]

For more information, please contact:

NAWQA Chief

U.S. Geological Survey

6417 Normandy Lane

Madison, WI 53719

(608) 276-3810

www: http://wwwdwimdn.er.usgs.gov

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet FS-208-96

Layout and illustrations: Michelle Greenwood

Banner graphic: Karen Lonsdorf

Northern-Forested 18,800 NA 88 8 0.50 0.31 0.05 0.01 0.87 0.00 0.08 9.3 0.9

Escanaba (1) 2,250 5,570 91 4 .57 .33 .05 .00 .95 .00 .06 .0 .0

Ford (2) 1,170 2,690 93 7 .35 .19 .05 .00 .59 .00 .07 .0 .0

Menominee (3) 10,180 13,910 90 6 .33 .21 .05 .01 .60 .00 .10 13.2 2.1

Popple (4) 430 0 96 2 .08 .09 .05 .00 .22 .00 .06 .0 .0

Oconto-Peshtigo 5,630 NA 71 25 2.13 1.38 .11 .01 3.63 .00 .18 3.9 .2

Fox-Wolf-Pensaukee 17,689 NA 35 53 5.26 3.94 .20 .08 9.48 .01 .32(.31)c

30.2(27.3)c4.8(9.3)c

Fox (5) 15,570 19,940 37 51 5.26 3.94 .20 .08 9.48 .00 .31 27.1 .9

White Creek (6) 8 0 0 100 11.09 8.83 .20 .00 20.11 .00 3.13 .0 .0

Direct Lake Michigan 9,430 NA 13 73 6.08 5.04 .20 .02 11.33 .20 .64(.44)c

33.5(3.7)c31.1(.1)c

Manitowoc (7) 1,360 6,350 13 83 6.73 7.22 .20 .01 14.15 .00 .46 1.6 .1

Milwaukee (8) 1,800 3,080 12 75 6.43 5.14 .20 .04 11.82 .00 .40 10.7 .4

Western Lake Michigan 51,500 NA 54 37 3.34 2.54 .14 .04 6.05 .04 .28(.24)c

27.5(15.0)c15.3(.6)c

References

Alexander, R.B., and Smith, R.A., 1990, County-level estimates of

nitrogen and phosphorus fertilizer use in the United States: U.S.

Geological Survey Open-File Report 90-130, 12 p.

Clesceri, N.L., Curran, S.J., and Sedlak, R.I., 1986, Nutrient loads to

Wisconsin lakes, Part I. Nitrogen and phosphorus export coefficients: Water Resources Bulletin, v. 22, no. 6, p. 983–990.

Cohn, T.A., DeLong, L.L., Gilroy, E.J., Hirsch, R.M., and Wells, D.K.,

1989, Estimating constituent loads: Water Resources Research,

v. 25, no. 5, p. 937–942.

Hirsch, R.M., Alley, W.M., and Wilber, W.G., 1988, Concepts for a

National Water-Quality Assessment program: U.S. Geological

Circular 1021, 42 p.

Murphy, T.J., 1974, Sources of phosphorus inputs from the atmosphere

and their significance to oligotrophic lakes: University of Illinois

at Urbana-Champaign Water Resources Research Center, Research Report 92, 45 p.

Panuska, J.C., and Lillie, R.A., 1995, Phosphorus loadings from

Wisconsin watersheds—recommended phosphorus export coefficients for agricultural and forested watersheds: Wisconsin Department of Natural Resources Research Management Findings, 38,

Publ-RS-738, 8 p.

Robertson, D.M., and Saad, D.A., 1996, Water-quality assessment of

the Western Lake Michigan Drainages—analysis of available

information on nutrients and suspended sediment, water years

1971–90: U.S. Geological Survey Water-Resources Investigation

Report 96-4012, 165 p.

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

Balsam Lake, northwestern Wisconsin: U.S. Geological Survey

Water-Resources Investigations Report 91-4125, 28 p.

U.S. Environmental Protection Agency, 1980, Modeling phosphorus

loading and lake response under uncertainty. A manual and compilation of export coefficients: Washington D.C., Office of Water

Regulations and Standards Criteria and Standards Division, EPA

440/5-80-011.

Basin/Subunit/Study Unit

(identifier number from

figures 1–3 in parentheses)

Point source

contribution

(percent of total)

Area

(km2

)

Percent

Forest Fert. Manure AtmDep.

Basin

point

sources

Total

inputs

to basin

Point

sources

to LM/GB (kg/ha) 100%

Exporta

LM/GB

Exportb Percent

Agric.

Phosphorus inputs (kg/ha) Phosphorus

export

Effective

area

(km2)

Annual inputs of phosphorus to the Western Lake Michigan Drainages


Total = 31,000,000 kilograms (6.09 kg/ha)

Allocation of total export from the Western Lake Michigan Drainages


Total = 1,421,000 kilograms (0.28 kg/ha)

100 percent of point sources 


exported


Point sources within the basin are treated as 


other nonpoint sources and are exported at a 


rate proportional to its percentage of total input. 


Only inputs to Lake Michigan and Green Bay


are 100 percent exported.


Point sources to 


Lake Michigan 


and Green Bay

Point sources in basin

INPUTS

EXPORT

Assumption 1: Assumption 2:

Fertilizers


(54.8%)

Manure


(41.7%)

Point sources (1.2%)

Atmosphere (2.2%)

Nonpoint


(72.5%)

(14.8%)

(12.7%)

Nonpoint


(84.7%)

(14.8%)

(0.5%)