Annual Nutrient Loadings, 

Primary Productivity, and 

Trophic State of Lake Koocanusa, 

Montana and British Columbia,

GEOLOGICAL SURVEY PROFESSIONAL PAPER izsa

Prepared for the s t/.S. Army Corps of 

Engineers, Seattle District

Annual Nutrient Loadings, 

Primary Productivity, and 

Trophic State of Lake Koocanusa, 

Montana and British Columbia, 1972-80

By PAUL F. WOODS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1283

Prepared for the U.S. Army Corps of 

Engineers, Seattle District

UNITEDSTATESGOVERNMENTPRINTINGOFFICE, WASHINGTON : 1982
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Woods, Paul F.

Annual nutrient loadings, primary productivity, and trophic state of Lake Koocanusa, Montana and British Columbia, 1972-80.

(Geological Survey Professional Paper 1283)

Bibliography: 24 p.

Supt. of Docs. No.: I 19.16: 1283

1. Primary productivity (Biology) Koocanusa, Lake (B.C. and Mont.) 2. Eutrophication Koocanusa, Lake (B.C. and Mont.) 3. Water

quality Koocanusa, Lake (B.C. and Mont.) 4. Koocanusa, Lake (B.C. and Mont.) 

I. United States Army Corps of Engineers. Seattle, Washington, District. II. Title. III. Series. 

QH105.M9W66 1982 574 82-600264
CONTENTS

Page

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

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

Regional setting ................................................... 3

Hydrology ....................................................... 4

Nutrient loadings .................................................. 5

Methodology .................................................. 6

Influent loadings ............................................... 7

Loadings discharged from Lake Koocanusa ............................ 8

Nutrient retention in Lake Koocanusa ................................ 8

Primary productivity ............................................... 10

Methodology .................................................. 10

Seasonal trend ................................................. 10

Annual trend .................................................. 12

Relationship of annual areal primary productivity and selected environmental variables . 12

Chlorophyll a distribution within the water column ...................... 18

Trophic state of Lake Koocanusa ...................................... 20

Conclusions ...................................................... 22

References ...................................................... 24

ILLUSTRATIONS

Page

FIGURE 1. Map showing location of Kootenai River drainage basin and Lake Koocanusa .......................... 2

2-4. Graphs showing:

2. Monthly and annual inflow to and releases from Lake Koocanusa, 1972-80 ......................... 5

3. Daily variation in surface altitude of Lake Koocanusa, 1972-80 .................................. 6

4. Daily values of areal primary productivity at the four limnological stations, Lake Koocanusa, 1972-80 ...... 13

5. Scatterplots of annual areal primary productivity versus maximum and mean annual euphotic zone depths, annual

areal phosphorus loading, and annual nitrogen retention coefficient for Lake Koocanusa, 1972-80 ........... 16

6. Graph showing annual variation of mean and maximum euphotic zone depths, areal

primary productivity, areal phosphorus loading, and nitrogen retention coefficient for Lake Koocanusa, 1972-80 . 17

7. Typical diagram used to quantify the percentage distribution of chlorophyllawithin the water column of Lake Koocanusa. . 19

8. Graph showing application of Lake Koocanusa data in table 17 to Vollenweider total phosphorus loading and mean

depth/hydraulic-residence time diagram ..................................................... 22

9. Graph showing application of Lake Koocanusa data in table 17 to Vollenweider total nitrogen loading and mean

depth/hydraulic-residence time diagram ..................................................... 23

III
IV CONTENTS

TABLES

Page

TABLE 1. Annual precipitation at climatological station 1 and annual mean air temperature at climatological station 2, 1972-80 . 3

2. Volume and surface area of Lake Koocanusa at selected reservoir surface altitudes ...................... 4

3. Lake-filling and hydraulic-residence times of Lake Koocanusa, 1972-80 ................................ 6

4. Annual loadings of total phosphorus that entered Lake Koocanusa, 1970-80 ............................ 7

5. Annual loadings of total nitrogen that entered Lake Koocanusa, 1970-80 .............................. 8

6. Comparison of 1973-78 loadings of total phosphorus (TP) and total nitrogen (TN) at two stations on the St.

Mary River, one upstream and one downstream from the fertilizer plant at Kimberley, British Columbia ..... 8

7. Annual loadings of total phosphorus (TP) and total nitrogen (TN) discharged from Lake Koocanusa, 1970-80 .... 9

8. Nutrient-retention coefficients for total phosphorus and total nitrogen in Lake Koocanusa, 1972-80 ............ 9

9. Statistical summary of daily areal primary productivity and euphotic zone depth at the four limnological stations, Lake

Koocanusa, 1972-80 ................................................................... 11

10. Statistical summary of daily areal primary productivity in Lake Koocanusa, 1972-80 ...................... 12

11. Annual areal primary productivity in Lake Koocanusa, 1972-80 ..................................... 12

12. Statistical summary of environmental variables used in correlation and multiple regression analyses of annual areal

primary productivity in Lake Koocanusa, 1972-80 ............................................. 14

13. Correlation coefficients between annual areal primary productivity and environmental variables XI through X12 . . 14

14. Multiple regression models for prediction of annual areal primary productivity in Lake Koocanusa ............ 15

15. Multiple regression model developed by Woods (1979) for prediction of daily areal primary productivity in Lake

Koocanusa ......................................................................... 18

16. Percentage distribution of chlorophyll a in three depth strata at the Forebay and Tenmile stations in 1980 ..... 20

17. Required data for using Vollenweider nutrient loading models to determine the trophic state of Lake Koocanusa . 21

METRIC CONVERSION TABLE

To convert International System (SI) of units in this report to inch-pound units, multiply by the

following factors:

Multiply SI unit By To obtain inch-pound unit

cubic kilometer (km3) 8. 107 x 105 acre-foot

cubic meter per second (m^s'1 ) 3.531 x 101 cubic foot per second

gram(g) 2.205 xlO"3 pound

gram per square centimeter 2.049 pound per square foot

(g-crrr2)

gram per square meter (g-m~2) 2.049x 10"4 pound per square foot 

gram per square meter per year 2.049x 10"4 pound per square foot per year (g-m-2-a-1 )

hectare (ha) 2.471 acre 

kilogram per hectare per year 8.922x 10" 1 pound per acre per year

(kg- ha^-a' 1 )

kilometer (km) 6.214 xlO" 1 mile 

lux (lx) 9. 290 x lO'2 foot-candle 

meter (m) 3.281 foot 

megagram (Mg) 1. 102 ton (short) 

megagram per cubic kilometer 1.359x 10"6 ton (short) per acre- foot

(Mg-knr3)

milligram (mg) 2.205xlO'6 pound 

milligram per cubic meter 6.245x 10"8 pound per cubic foot

(mg-m-3) 

milligram per square meter per day 2.049 pound per square foot per day

millimeter (mm) 3.937xlO"2 inch

square kilometer (km2) 3.861X10"1 square mile

Temperature in degrees Celsius (°C) can be converted to degrees Fahrenheit (°F) by the following

equation:

°F = 1.8x°C + 32

National Geodetic Vertical Datum of 1929 (NGVD of 1929): A geodetic datum derived from 

a general adjustment of the first-order level nets of both the United States and Canada, formerly 

called "mean sea level." NGVD of 1929 is referred to as sea level in this report.
ANNUAL NUTRIENT LOADINGS, PRIMARY PRODUCTIVITY, AND

TROPHIC STATE OF LAKE KOOCANUSA, 

MONTANA AND BRITISH COLUMBIA, 1972-80

By PAUL F. WOODS

ABSTRACT

Limnological data collected at Lake Koocanusa were used to investigate the relationship of nutrient loadings, primary productivity, and 

trophic state of the reservoir during 1972-80. The reservoir, on the 

Kootenai River, was impounded by Libby Dam on March 21, 1972. 

Manipulation of the 7.16-cubic-kilometer reservoir for flood control, 

its primary function, created large fluctuations in reservoir volume 

and produced annual lake-filling times that ranged from 0.14 to 0.66 

year.

Loadings of nitrogen and phosphorus prior to and following impoundment of Lake Koocanusa were found to be large enough to 

predict eutrophic conditions. Beginning in 1976, total phosphorus 

loadings, but not total nitrogen loadings, were substantially reduced 

following improvements in waste-water treatment at a fertilizer plant 

located upstream from the reservoir. The closure of Libby Dam substantially reduced loadings of nitrogen and phosphorus downstream 

from Lake Koocanusa. On the average, the reservoir retained 63 percent of its influent loading of total phosphorus and 25 percent of its 

influent loading of total nitrogen.

Daily areal and volumetric primary productivity varied widely in 

each year at four sampled limnological stations. During the 9 years 

studied, daily areal primary productivity, in milligrams of carbon 

fixed per square meter, ranged from 0.4 to 420.0; the mean of the 

313 sampled days was 128.5. Annual areal primary productivity 

ranged from 23.2 to 38.5 grams of carbon fixed per square meter 

and thereby categorized Lake Koocanusa as oligotrophic.

The relationship of annual areal primary productivity and 12 

selected environmental variables was determined by multiple regression analysis. One of the models that was derived used two variables annual euphotic zone depth and annual areal phosphorus loading and accounted for 62.0 percent of the variation in annual areal 

primary productivity.

The distribution of chlorophyll a within the water column indicated 

that, on the average, more than one-half of the phytoplankton in 

the reservoir was beneath the euphotic zone. These results support 

the hypothesis that the reservoir's weak thermal structure had allowed circulation of phytoplankton out of the euphotic zone.

The trophic state of Lake Koocanusa was categorized as eutrophic 

when based on the relationship of the nutrient loadings and the reservoir's ratio of mean depth to hydraulic-residence time. This result 

conflicted with the oligotrophic ranking the reservoir received based 

on its areal primary productivity. The discrepancy in trophic state 

was attributed mainly to the failure of nutrient loading models to 

adequately account for physical processes within reservoirs. Part of 

the nutrient loading that entered Lake Koocanusa was unavailable 

to phytoplankton because the nutrients were carried beneath the 

euphotic zone by large volumes of interflow and underflow. Another

part of the nutrient loading was adsorbed to suspended sediment and 

removed from the water column. Thus, phytoplankton primary productivity was controlled not only by nutrients, but also by other limnological processes.

INTRODUCTION

Libby Dam was constructed as part of a treaty between the United States and Canada to cooperatively 

develop the water resources of the Columbia River 

drainage basin. Construction of the dam on the 

Kootenai (spelled Kootenay in Canada) River began in 

1966, and Lake Koocanusa was officially impounded on 

March 21, 1972. The reservoir has a volume of 7.16 

km3 and provides flood storage, hydroelectric power 

production, and recreation benefits. The 148-km-long 

reservoir straddles the United States-Canadian border 

and impounds water from about 23,271 km2, or 47 percent, of the Kootenai River drainage basin (fig. 1). 

Three Canadian rivers, the Kootenai, Elk, and Bull, 

supply 87 percent of the reservoir's inflow (Bonde, 

1979) and, therefore, exert a major influence on the 

limnology of Lake Koocanusa.

Preimpoundment water-quality studies indicated the 

presence of large concentrations of total phosphorus, 

orthophosphorus1 , and total nitrogen in the Kootenai 

River, which were attributed to industrial point-source 

discharges in the Canadian part of the drainage basin 

(Bonde and Bush, 1975). Using the preimpoundment 

data, these authors calculated that the reservoir's surface would receive areal loadings of 10 g-m"2 of total 

phosphorus and 20 g-m"2 of total nitrogen. Areal nutrient loadings have been used in conjunction with mean 

depth and hydraulic residence time to estimate a water 

body's susceptibility to eutrophication (Vollenweider,

'Rigler (1973) reported that methodological problems preclude determination of the actual 

concentration of orthophosphate-phosphorus in natural water samples . He suggested use 

of the term "soluble reactive phosphorus" as a more accurate operational definition. However, the term orthophosphorus is retained for use in this report because it is the terminology 

currently used by the Geological Survey.

1
NUTRIENT LOADINGS, PRODUCTIVITY, AND TROPHIC STATE OF LAKE KOOCANUSA, 1972-SO

1968, 1975). Application of Vollenweider's methods to 

Lake Koocanusa by Bonde and Bush (1975) indicated 

that annual areal loadings of 2.0 g-m"2 of total phosphorus and 8.0 g-m of total nitrogen would be sufficient 

to cause concern for eutrophication of the reservoir. 

These values were substantially less than the predicted

118°

ALBERTA

BOUNDARY 

PINKHAM

TENMILE 

FORE BAY 

LIBBY DAM

EXPLANATION

LIMNOLOGICAL STATION 

AND NAME

STREAMFLOW-SAMPLING 

STATION AND NUMBER

O CLIMATOLOGICAL STATION 

AND NUMBER

48°

50 KILOMETERS

30 MILES

FIGURE 1. Location of Kootenai River drainage basin and Lake Koocanusa.
REGIONAL SETTING

annual loadings; therefore, Bonde and Bush considered 

Lake Koocanusa to be susceptible to eutrophication. 

However, they cautioned that prediction of the actual 

response of the reservoir to such nutrient loadings 

would be difficult.

Bonde and Bush (1975) reported that blooms of 

Aphanizomenon flos- aquae, a blue-green alga characteristic of eutrophic waters, occurred in Lake 

Koocanusa in the autumns of 1974 and 1975 and considered these symptomatic evidence of eutrophication. 

However, Woods (1979) reported that annual primary 

productivity in the reservoir during 1972 through 1975 

was characteristic of oligotrophic, not eutrophic, water 

bodies. Preliminary evaluations by Hobbie (1976) and 

Bush and Bonde (1977) revealed that primary productivity in Lake Koocanusa was predominantly controlled 

by physical limnological processes. Additional studies 

by Woods (1979, 1981) attributed the reservoir's oligotrophy to suppression of phytoplankton photosynthesis 

by circulation of phytoplankton out of the euphotic zone.

Subsequent to the 1972-75 period evaluated by 

Woods (1979), waste-water treatment was improved at 

the industrial point-source discharges in Canada. Beginning in 1976, these improvements resulted in substantial reductions in loadings of total phosphorus delivered into Lake Koocanusa (Bonde, 1979).

Although the limnological data collected during 1972 

through 1975 have been evaluated (Woods, 1979, 1981; 

Woods and Falter, 1982), little such work has been done 

on the post-1975 data. The U.S. Army Corps of Engineers, therefore, requested that the U.S. Geological 

Survey further evaluate some selected data. The Corps 

of Engineers was primarily interested in furthering 

their understanding of the limnological processes that 

control the trophic state of Lake Koocanusa. Annual 

nutrient loadings and annual areal primary productivity 

were specified for intensive evaluation, because both 

have been extensively used to classify the trophic state 

of lakes and reservoirs.

The purpose of this resultant report is to categorize 

the trophic state of Lake Koocanusa based on annual 

nutrient loadings and annual areal primary productivity. Seasonal and annual trends in primary productivity 

are described, as well as annual trends in loadings of 

nitrogen and phosphorus. Control of annual areal primary productivity by selected limnological processes in 

Lake Koocanusa is evaluated in relation to results derived from multiple regression analyses and an analysis 

of the distribution of chlorophyll a within the water 

column. The report addresses the 1972-80 postimpoundment phase and is based on preimpoundment data collected after 1967 and postimpoundment data collected 

from 1972 through 1980.

REGIONAL SETTING

Lake Koocanusa lies within the Kootenai River drainage basin, an area of 49,987 km2 occupying parts of 

Idaho, Montana, and British Columbia. Altitudes in the 

basin range from 3,618 m above sea level at Mt. Assiniboine to 418 m at the confluence of the Kootenai 

and Columbia Rivers near Castlegar, British Columbia. 

The surface of Lake Koocanusa at maximum pool is at 

an altitude of 749.50m.

The Kootenai River, the principal tributary to Lake 

Koocanusa, arises 64 km west of Banff, Alberta, and 

flows southward to the Purcell Mountains. Near the 

Purcells, the river is impounded by Libby Dam 4.5 km 

upstream from the Fisher River. The Kootenai River 

eventually enters Kootenay Lake, and thence the Columbia River near Castlegar, British Columbia.

Between Canal Flats, British Columbia, and the Tobacco River, the Kootenai River flows along the Rocky 

Mountain Trench a graben having a floor about 11 km 

wide and a vertical displacement of several hundred 

meters. The trench is partly filled with Pleistocene and 

Holocene deposits of unconsolidated silt, sand, and 

gravel into which the Kootenai River has incised about 

60 to 90 m (Coffin, 1970). Except for the relatively flat 

terrain of the Rocky Mountain Trench, the topography 

upstream from Libby Dam is dominated by rugged 

mountains that are densely forested with conifers.

Climatically, the Kootenai River basin is subjected 

to maritime influences in the winter and continental influences in the summer. These influences, in conjunction with the mountainous terrain, result in complex 

weather patterns. The Rocky Mountain Trench receives

TABLE 1. Annual precipitation at climatological station la and 

annual mean air temperature at climatological station 2b , 1972-80

Annual precipitation, in millimets

Annual mean

Total Departure from normal in degrees Celsius

1972

1973

1974

1975

1976

1977

1978

1979

1980

485.6

357.4

460.2

527.6

354.3

403.1

424.4

311.7

551.9

-7.2

-135.4

-32.6 

+ 34.8

-138.5

-89.7

-68.4

-181.1 

+ 59.1

6.2 

6.6 

7.4 

6.1 

7.0 

7.2 

6.2 

7.1 

7.1

a Climatological station 1 is "Libby 1 NE Ranger Station" in

reports of the U.S. Department of Commerce (issued annually).

b Climatological station 2 is "Libby Dam" in reports of the

U.S. Department of Commer

long 

s of

^-Departure f 

492.8 mm for 83 y

(issued annually).

L average precipitation of
NUTRIENT LOADINGS, PRODUCTIVITY, AND TROPHIC STATE OF LAKE KOOCANUSA, 1972-80

an average of 400 mm of precipitation per year and 

is considered semiarid, whereas the Rocky Mountains 

annually receive an average of 1,000 mm of precipitation (Water Resources Service, 1976). About 70 percent 

of the basin's precipitation falls as snow during 

November through March (Bonde and Bush, 1975). At 

<climatological station 1 (fig. 1), precipitation has been 

recorded for 83 years and temperature for 73 years 

(U.S. Department of Commerce, issued annually). 

Based on these data, the mean annual precipitation near 

Libby Dam is 492.8 mm and the mean annual temperature is 7.3°C.

Annual precipitation and annual mean air temperatures recorded at climatological stations 1 and 2 (fig. 

1) near Libby Dam during 1972-50 are listed in table 

1. Precipitation ranged from 311.7 to 551.9 mm. Except 

for 1975 and 1980, precipitation was less than normal 

over the 9 years, particularly in 1973, 1976, and 1979. 

Yearly variation in air temperature was small, ranging 

from 6.1° to 7.4°C, but the monthly values published 

by the U.S. Department of Commerce (issued annually) 

varied widely.

HYDROLOGY

The predominant flood-control function of Lake 

Koocanusa necessitates substantial reductions in volume during the autumn and winter to provide adequate 

capacity for spring snowmelt runoff. This operational 

schedule produces a wide range of reservoir volume and 

surface area (table 2). At maximum pool, the reservoir 

contains 7.16 km3 and has a surface area of about 188 

km2. A drawdown of 52.4 m to minimum operational 

pool reduces the volume to 1.08 km3 and the surface

TABLE 2. Volume and surface area of Lake Koocanusa at 

selected reservoir surface altitudes

Surface 

altitude, 

in meters

Volume, ir

cubic 

kilometers

a 749,

740,

730,

720,

710,

700,

b 697.

690,

680.

C 671,

.50

.00

.00

.00

,00

.00

.08

,00

.00

. 17

7.16

5.52

4.08

2.91

1.99

1.26

1.08

.71

.34

.14

100.0

77.1

57.0

40.6

27.8

17 .6

15.1

9.9

4.7

2.0

188.:

159.4

131.5

103.2

80.9

64.8

59.1

46.5

27.5

16.2

100.

84.

69.

54.

43.

34.

31.

24.

14.

8.

,0

.7

.9

.8

,0

,4

,4

,7

.6

,6

Maximum pool.

bMinimum operational pool. 

°Dead storage.

area to 59.1 km2 .

Annual inflow to the reservoir exceeded or equaled 

annual outflow from Libby Dam except in 1977 and 

1979, the 2 years in which inflow was about one-half 

of normal (fig. 2). Maximum inflow occurred in May 

or June, whereas, in general, outflow was largest during reservoir drawdown. This cyclic pattern of inflow 

and outflow produced large fluctuations in reservoir volume and surface altitude (fig. 3). The reservoir surface 

generally was at its yearly maximum altitude in July, 

August, and September and at its minimum during 

March and April.

The reservoir attained its maximum surface altitude 

for less than 7 months during the 9 years graphed in 

figure 3. Three months was the longest period in which 

the reservoir surface was at a constant altitude. Most 

of the 9 years was characterized by a constantly changing reservoir surface altitude and, hence, volume.

Reservoir volume, inflow, and outflow can be related 

mathematically as lake- filling time and hydraulic-residence time. Lake-filling time represents the time required to replace the volume of a reservoir at a given 

inflow (Daley and others, 1981); whereas hydraulic-residence time represents the time required to replace the 

volume of a reservoir at a given outflow (Vollenweider, 

1975). The two are calculated by the following equations:

« = f a)

F = I ®

where

R is lake-filling time, in years; 

F is hydraulic-residence time, in years; 

V is reservoir volume, in cubic kilometers; 

7 is annual inflow, in cubic kilometers; and 

0 is annual outflow, in cubic kilometers.

The theoretical lake-filling time of Lake Koocanusa 

is 0.67 year, based on a reservoir volume of 7.16 km3 

and an assumed annual inflow of 10.65 km3 . However, 

the reservoir experienced a large range in volume, inflow, and outflow from 1972 through 1980, which yielded 

annual lake-filling times that ranged from 0.14 

to 0.66 year and annual hydraulic-residence times that 

ranged from 0.14 to 0.62 year (table 3). In addition to 

the annual variation, the reservoir exhibited monthly 

variation in volume, inflow, and outflow. Lake-filling 

and hydraulic-residence times, therefore, were calculated using monthly data instead of annual data. The 

results, in table 3, indicated a wide range in values. 

For example, the mean monthly lake filling time in 1976 

was 0.71 year, but during that year, the value ranged 

from 0.13 to 1.54 years. The minimum value resulted
NUTRIENT LOADINGS

12.87 8.38 13.91

ANNUAL INFLOW, IN CUBIC KILOMETERS 

10.09 . 11.68 . 6.21 10.17 6.95 9.48

12.71'

ANNUAL RELEASES, IN CUBIC KILOMETERS 6

.9 ' 09. A1 '.6.8. .T'88 .9.'P4.

7.45 8.49

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

1972 1973 1974 1975 1976 1977 1978 1979 1980

Calculated from change in contents

Measured at streamflow-sampling station 2, 

located I.I kilometers downstream from Libby Dam

April through December

FIGURE 2. Monthly and annual inflow to and releases from Lake Koocanusa, 1972-80.

from springtime snowmelt runoff entering a partly 

drawndown reservoir in May; the maximum lake-filling 

time of 1.54 years occurred in December as small inflow 

entered a large reservoir volume.

NUTRIENT LOADINGS

The relationship between nutrient loadings and the 

trophic state of a lake or reservoir has been extensively 

studied and was reviewed in recent reports by East 

and Lee (1978) and Reckhow (1979). A model developed 

by Vollenweider (1968, 1975) was discussed in these two 

reports. This model, based on areal loadings of total 

phosphorus or total nitrogen, mean depth, and hydraulic-residence time, plots a point onto a graph from which 

the trophic state of the lake or reservoir may be estimated. In Lake Koocanusa, loadings of both nutrients

were large enough to produce a eutrophic ranking 

(Bonde and Bush, 1975).

The source of much of the phosphorus loading to Lake 

Koocanusa was a fertilizer plant near Kimberley, 

British Columbia (Bonde and Bush, 1975). Daley and 

others (1980) cited this plant as a major source of orthophosphate and, to a lesser degree, ammonia nitrogen 

that has entered Kootenay Lake, which is about 230 

km downstream from Libby Dam. The plant, in operation since 1953, discharged wastes to the St. Mary 

River, a tributary of the Kootenai River. Fertilizer production was doubled in 1962 and again increased in 1965. 

Water pollution control at the plant was improved in 

1969, but it was not operating optimally until 1975 

(Daley and others, 1980). The effect of the 1969 waterpollution control measures is evident in the history of 

orthophosphate loading in the Kootenai River at a station 6 km downstream from Libby Dam (streamflow-
NUTRIENT LOADINGS, PRODUCTIVITY, AND TROPHIC STATE OF LAKE KOOCANUSA, 1972-^80

samp 

thoph 

the a 

Mg(I

750 

700

750

700 

750

700 

5 750

700 

750

700 

: 750

700 

750

700

750

700 

750

700

ling station 1, fig. 1). In 1968, 3,259 Mg of orlosphate passed the station, but from 1969 to 1971, 

nnual loading of orthophosphate was less than 900 

Sonde and Bush, 1975).

~ 1972 ~

Initial impoundment / *«TO7.0 m.t.r. NV 

March 21 / \

1973

~ / 7= 708.4 mtttrt

f" , ( , t _ , , f t , j

1974 *^ ^_^^

r^^ ». .X X«72».0 mtttrt

" ^^\^ y 7=727.1 mtttrt ]

1 I I 1 1 1 I 111 F

1976 s~- ' ' - _^_^ - 

. ^"\^^ / ~X- 732. 7 mtttrt

" 1977

7=728.9 mtttrt

i i i i i i i i i i i

1978 ^--- ~~ - ^^^

*--- ^___^_^ ^^^__^s^ X=732.4 mtttrt

1979 ^^_- - __^^^

K X=733.9mtt»r»

! I 1 1 1 1 1 1 1 1 I

I960 ^" * - ^__^

TABLE 3. Lake-filling and hydraulic-residence times of Lake 

Koocanusa, 1972-80

Year Monthly Monthly

Mini- Maxi- Mini- Maxl1972 b 0.14 0.17 0.04 0.52 0.14 0.14 0.02 0.37 

1973 .22 .40 .10 .88 .33 .49 .11 1.29 

1974 .28 .61 .09 1.28 .29 .33 .13 .67 

1975 .37 .63 .11 1.13 .41 .78 .10 2.66 

1976 .38 .71 .13 1.54 .38 .55 .13 1.56 

1977 .64 .93 .26 1.64 .50 .59 .24 1.42 

1978 .43 .75 .18 1.33 .48 .63 .24 1.28 

1979 .66 1.08 .22 1.78 .62 .97 .22 2.08 

1980 .52 .94 .17 1.47 .58 .78 .29 2.07

a b d 12 hi 1 f 1972

b

METHODOLOGY

Nutrient loadings that annually entered Lake 

Koocanusa were estimated to be the sum of loadings 

from gaged streamflow, ungaged drainage area, atmospheric deposition, and the Eureka sewage-treatment 

plant. Nutrient loadings from ground water and nutrient fluxes due to changes in reservoir bank storage 

were not quantified because of a lack of data. 

The only nutrient loading estimated from gaged 

streamflow was for streamflow-sampling station 3 on 

the Kootenai River in British Columbia (fig. 1). The 

drainage area upstream from this station is 11,266 km2 

and the streamflow includes the effluent from the fertilizer plant at Kimberley. Annual nutrient loadings 

from this station were summations of daily loadings that 

are calculated as follows:

L = CxQxf (3) 

where 

L is daily loading, in megagrams; 

C is daily nutrient concentration, in milligrams per 

liter; 

Q is mean daily streamflow, in cubic meters per 

second; and 

/ is 0.08640, a factor to convert equation units to 

megagrams per day.

Daily nutrient concentrations were estimated from

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

FIGURE 3. Daily variation in surface altitude of Lake Koocanusa, 

1972-80. X denotes mean annual reservoir surface altitude, in 

meters.

measured in the gaged stream. Loadings of nutrients 

discharged from Lake Koocanusa were estimated with 

equation 3. The annual loadings of total phosphorus and 

total nitrogen that entered Lake Koocanusa prior to
NUTRIENT LOADINGS

impoundment were estimated by Bonde and Bush 

(1975) in a manner similar to equation 3.

Nutrient loading from the 12,005 km2 of drainage 

area that was ungaged was estimated by the method 

of watershed-export coefficients of Reckhow, Beaulac, 

and Simpson (1980). Sufficient data were available from 

the Bull, Elk, and Tobacco Rivers to permit calculating 

loadings per unit area. Streamflow and water-quality 

data for 1972-77 were input to equation 3 and the results were summed to derive annual loadings of total 

phosphorus and total nitrogen. The annual loadings for 

each stream then were divided by their gaged drainage 

area. The median value of the derived watershed-export coefficients for the three streams was 0.144 

kg-ha^-a'1 (range = 0.022-0.796, number of samples = 

13) for total phosphorus and 1.35 kg-ha^-a"1 

(range = 0.52-1.75, number of samples = 14) for total nitrogen. These two median watershed-export coefficients were multiplied by the ungaged drainage area 

of 1,200,502 ha to obtain annual loadings of total phosphorus and total nitrogen in kilograms. These values 

then were multiplied by 0.001 to convert them into 

megagrams.

Atmospheric deposition of total phosphorus and total 

nitrogen received by the surface of Lake Koocanusa 

was estimated using the product of the reservoir's mean 

annual surface area in hectares, a forest-atmospheric 

input coefficient in kilograms per hectare per year, and 

a 0.001 factor which yielded annual loading in megagrams. Forest-atmospheric input coefficients were 0.27

-i kg-ha'^a' 1 for total phosphorus and 0.99 kg-ha^-a 

for total nitrogen the only values cited in Reckhow, 

Beaulac, and Simpson (1980) that were judged by this 

author as geographically applicable to the study area. 

Mean annual surface area, in hectares, was derived 

from statistical analysis of daily measurements of surface altitude. Mean annual reservoir surface altitude 

was converted to surface area using area-capacity 

curves for Lake Koocanusa.

Loadings of total phosphorus and total nitrogen from 

the sewage-treatment plant at Eureka, Montana (fig. 

1), were obtained from the U.S. Environmental Protection Agency (1977). Annual loadings of 1.8 Mg of total 

phosphorus and 9.1 Mg of total nitrogen were measured 

during 1975 at the Eureka plant.

Variations in annual loadings of total phosphorus and 

total nitrogen were due, in part, to variations in annual 

streamflow. To remove this effect, each annual loading 

was divided by its respective annual streamflow to obtain a loading rate per unit volume of streamflow, expressed as megagrams per cubic kilometer of 

streamflow.

The relationship between influent loadings of total 

phosphorus and total nitrogen and the loadings of these

two nutrients discharged through Libby Dam was quantified with a nutrient-retention coefficient (Dillon and 

Rigler 1974), calculated thusly:

C = 1.0 - (4)

where

C is a nutrient-retention coefficient;

E is the mass of nutrient, in megagrams, discharged from the reservoir in a year; and

/ is the mass of nutrient, in megagrams, that entered the reservoir in a year.

INFLUENT LOADINGS

Listed in tables 4 and 5 for total phosphorus and total 

nitrogen are the annual loadings and loading rates per 

unit volume of streamflow that entered Lake Koocanusa 

via gaged streamflow, ungaged drainage area, atmospheric deposition, and the Eureka sewage-treatment 

plant. From 1972 through 1975, the influent loading of 

total phosphorus ranged from 1,188 to 1,626 Mg. A 

large reduction occurred in 1976; the 1976 loading of 

total phosphorus was 39.4 percent of that estimated for 

1975. For 1977-80, the loading of total phosphorus varied from 362 to 498 Mg. The loading rate per unit volume of streamflow for total phosphorus generally followed the same trend as the annual loadings of total 

phosphorus. The changes in total loading of total phosphorus primarily resulted from changes in loadings from

TABLE 4. Annual loadings of total phosphorus that entered Lake 

Koocanusa, 1970-80a

Loading, in megagrams

Year

1970

1971

1972

1973

1974

1975

1976

1977

1978

1979

1980

Totalb

1,905

1,924

1,188

1,626

1,485

1,304

514

362

498

416

428

Gaged inflow Atmospheric

1,905 

1,924 

1,011 2.0

1,449 2.1

1,307 3.4

1,126 3.4

336 3.7

184 3.4

320 3.7

238 3.8

250 4.0

Loading rate, in 

megagrams per cubic 

kilometer of streamflow

255.1

162.1

92.2

194.1

106.8

129.2

44.0

58.3

49.0

59.8

45.2

aPreimpoundment loadings for 1970-71 calculated for streamflow-sampling station 1,

located 6 kilometers downstream from Libby Dam. 

^Total loadings for 1972-80 include constant annual loadings of 172.6 megagrams frc

ungaged inflow and 1.8 megagrams from Eureka sewage-treatment plant.
NUTRIENT LOADINGS, PRODUCTIVITY, AND TROPHIC STATE OF LAKE KOOCANUSA, 1972-^80

TABLE 5. Annual loadings of total nitrogen that entered Lake 

Koocanusa, 1970-80"

Loading, in megagranis

Loading rate, in

Year Total b Gaged inflow Atmospheric kilometer of streamflow

1970 2,825 2,825 -- 378.3 

1971 4,057 4,057 341.8

1972 4,679 3,042 7.5 363.5 

1973 3,228 1,590 7.8 385.3 

1974 4,051 2,409 12.4 291.3 

1975 2,798 1,156 12.3 277.2 

1976 3,089 1,445 13.5 264.4 

1977 2,451 809 12.6 395.0

1978 2,891 1,248 13.4 284.4 

1979 2,707 1,063 14.0 389.5 

1980 3,258 1,613 14.6 343.6

aPre impoundment loadings for 1970-71 calculated for streamflow-sampling station

bTotal loadings for 1972-80 include constant annual loadings of 1,621 megagrams from 

ungaged inflow and 9.1 megagrams from Eureka sewage-treatment plant.

gaged streamflow, which reflected the reduced loadings 

of total phosphorus discharged by the fertilizer plant. 

Data on loadings of total phosphorus and total nitrogen 

during 1973-78 in the St. Mary River upstream and 

downstream from the fertilizer plant (streamflow-sampling stations 4 and 5, fig. 1) readily showed the reduced loading of total phosphorus that commenced in 

1976 (table 6). The loadings from the Eureka sewagetreatment plant and atmospheric deposition were too 

small to significantly alter the total loading. Ungaged 

drainage areas contributed a constant yearly load of 

172.6 Mg of total phosphorus. Variations in loadings 

from ungaged drainages may have significantly affected 

the total loading; however, such variations could not 

be estimated because of the lack of data. 

The declining trend in influent loading that was readily apparent for total phosphorus did not occur for loadings of total nitrogen. During 1972-80, influent loadings 

of total nitrogen ranged from 2,451 to 4,679 Mg; nitrogen loading was not largely reduced in 1976 (table 5) 

as was total phosphorus loading. Variations in the loading of total nitrogen were primarily due to variations 

in loading of total nitrogen in gaged inflow, because 

ungaged inflow was estimated as a constant loading and 

loadings from the Eureka sewage-treatment plant and 

atmospheric deposition were too small to significantly 

affect total loading. The loading rate per unit volume 

of streamflow for total nitrogen showed no definite 

trend (table 5). Water-pollution-control measures instituted at the fertilizer plant apparently had little effect

TABLE 6. Comparison of 1973-78 loadings of total phosphorus 

(TP) and total nitrogen (TN) at two stations on the St. Mary 

River, one upstream and one downstream from the fertilizer plant 

at Kimberley, British Columbia"

Annual loading^ In megagrams

Percentage upstream

downstream loading

TP TN TP TN TP TN 

Year

1973 8.1 175.4 1,806 532.5 22,270 303.7 

1974 15.5 26*4.1 1,671 937.3 10,770 354.9 

1975 7.9 196.1 1,215 579.4 15,340 295.4 

1976 10.5 259.0 459.4 433.4 4,388 167.3 

1977 3.6 125.0 165.2 291.3 4,538 233.0 

1978 8.9 178.1 226.5 411.2 2,548 230.9

a Loading data from James Helms (U.S. Army Corps of Engineers, written commun., 

1981).

b Streamflow-sampllng station 4 (fig. 1)

c Streamflow-sampllng station 5 (fig. 1)

on the amount of total nitrogen carried per unit volume 

of streamflow, because loadings of total nitrogen upstream and downstream from the fertilizer plant did 

not substantially change during 1973-78 (table 6).

LOADINGS DISCHARGED FROM 

LAKE KOOCANUSA

After closure of Libby Dam in March 1972, loadings 

of total phosphorus substantially declined downstream 

from the dam; reductions in loading of total nitrogen 

were not as apparent, although some reduction occurred (table 7). Loading rates per unit volume of 

streamflow for total phosphorus and total nitrogen 

showed trends similar to those for their annual loadings. The variations in loading of total phosphorus and 

total nitrogen discharged from Lake Koocanusa were 

due, in part, to the water-pollution-control measures 

instituted at the fertilizer plant, which were previously 

discussed. However, the loadings that were input at 

the upstream end of the reservoir were subjected to 

sedimentation and chemical-biological transformations 

prior to their release from Libby Dam. Because of these 

two processes, variations in loads discharged from the 

dam cannot be directly attributed to variations in loads 

from the fertilizer plant.

NUTRIENT RETENTION IN 

LAKE KOOCANUSA

The combined effects within the reservoir on influent 

loadings of total phosphorus and total nitrogen were 

estimated with nutrient-retention coefficients (table 8).
NUTRIENT LOADINGS

TABLE 7. Annual loadings of total phosphorus (TP) and total 

nitrogen (TN) discharged from Lake Koocanusa, 1970-80a

Loading, in megagrams

1970

1971

1972

1973

1974

1975

1976

1977

1978

1979

1980

1,905

1,924

997

554

706

326

359

125

82

45

50

2,825

4,057

4,004

1,702

3,378

1,876

2,529

2,359

2,229

2,201

1,626

255.1

162.1

78.5

98.5

53.2

35.9

30.8

15.9

9.0

6. 1

5.9

378.

341.

315.

302.

254.

206.

216.

299.

246.

295.

191,

,3

8

1

5

,8

3

,5

4

7

3

,5

static 

0 calc :ulated

bby Dam.

These coefficients indicate that total phosphorus was 

more readily retained in the reservoir than was total 

nitrogen. During 1972-80, an average of 63 per cent 

of influent total phosphorus was retained, whereas 25 

percent of the total nitrogen loading was retained.

The large retention of total phosphorus in Lake 

Koocanusa was partly accounted for by the tendency 

for phosphorus to adsorb to clay minerals (Golterman 

1975; Lee, 1970). Upstream from Lake Koocanusa, the 

Kootenai River traverses sedimentary and glacial deposits containing clay and silt (Crozier and Leinweber, 

1975). Eroded sediments were transported into the reservoir and were deposited; part of the influent load 

of total phosphorus was, thereby, deposited. The efficiency of sediment trapping by reservoirs was empirically related to lake-filling time by Brune (1953). Based 

on Brune's graphs and Lake Koocanusa's theoretical 

lake-filling time of 0.67 year, the sediment trapping efficiency of the reservoir exceeds 95 percent. This value 

was verified by suspended-sediment measurements at 

streamflow-sampling stations 1 and 2, located 

downstream from Libby Dam. The average annual loading of suspended sediment per cubic kilometer of 

streamflow was 131,792 Mg from 1968 through 1971 and 

9,288 Mg from 1972 through 1975. Postimpoundment 

loadings of suspended sediment, therefore, were reduced an average of 122,504 Mg, or by 93 percent, following impoundment of Lake Koocanusa.

The chemistry of phosphorus in Lake Koocanusa sediments was investigated by Iskandar and Shukla (1981). 

Their samples indicated a very small organic matter 

content, which they stated was characteristic of oligoTABLE 8. Nutrient-retention coefficients for total phosporus and 

total nitrogen in Lake Koocanusa, 1972-80

Ion coefficient

Total phosphorus Total nitrogen

1972

1973

1974

1975

1976

1977

1978

1979

1980

0.16

.66

.52

.75

.30

.66

.84

.89

.88

0.14

.47

.17

.33

.18

.04

.23

.19

.50

trophic lake sediments. The total phosphorus content of 

the silty clay sediments was composed of 87 to 98 percent inorganic phosphorus. These authors determined 

experimentally that the sediments had limited ability 

to adsorb additional phosphorus and the sediments desorbed only small amounts of phosphorus. Iskandar and 

Shukla concluded that Lake Koocanusa sediments function as a phosphorus sink. This conclusion appears 

reasonable because of the 7,821 Mg of total phosphorus 

that entered the reservoir during 1972-80, the reservoir 

discharged 3,244 Mg during the same period.

Part of the loading of total nitrogen also may have 

sorbed to clay minerals that were deposited in the reservoir. Although nitrate has little tendency to sorb to 

clay minerals (Lee, 1970), Keeney (1972) reported that 

organic nitrogen compounds readily sorb to clays and 

other inorganic colloids. During 1972-80, total nitrogen 

loading can be divided, on the average, into the following nitrogen forms: Nitrite plus nitrate, 45.6 percent; 

organic nitrogen, 43.1 percent; and ammonia, 8.2 percent. Presumably, less than 43.1 percent of the total 

nitrogen loading would have been adsorbed and deposited during 1972-80.

Further consideration of the retention of nitrogen and 

phosphorus loadings in Lake Koocanusa was difficult 

because both are nonconservative nutrients required by 

algae for photosynthesis and growth. The two nutrients 

are thereby involved in a complex sequence of uptake, 

utilization, and remineralization. For example, a bluegreen alga, Aphanizomenon flos-aquae, found in Lake 

Koocanusa has been reported by Fogg (1974) to fix atmospheric nitrogen and, thereby, may have provided 

an additional source of nitrogen to the reservoir. However, an undetermined amount of nitrogen was removed 

to the reservoir's sediment via denitrification reactions. 

Although the chemically and biologically induced processes that affect nitrogen and phosphorus within aqua-
10 NUTRIENT LOADINGS, PRODUCTIVITY, AND TROPHIC STATE OF LAKE KOOCANUSA, 1972-80

tic ecosystems are widely recognized, they are infrequently quantified. Detailed nutrient budgets that 

quantify these processes require data that were not 

available for Lake Koocanusa.

PRIMARY PRODUCTIVITY 

METHODOLOGY

Primary productivity was measured at the four limnological stations (fig. 1) commencing in late July 1972; 

sampling was discontinued at Pinkham after July 1976. 

Most of the 313 samples of primary productivity were 

obtained during May through October; only nine were 

taken in April and nine in November. Pinkham and 

Boundary generally were not sampled until late May 

or early June, because reservoir drawdown created insufficient depth for sampling prior to this time.

Primary productivity was measured with the carbon14 method for phytoplankton described by Greeson, 

Ehlke, Irwin, Lium, and Slack (1977). Five water samples were retrieved from depths at which the light intensity was equal to or larger than 0.1 percent of that 

incident at the reservoir surface. Light measurements 

were made with an unfiltered submersible photometer 

immediately prior to retrieval of the water samples. 

The water samples were then transferred to light and 

dark BOD bottles and inoculated with radioactive carbonate (14C03-2). They were then incubated for 3 to 4 

hours at the depth from which they were withdrawn. 

The light intensities used for incubation generally included five of the following seven values: 90, 60, 30, 

15, 5, 1, and 0.1 percent. During the study, incubations 

generally were conducted during the period spanning 

0900 to 1500 hours.

The value calculated for each water sample represents daily primary productivity, expressed as milligrams of carbon fixed per cubic meter (mg-C-m"3). Values of daily primary productivity measured within the 

sampled depth profile were integrated to obtain daily 

areal primary productivity, expressed as milligrams of 

carbonifixed per square meter (mg-C-m"2).

Only primary productivity by phytoplankton was 

measured in this study. No estimates of periphyton primary productivity were available for Lake Koocanusa. 

However, in large and deep lakes one generally can 

assume that the majority of annual primary productivity is by phytoplankton (Likens, 1975).

Annual areal primary productivity, in grams of carbon fixed per square meter (g-C-m"2), was estimated 

for Lake Koocanusa to permit comparisons with other 

lakes and reservoirs. Annual areal primary productivity 

was calculated by integrating the area under a curve 

defined by values of total daily primary productivity 

for the reservoir. The total daily values were derived

with the following equation :

A = (RF x PF) + (RT xPT) + (RP x Pp) + (RB x PB} (5) 

where

A is total daily primary productivity, in grams

of carbon fixed per square meter; 

RF is daily primary productivity, in grams of carbon fixed per square meter, at Forebay station 

(fig. i);

RT is daily primary productivity, in grams of carbon fixed per square meter, at Tenmile station;

RP is daily primary productivity, in grams of carbon fixed per square meter, at Pinkham station;

RB is daily primary productivity, in grams of carbon fixed per square meter, at Boundary station;

PF is fraction of reservoir surface area associated 

with Forebay primary productivity value;

PT is fraction of reservoir surface area associated 

with Tenmile primary productivity value;

PP is fraction of reservoir surface area associated 

with Pinkham primary productivity value; and

PB is fraction of reservoir surface area associated 

with Boundary primary productivity value.

Days on which primary productivity was measured 

were used in equation 5; however, samples were not 

taken during much of the winter so this period required 

estimation. Based on primary productivity measurements at Forebay in November and December, the following estimates of daily areal primary productivity, 

in grams of carbon fixed per square meter, were used 

for each limnological station during winter months: 

November, December, and March, 0.01; January and 

February, 0.005. The months of April through July occasionally lacked a measured value of daily areal primary productivity at one or more limnological stations. 

In such instances, the average daily areal primary productivity at the limnological station during the missing 

month was used as input to equation 5.

SEASONAL TREND

Daily areal primary productivity varied considerably 

among limnological stations and years and ranged from 

0.4 to 420.0 mg-C-m'2 (table 9). Statistical tests used 

to detect significant differences among limnological stations and years were not performed because of differences in the period sampled at each limnological station. 

Instead, the mean, minimum, and maximum values of 

daily areal primary productivity for each month are 

given (table 9). With the four limnological stations combined, August had the largest mean daily areal primary 

productivity and November had the smallest.

Mean values of daily areal primary productivity for 

April through June were substantially smaller at
PRIMARY PRODUCTIVITY

TABLE 9. Statistical summary of daily areal primary productivity and euphotic zone depth at 

the four'limnological stations, Lake Koocanusa, 1972-80

11

Station

Boundary

Pinkham

Tenmile

For ebay

All

All Apr

Month

June

July

Aug.

Sept .

Oct.

Apr .

May

June

July

Aug.

Sept.

Oct.

Nov.

Apr .

May

June

July

Aug.

Sept.

Oct.

Nov.

Apr .

May

J une

July

Aug.

Sept .

Oct.

Nov.

Apr .

May

June

July

Aug.

Sept.

Oct .

Nov.

. -Nov .

in

Daily areal primary productivity, 

milligrams of carbon 

fixed per 

square meter

Mean

82.

135.

179 .

136.

90.

75.

10.

83.

119 .

131.

154.

65 .

6.

119.

109.

171.

147.

152.

138.

109.

20.

168.

139.

165.

141.

171.

126.

107.

31.

136.

104.

141.

138.

163.

136.

97 .

22.

128 .

6

1

7

4

3

0

2

0

7

0

3

7

7

8

4

3

7

2

2

1

0

5

4

1

9

2

3

3

0

4

2

0

6

4

2

7

0

5

Mini mum

0

75

96

20

24

75

18

28

86

96

24

1

57

16

100

41

43

20

44

5

23

62

92

12

63

37

40

3

23

12

43

20

24

1

.4

.0

.0

.0

.0

.0

.5

.0

.0

.0

.0

.0

.4

.0

.0

.0

.0

.0

. 0

.0

.1

.0

.0

.0

.0

. 0

.0

.0

.2

.0

.5

.4

.0

.0

. 0

.0

.4

.4

Maxim

150.

340.

290.

230.

220.

75.

36.

230.

270.

170.

250.

120.

12.

250.

390.

280.

220.

220.

230.

200.

43.

340.

320.

400.

370.

420.

190.

230.

69.

340.

390.

400.

370.

420.

250.

230.

69.

420.

ium

0

0

0

0

0

0

0

0

0

o

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Euphotic zone depth, 

in meters

Mean

4.9

7.3

9.6

10.2

8.7

1.4

1.1

2.8

5 .1

8.4

9. 7

7.6

4.8

2.6

4.6

5.8

8.0

11.3

12.5

11.7

7.2

4.0

6.0

6.3

7 .8

11.4

11.8

11.0

5.2

3.1

4.5

5.5

7.4

10.5

11.3

10. 1

5.8

8.3

Minimum

0.4

2. 1

5.8

5. 5

5.9

1.4

.4

1. 1

1.7

5.4

8.5

5.8

.5

1.4

.6

1.7

2.3

5.9

8.5

7.7

4.9

2.7

2.3

2. 3

1. 8

7.1

9.0

7 .6

3.7

1.4

.4

.4

1.7

5.4

5.5

5.8

.5

.4

Maximum

1 1 .

12.

13.

16.

14.

1.

1.

4.

7.

10.

10.

10.

9.

3.

9.

10.

14.

18.

18.

19.

11.

4.

11.

10.

12.

18.

16.

16 .

7 .

4.

11.

11.

14.

18.

18.

19.

11.

19.

4

2

5

0

8

4

8

9

8

7

5

3

2

7

5

7

5

0

4

8

2

7

2

8

2

0

0

8

9

7

2

4

5

0

4

8

2

8

Numbe r

of

Samp le s

9

17

15

17

15

1

6

5

7

6

6

7

2

4

14

15

15

16

17

16

3

4

14

15

16

16

15

15

4

9

34

44

55

53

55

54

9

313
12 NUTRIENT LOADINGS, PRODUCTIVITY, AND TROPHIC STATE OF LAKE KOOCANUSA, 1972-80

Boundary and Pinkham than at Tenmile and Forebay. 

This condition is attributed to the fact that areal primary productivity is dependent on euphotic zone depth 

and the depths of the euphotic zone during April 

through June were smallest at Boundary and Pinkham. 

Mean euphotic zone depths during April and May at 

Pinkham were equal to or less than 1.4 m, whereas 

they were equal to or greater than 2.6 m at Tenmile 

and Forebay. Although not measured in April and May, 

euphotic zone depths at Boundary probably were similar to those at Pinkham. The shallow euphotic zones 

during April and May occurred when the reservoir received large loadings of suspended sediment that were 

reported by Bonde and Bush (1975 ) to be historically 

carried by the Kootenai River during spring runoff.

The monthly based means for daily areal primary productivity in table 9 failed to show the variation among 

years; such variation is evident in table 10. Because 

sampled periods did not coincide, statistical tests were 

not used to locate significant differences among the 

years. Mean daily areal primary productivity varied 

from 106.7 to 165.5 mg-C-m'2, but the overall range 

during the 9 years was 0.4 to 420.0 mg-C-m"2 . The 

variation in daily areal primary productivity within each 

year at the four limnological stations is apparent from 

figure 4. No common pattern among stations or years 

could be discerned, owing to the wide variation in daily 

areal primary productivity. Such variation was due 

to the effect of environmental variables on daily areal 

primary productivity; however, evaluation of the effects 

of such variables on daily areal primary productivity 

was beyond the scope of this report.

ANNUAL TREND

Annual areal primary productivity ranged from 23.2 

to 38.5 and averaged 29.5 g-C-nr2 during 1972-^80 (table 

11). The 95-percent confidence interval for the mean 

was determined to be 25.6 to 33.4 g-C-m'2 . The years 

with annual primary productivity outside of the confidence interval included 1973-76 and 1980.

Primary productivity in Lake Koocanusa can be compared to general ranges of phytoplankton primary productivity of lakes in various trophic categories that 

were listed in Wetzel (1975). Wetzel listed mean primary productivity, in milligrams of carbon fixed per 

square meter per day, which is identical to average 

daily areal primary productivity listed in table 11. 

Three of the trophic categories listed by Wetzel (1975), 

based on mean primary productivity, in milligrams of 

carbon fixed per square meter per day, were: Oligotrophic, 50-300; mesotrophic, 250-1,000; and eutrophic, 

greater than 1,000. Lake Koocanusa was clearly in the 

oligotrophic category, based on its average daily areal 

primary productivity that ranged from 63.6 to 105.5 

mg-C-m-2 (table 11).

TABLE 10. Statistical summary of daily areal primary productivity in Lake Koocanusa, 1972-80

Period

sampled

1972

1973

1974

1975

1976

1977

1978

1979

1980

July

Apr.

Apr.

Apr.

Apr.

May

May

May

May

24 -

24 -

23 -

16 -

12 -

4 -

5 -

15 -

14 -

Nov.

Nov.

Nov.

Oct.

Oct.

Oct.

Oct.

Oct.

Oct.

15

30

20

30

28

26

25

17

16

122.8

120.5

106.7

115.5

114.1

139.8

146.2

143.6

165.5

1.4

.5

2.1

.4

23.0

20.0

80.0

62.0

45.0

290.0

390.0

400.0

320.0

420.0

280.0

230.0

370.0

340.0

19

28

51

48

39

30

33

32

33

RELATIONSHIP OF ANNUAL AREAL

PRIMARY PRODUCTIVITY AND 

SELECTED ENVIRONMENTAL VARIABLES

Variations in annual areal primary productivity are 

caused by numerous environmental variables that directly or indirectly control the primary productivity of 

phytoplankton. The relationship of environmental 

variables and annual areal primary productivity in Lake 

Koocanusa was investigated with correlation and multiple regression analyses. Annual-based values for 12 environmental variables (table 12) were derived from the 

data base for use in these statistical analyses. Although 

table 12 is not an exhaustive listing, each of the 12 

variables has a direct or indirect causal relationship 

with phytoplankton primary productivity.

Correlation coefficients for each environmental variTABLE 11. Annual areal primary productivity in Lake 

Koocanusa, 1972-80

Annual areal 

primary productivity, 

n grams of carbon fixed 

per square meter_____

Average dally areal

In milligrams of carbon 

fixed per square meter

1972 a

1973

1974

1975

1976

1977

1978

1979

1980

27.1

38.5

25.3

24.4

23.2

30.6

30.7

30.0

35.3

29.5

74.2

105.5

69.3

66.8

63.6

83.8

84.1

82.2

96.7

80.7

Includes March 21 to December 31.
PRIMARY PRODUCTIVITY 13

350 

300 

250 

200 

150 

IOO 

50

BOUNDARY

0

300

\

250 

2OO

100

50 J

PINKHAM

(Station discontinued after July 1976)

g 400

S 350

<

° 300

< 250

o

_J 200

* .50

100

TENMILE

50

-°- 

450

350

3OO

250

200

ISO

100

50

0

FOREBAY

1972 1973 1975 1976 1977 1978 1979 1980

FIGURE 4. Daily values of areal primary productivity at the four limnological stations, Lake Koocanusa, 1972-80.

able and annual areal primary productivity are listed 

in table 13. The two largest correlation coefficients exceeded 0.500 and were for annual nitrogen retention 

coefficient (X12) and mean annual euphotic zone depth 

(XI). However, none of the correlation coefficients were 

significantly different from 0.00 when judged against 

a significance level of 0.05 or less. Therefore, no single

environmental variable was usable as a predictor of annual areal primary productivity. Combinations of the 

12 environmental variables, therefore, were analyzed 

by multiple linear regression analysis, using annual 

areal primary productivity as the dependent variable.

Variable combinations were initially screened with 

the RSQUARE procedure of the Statistical Analysis
14 NUTRIENT LOADINGS, PRODUCTIVITY, AND TROPHIC STATE OF LAKE KOOCANUSA, 1972-80

TABLE 12. Statistical summary of environmental variables used 

in correlation and multiple regression analyses of annual area/ 

primary productivity in Lake Koocanusa, 1972-80

Variable

Variable label

Annual areal primary productivity, in 

grains carbon fixed per square meter

Mean annual euphotic zone depth, in meters 8.5

X2 Maximum annual euphotic zone depth, in 13.8 

meters

X3 Mean annual water temperature at 3 meters 

depth, in degrees Celsius

Maximum annual water temperature at 3 

meters depth, in degrees Celsius

X5 Mean monthly lake filling time, in years .69

X6 Range of monthly lake filling time, in 1.14 

years

5.9

10.7

12.5

19.8

.17

.48

1.08

1.56

X7

X8

X9

X10

Xll 

X12

Mean monthly hydraulic residence time,

in years

Range of monthly hydraulic residence

time, in years

Annual areal phosphorus loading, in

grams per square meter

Annual areal nitrogen loading, in

grams per square meter

Annual phosphorus retention coefficient

.58

1.33

8.3

29.1

.63 

.25

.14

.36

2.8

19.1

.16

.04

.97

2.57

20.7

61.7

.89 

.50

System (SAS Institute, Inc., 1979). This computer procedure determines all possible linear regressions for a 

dependent variable and a set of independent variables, 

and then lists the coefficient of multiple correlation for 

each regression. Although 12 independent variables 

were available, there were only nine observations on 

each variable, which required that less than nine variables be used in each regression. Based on the advice 

of R. K. Steinhorst (University of Idaho, oral commun., 

1981), only two- and three-variable regressions were 

developed from the data base. Those two- and threevariable regressions with the largest coefficient of multiple determination were selected for further analysis 

using the GLM procedure in the Statistical Analysis 

System. This computer procedure provides the parameter estimates for a regression model, the analysis of 

variance table, hypothesis tests, and residuals.

The two- and three-variable regression models that 

were derived from the 12 environment variables and 

that had the largest coefficient of multiple determination are listed in table 14. In the two-variable model, 

annual areal primary productivity varied directly with 

mean annual euphotic zone depth and annual areal phosphorus loading. In the three-variable model, the dependent variable varied directly with mean annual euphotic 

zone depth and annual nitrogen retention coefficient and 

it varied inversely with maximum annual euphotic zone

TABLE 13. Correlation coefficients between annual areal primary productivity and environmental variables XI through Xl2°-

[Number of samples is nine]

able

no.

XI

X2

X3

X4

X5

X6

X7

X8

X9

XI 0

Xll

XI 2

coefficient

0.522

.355

.392

.418

.100

-.106

.187

-.012

.185

.043

.493

.605

Significance level of

0.150

.349

.296

.263

.800

.786

.630

.976

.633

.913

.178

.084

Variables defined in table 12.

depth. Mean annual euphotic zone depth was the dominant independent variable in both regression models, 

based on the relative magnitude of standardized partial 

regression coefficients. The two regression models account for 62.0 and 78.4 percent of the variation in the 

dependent variable. The overall regression models and 

their partial regression coefficients were significantly 

different from 0.00 at significance levels less than 0.061.

The residuals for each regression model were graphically analyzed according to methods of Draper and 

Smith (1966). These analyses revealed no apparent violations in the assumptions of regression analysis. Scatter plots of each independent variable and the dependent variable exhibited linear trends; therefore, the independent variables were not transformed.

The two regression models in table 14 provided an 

empirical method for predicting annual areal primary 

productivity in Lake Koocanusa. The models were developed with a statistical technique that only quantifies 

the variation between dependent and independent variables. The method does not prove the existence of 

cause-and-effect relationships between the variables. 

The relationships may be spurious if the variables varied systematically, but independent of one another. 

Therefore, the two regression models were examined 

further to determine if cause-and-effect relationships 

were plausible between the dependent and independent 

variables. Correlations between the dependent and independent variables were examined with scatterplots 

(fig. 5) and the temporal variation of the variables was 

plotted (fig. 6) to assess systematic covariation.

Mean annual euphotic zone depth, the dominant variable in both regression models, was directly related to
PRIMARY PRODUCTIVITY 15

TABLE 14. Multiple regression models for prediction of annual 

areal primary productivity in Lake Koocanusa

TWO INDEPENDENT VARIABLES

Y = 7.725 + (2.020xXl) + (0.548xX9)

Significance level 

partial of partial

0.023

0.057

THREE INDEPENDENT VARIABLES

Y - 23.852 + (4.310xXl) + (-2.583xX2) + (18.120xX12)

Significance level 

partial of partial

XI 4.310

X2 -2.583

X12 18.120

1.993

-1.662

0.552

annual areal primary productivity (table 14). This relationship appeared plausible because areal primary 

productivity might be expected to increase if the depth 

of the euphotic zone increased. Mean annual euphotic 

zone depth plotted a positive linear trend with annual 

areal primary productivity, although the value for 1973 

appeared to be outside of the trend (fig. 5). Over time, 

mean annual euphotic zone depth had an increasing 

trend similar to that of annual areal primary productivity, excepting the large 1973 value of the latter variable 

(fig. 6).

Maximum annual euphotic zone depth was inversely 

related to annual areal primary productivity in the 

three-variable regression model (table 14). This result 

does not seem plausible because mean and maximum 

euphotic zone depths were positively correlated (correlation coefficient = 0.953, significance level less than 

0.0001, number of samples = 9) and, therefore, would 

be expected to affect annual areal primary productivity 

in a similar manner. Maximum annual euphotic zone 

depth plotted a positive linear trend with annual areal 

primary productivity, although the values for 1973 and 

1979 appeared to plot outside of the trend (fig. 5). Similar temporal trends, excepting 1973, were exhibited by

maximum annual euphotic zone depth and annual areal 

primary productivity (fig. 6). Based on this conflicting 

evidence, the occurrence of this variable in the regression model appeared to be spurious.

Annual areal phosphorus loading was directly related 

to annual areal primary productivity in the two-variable 

regression model (table 14). The relationship appeared 

to be plausible because increased phosphorus loading 

might be expected to result in increased annual areal 

primary productivity. The scatterplot of annual areal 

phosphorus loading showed a positive linear trend, 

which was due largely to the location of the 1973 value 

(fig. 5). If the 1973 value were deleted, the trend would 

likely have been negatively linear. Annual areal phosphorus loading had a decreasing trend with time (fig. 

6), as would be expected from the reductions in total 

phosphorus loading listed in table 4. During 1972-76 

and 1977-79, annual areal phosphorus loading and annual areal primary productivity covaried systematically, 

which largely explains the direct relationship derived 

by the regression modeling technique.

In the three-variable regression model, annual nitrogen retention coefficient was directly related to annual 

areal primary productivity (table 14). This relationship 

appeared to be intuitively correct, because additional 

retention of nitrogen might be expected to increase annual areal primary productivity. However, the positive 

linear trend in the scatterplot (fig. 5) was produced by 

the values for 1973 and 1980; if these two were not 

present, the trend would have been negatively linear. 

Figure 6 shows that the annual nitrogen retention coefficient varied sporadically with time, but during 1972- 

74 and 1979-80 it varied systematically with annual 

areal primary productivity. The inclusion of annual nitrogen retention coefficient into the regression model appeared to be largely dependent upon the bias created 

by its two largest values, which corresponded to the 

two largest values of annual areal primary productivity.

Of the four independent variables, only mean annual 

euphotic zone depth appeared likely to have a causeand-effect relationship with annual areal primary productivity. The direct relationships of annual areal phosphorus loading and annual nitrogen retention coefficient 

with annual areal primary productivity were strongly 

biased by the coincidence of their largest values. The 

inverse relationship of maximum annual euphotic zone 

depth and annual areal primary productivity was 

deemed spurious because it was intuitively incorrect.

Mean annual euphotic zone depth was indicative of 

the depth of light penetration into Lake Koocanusa. The 

attenuation of light by water can be quantified with 

an extinction coefficient that composites effects due to 

absorption of light by water and the scattering of light 

by suspended and dissolved matter. The extinction coef-
16 NUTRIENT LOADINGS, PRODUCTIVITY, AND TROPHIC STATE OF LAKE KOOCANUSA, 1972-80

40

35

30

20

O 73 73

Q80

O MEAN ANNUAL EUPHOTIC ZONE DEPTH AND YEAR

 MAXIMUM ANNUAL EUPHOTIC ZONE DEPTH AND YEAR

080

770

O 78 770

.78

79' 79'

072

74 

O75

Q76

 72

.74

 75

 76

8 10 12 

DEPTH, IN METERS

14 16 18 20 22

40

ANNUAL NITROGEN RETENTION COEFFICIENT 

O.I 0.2 0.3 0.4 0.5

35

30

25

20

O ANNUAL AREAL PHOSPHORUS LOADING AND YEAR

 ANNUAL NITROGEN RETENTION COEFFICIENT AND YEAR

O'O

 73 Q73

 80

77* 770

079 79

 72 072

O76

 74

 76

074

O75

46 8 10 12 14 l«

ANNUAL AREAL PHOSPHORUS LOADING, IN 

GRAMS OF PHOSPHORUS PER SQUARE METER

20 22

FIGURE 5. Scatterplots of annual areal primary productivity versus maximum and mean annual euphotic zone depths, annual areal phosphorus loading, and annual areal nitrogen retention coefficient 

for Lake Koocanusa, 1972-80.
PRIMARY PRODUCTIVITY 17

ID

CO

DC

ui 10

UJ

2

z

x 5

K 

O.

UJ

0

0 

w 20

K

UJ

K

UJ

2

5 15

x"

a.

UJ

O

10

OK 40

UJ uj

Z u 

r 2 35

O ui

S

" < 30

0

2 W 25 f- K

K ^

O " 9O

1 1 1 1 1 I 1 1 1

MEAN ANNUAL EUPHOTIC ZONE DEPTH

0

O

o

o

o '

1 1 1 1 1 1 1 1 1

MAXIMUM ANNUAL EUPHOTIC ZONE DEPTH O

0

o

o

0 0 ° 0 0 1 1 1 1 1 1 1 1

ANNUAL AREAL PRIMARY PRODUCTIVITY

O

O -

0 0 0

O

° 0

1 1 1 1 1 1 1 1 1

»

i £ a. ui

m 2

0 

X UJ

a. ac

0 0

(0

(O

2 ac.

< UJ

ac a.

<9

lz

UJ

o

U.

U. 

UJ 

0

U

25

20

15

10

5

0

0.6

0.5

0.4

0.3

0.2

O.I

n

i 1 1 1 1 1 1 i i

- ANNUAL AREAL PHOSPHORUS LOADING

O

0

o o

0 0 0 0 0

1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1

ANNUAL NITROGEN RETENTION COEFFICIENT

0 ° -

-

o

0

~ /7\ O O

-

o u

1 i 1 1 1 T 1 1 1

1972 1973 1974 1975 1976 1977 1978 1979 1980

FIGURE 6. Annual variation of mean and maximum euphotic zone depths, areal primary productivity, 

areal phosphorus loading, and nitrogen retention coefficient for Lake Koocanusa, 1972-80.

ficient within the euphotic zone of Lake Koocanusa and 

the surface illumination were independent variables in 

a multiple regression derived by Woods (1979) to predict daily areal primary productivity measured in Lake 

Koocanusa from 1972 to 1975. Woods used 93 daily 

values instead of the 9 annual values reported here. 

The regression model is listed in table 15; derivation

of the variables was discussed by Woods (1979). The 

regression model showed that daily areal primary productivity was inversely related to euphotic extinction 

coefficient and directly related to surface illumination. 

These relationships appeared plausible because a reduction in euphotic zone depth caused by an increase in 

euphotic extinction coefficient would likely reduce daily
18 NUTRIENT LOADINGS, PRODUCTIVITY, AND TROPHIC STATE OF LAKE KOOCANUSA, 1972-80

TABLE 15. Multiple regression model developed by Woods (1979)

for prediction of daily areal primary productivity in Lake

Koocanusa

Y--62.12+(-12.95xXl)+(0.17xX2)+(1.04xX3)+(2.12xX4)+(0.01xX5)+(0.05xX6)

0.707

Ion - 0.500 

assion = <0.001

-0.677 

0.391 

0.315 

0.276 

0.265 

0.208

<0.001 

<0.001 

<0.001 

<0.005 

<0.005 

<0.025

areal primary productivity; increased surface illumination should increase daily areal primary productivity. 

Results of multiple regression analyses for daily and 

annual values were consistent in that variables related 

to light penetration into the euphotic zone were important predictors of areal primary productivity.

Woods (1979) concluded from analyses of data collected from 1972-75 at Lake Koocanusa that the quantity of light received by phytoplankton was the major 

environmental influence on primary productivity in the 

reservoir although no single environmental variable 

accounted for all variation. Three processes were identified as having had an effect on the light available to 

phytoplankton. First, a weak thermal structure in the 

reservoir allowed phytoplankton to be circulated out of 

the euphotic zone into light conditions inadequate for 

photosynthesis to exceed respiration. The operational 

schedule of the reservoir, in conjunction with large seasonal inflow and outflow, produced the weak thermal 

structure. Second, the largely turbid inflows during 

spring runoff substantially reduced euphotic zone 

depths and, thereby, intensified the problem of light 

limitation experienced by phytoplankton that was circulated out of the euphotic zone. Third, the phytoplanktonic light environment was largely dependent upon 

seasonally and meteorologically induced variations in incident solar radiation.

The foregoing conclusions of Woods (1979) also may 

be applicable to the environmental control of daily areal 

primary productivity during 1976-80 in Lake 

Koocanusa. However, the data analyses necessary to 

determine the environmental control of daily areal primary productivity during 1976-80 were beyond the

scope of this report. Evaluation of the relationship of 

daily values of primary productivity in conjunction with 

concurrent values for physical, chemical, and biological 

variables likely would yield valuable insight into the 

reasons why Lake Koocanusa was oligotrophic during 

1972-80, in spite of its potentially eutrophic loadings 

of nitrogen and phosphorus.

CHLOROPHYLL a DISTRIBUTION WITHIN 

THE WATER COLUMN

Based on his analysis of data collected during 1972-75 

at Lake Koocanusa, Woods (1979) hypothesized that 

phytoplankton was circulated beneath the euphotic 

zone. To test this hypothesis the limnological sampling 

program at Lake Koocanusa was modified in the 1980 

water year. In that chlorophyll a is a primary photosynthetic pigment found in all algae (Wetzel, 1975), its 

distribution in the water column was used to test this 

hypothesis. Such tests were not feasible prior to 1980 

because chlorophyll a was not sampled beneath the 

euphotic zone prior to 1980. In 1980, however, 

chlorophyll a was sampled at two points beneath the 

euphotic zone one each at two and three times the 

depth of 0.1 percent light. In addition, five samples of 

chlorophyll a were collected in the euphotic zone one 

each at the carbon-14 incubation depths. Prior to midMay 1978 chlorophyll a in Lake Koocanusa was 

analyzed according to methods of Slack, Averett, 

Greeson, and Lipscomb (1973). This procedure did not 

correct for the degradation products of chlorophyll a; 

hence, one could not ascertain if the results were representative of living or dead chlorophyll-bearing organisms. Following mid-May 1978, chlorophyll a sampled from Lake Koocanusa was determined according 

to methods of Greeson, Ehlke, Irwin, Lium, and Slack 

(1977). These results indicated the amount of 

chlorophyll a that was contained in living chlorophyllbearing organisms.

Chlorophyll a concentrations were sampled at 

Forebay and Tenmile in 1980. The volume of chlorophyll 

a in each of three depth strata then was determined 

from a plot of chlorophyll a concentration versus depth 

(fig. 7). The percentage of the total volume of 

chlorophyll a in the sampled water column then was 

determined for each of the three depth strata. The 

three depth strata were as follows: Surface to 1.0 percent light depth, 1.0 percent to 0.1 percent light depth, 

and 0.1 percent light depth to a depth equal to the 

depth of 0.1 percent light plus one-half of the depth 

difference between 1.0 percent and 0.1 percent light 

depth. The 1.0 percent light depth was chosen as the 

lower boundary of the upper stratum because the 1.0 

percent light depth is often defined as the lower boundary of the euphotic zone (Greeson and others, 1977).
PRIMARY PRODUCTIVITY 19

The 0.1 percent light depth was chosen as a boundary 

because it was often the depth at which the lowermost 

carbon-14 sample was incubated in Lake Koocanusa.

The percentage of chlorophyll a in each stratum varied during the sampled interval; the 95-percent confidence intervals indicate that the middle stratum varied 

less than the other two strata (table 16). The mean 

percentage distribution of chlorophyll a in the three 

strata was similar at both limnological stations. On the 

average, less than one-half of the sampled chlorophyll

CHLOROPHYLL a CONCENTRATION, IN MICROGRAMS PER LITER 

O.I 0.2 0.3 0.4

.''DEPTH OF O.I PERCENT LIGHT .

» » ' *» ' s*» ' *»'»*»' * * ' * I*

*» ' »*»*' *» ' ' ' * ' * *l* * \* " ~ *

V ' v* - v*'- -LOWER 'STRATUM

FIGURE 7. Typical diagram used to quantify the percentage distribution of chlorophyll a within the water

column of Lake Koocanusa.
20 NUTRIENT LOADINGS, PRODUCTIVITY, AND TROPHIC STATE OF LAKE KOOCANUSA, 1972^80

TABLE 16. Percentage distribution of chlorophyll a in three depth 

strata at the Forebay and Tenmile stations in 1980

Date 

sampled

Upper 

stratu

Middle 

stratum2

Lower 

stratum3

Forebay I

(

(

lay 15

lay 30

Fune 11

une 30

uly 10

uly 24

ugust 7

ugust 21

>eptember 4

September 18

October 2

Jctober 16

24.5

26.5

38.1

34.1

41.4

63.9

45.8

74.7

41.3

24.0

40.8

35.8

13.0

18.0

16.4

13.7

21.1

20.5

20.0

16.0

21.8

15.2

15.4

20.8

62.5

55.5

45.5

52.2

37.5

15.6

34.2

9.3

36.9

60.8

43.8

43.4

Mean 

95 pe

40.9 

onfidence interval 31.3 - 50.6

May 14 

May 29 

June 10 

June 24 

July 22 

August 5 

August 19 

September 2 

September 16 

September 30 

October 14

40.5

42.9

30.4

44.5

61.4

40.0

32.5

58.2

50.3

64.2

8.1

Mean 43.0 

95-percent confidence interval 32.2 - 53.

17.7 

15.7 - 19.

28.2

19.3

11.5

12.7

17.5

17.7

16.2

11.8

17.0

17.9

6.2

16.0 

12.2 - 19.

41.4 

31.0 - 51.

31.3

37.8

58.1

42.8

21.1

42.3

51.3

30.0

32.7

17.9

85.7

41.0 

28.2 - 53.

'Surface to depth of 1.0 percent light

2 Depth of 1.0 percent light to depth of 0.1 percent light

^Depth of 0.1 percent light to depth equal to depth of 0.1 percent light plus onehalf of depth difference between 1.0 and 0.1 percent light depths.

a was contained within the euphotic zone and, thereby, 

provided evidence that phytoplankton was circulated 

out of the euphotic zone.

The importance of circulation of phytoplankton out 

of the euphotic zone in Lake Koocanusa was discussed 

by Woods (1981). He concluded that the reduction in 

phytoplankton photosynthesis resulting from reduced 

light intensities below the euphotic zone was an important factor in suppressing the reservoir's primary productivity to oligotrophic levels.

TROPHIC STATE OF LAKE KOOCANUSA

The trophic state of Lake Koocanusa can be 

categorized with one or more of the available trophic 

state indices. Most indices define discrete ranges for 

their categories of trophic state. A lake or reservoir 

generally can be placed within one of the following 

categories: Oligotrophic, mesotrophic, eutrophic, and 

hypereutrophic. Comparison of different trophic state 

indices, however, is often misleading because of the variety of variables used in their development. The use 

of different variables has arisen, in part, because three 

general approaches to trophic state determination are 

recognized. One approach considers nutrient supply as 

a stimulus to the water body. This approach uses variables such as nutrient concentrations within the water 

body or nutrient loads delivered to the water body from 

its drainage basin. The second approach uses the biological condition of the water body as an indicator of its 

trophic state. This approach uses variables such as primary productivity or chlorophyll a concentrations. The 

third approach combines the first two approaches to 

develop multivariate indices of trophic state.

The trophic state of Lake Koocanusa during 1972-80 

was determined from data on primary productivity and 

loadings of total phosphorus and total nitrogen delivered into the reservoir. Primary productivity was used 

because it is a biological process that integrates the 

effects of numerous environmental variables controlling 

production of organic matter. Nutrient loadings were 

used because they have been emphasized in most 

studies of the aquatic environment of Lake Koocanusa, 

and Bonde and Bush (1975) used them to predict the 

trophic state of the reservoir prior to impoundment. 

The use of primary productivity and nutrient loadings 

for predicting trophic state was deemed appropriate in 

light of the conclusion drawn by Schindler (1978) that 

the relationship between nutrient supply and algal productivity has been well documented by numerous researchers.

A previous section of this report discussed primary 

productivity data that were used to classify the trophic 

state of Lake Koocanusa. The conclusion was drawn 

that Lake Koocanusa was oligotrophic from 1972-80 because its average daily areal primary productivity 

ranged from 63.6 to 105.5 mg-C-nr2 (table 11) and, 

therefore, was clearly within the range of 50 to 300 

mg-C-m"2 that Wetzel (1975) considered oligotrophic.

The nutrient loading approach developed by Vollenweider (1968, 1975) was the other method used to classify the trophic state of the reservoir. Vollenweider 

(1968) was able to graphically separate 20 lakes into 

categories of oligotrophic, mesotrophic, and eutrophic 

by plotting logarithmic areal total phosphorus or total 

nitrogen loading of each lake against its logarithmic 

mean depth. Mean depth is the quotient of lake volume 

divided by surface area of the lake. Vollenweider also 

empirically determined a lower boundary line for permissible loading that quantified the maximum areal 

loading of total phosphorus or total nitrogen that a lake 

could receive and remain oligotrophic. Also determined 

was an upper boundary line for excessive loadings that 

would yield a eutrophic lake. Vollenweider (1975) modified his original model by adding hydraulic-residence 

time to allow for hydraulic effects in addition to the 

effects of nutrient loading and lake morphometry. The 

boundary lines for permissible and excessive loadings 

of total phosphorus were revised in the modified model; 

however, boundaries were not revised for total nitrogen
TROPHIC STATE 21

because the criteria needed to develop such boundary 

lines were incomplete (Rast and Lee, 1978).

Data needed to use Vollenweider's model include the 

yearly areal loading of total phosphorus or total nitrogen and the ratio of mean depth to hydraulic-residence 

time. Such data for Lake Koocanusa are listed in table 

17; the data include 1972 through 1980 and the theoretical values for a reservoir containing 7.16 km3 and discharging an annual outflow of 10.65 km3 , which is equal 

to its mean annual inflow. The theoretical nutrient loadings in table 17 are those reported by Bonde and Bush 

(1975) in their preimpoundment study of the reservoir.

The trophic state of Lake Koocanusa, based on application of Vollenweider's phosphorus loading model, is 

depicted in figure 8. The data in table 17 produced 

10 points on figure 8; all 10 were within the eutrophic 

zone, but they were clustered into two groups. The 

theoretical value and those for 1972 through 1975 

grouped higher into the eutrophic zone than the post1975 group. The post-1975 group was lower than the 

pre-1975 group as a consequence of the 1976 reduction 

in phosphorus loading (table 4).

The relationship of areal total nitrogen loading and 

the ratio of mean depth to hydraulic residence time is 

depicted in figure 9 for Lake Koocanusa and 34 lakes 

analyzed by Rast and Lee (1978). The upper and lower 

boundary lines were not available for delineation of 

the three regions of trophic state, but the 10 points 

for Lake Koocanusa clearly occur within the cluster of 

eutrophic lakes. In contrast to figure 8, the 10 points 

were not split into two groups. The occurrence of the 

points in a single group was a consequence of the lack 

of reduction in nitrogen loading to the reservoir (table 

5).

TABLE 17. Required data for using Vollenweider0 nutrient loading models to determine the trophic state of Lake Koocanusa

Ratio of 

mean depth to

hydraulicHydraulic- residence 

residence time,

ading,

Year

Mean dept 

In meters years per year phosphorus

Total 

nltroge

1972

1973

1974

1975

1976

1977

1978

1979

1980

23.2

23.9

30.6

29.7

32.6

30.7

32.5

32.8

33.5

Theoretical 38.4

0.14 

.33 

.29 

.41 

.38 

.50 

.48 

.62 

.58 

.67

165.7

72.4

105.5

72.4

85.8

61.4

67.7

52.9

57.8

57.3

15.7

20.7

11.9

10.5 

3.8 

2.8 

3.7 

2.9 

2.9

10.O b

61.7

41.1

32.4

22.5

22.6 

19.3 

21.3 

19.1 

22.1 

20.0t>

aVollenwelder (1975)

bFrom Bonde and Bush (1975)

The eutrophic ranking that was derived for Lake 

Koocanusa from figures 8 and 9 conflicted with the 

oligotrophic ranking that the reservoir received for its 

values of primary productivity. The following discussion 

develops possible explanations for this discrepency in 

trophic state ranking.

Two recent papers by Winter (1981a, 1981b) have emphasized measurement errors that are associated with 

water and chemical balances of lakes and reservoirs. 

Sufficient data were not available for Lake Koocanusa 

for estimating the magnitude of such errors for computations of annual areal loadings of total phosphorus and 

total nitrogen. However, the effect of these errors on 

the categorization of trophic state can be visualized with 

figures 8 and 9. If the annual areal loadings of total 

phosphorus in table 17 were assumed to be double their 

true values, the downward shifts in the 10 points that 

would be plotted on figure 8 for the true values still 

would be inadequate to place the reservoir into the 

oligotrophic zone. A similar conclusion was drawn for 

annual areal nitrogen loadings depicted in figure 9. Undoubtedly, errors existed in the water and chemical balances computed for both the reservoir and its various 

sources of inflow, but these errors did not fully account 

for the discrepency in trophic state.

Only part of the nutrient loading that enters a lake 

or reservoir may become available to phytoplankton in 

the euphotic zone if the lake or reservoir experiences 

significant amounts of stratified interflow or underflow 

beneath the euphotic zone (Rast and Lee, 1978). Inflow 

to Lake Koocanusa during its annual filling phase was 

routed primarily as interflow or underflow (Woods, 

1979). During the filling of Lake Koocanusa, the 

monthly loadings of total phosphorus and total nitrogen 

were at their largest for the year (Woods and Falter, 

1982). Hence, some of the reservoir's nutrient supply 

may have been inaccessible to phytoplankton in the 

euphotic zone.

In addition to this hydrodynamic loss of nutrients, 

another part of the influent loading of total phosphorus 

may have been adsorbed and deposited by suspended 

sediment. Iskandar and Shukla (1981) concluded that 

Lake Koocanusa sediments functioned as a phosphorus 

sink.

The hydrodynamic and sedimentation processes within the reservoir help explain why potentially eutrophic 

nutrient loadings failed to stimulate primary productivity to eutrophic levels. However, the primary productivity of the reservoir was not solely controlled by nutrient supply; physical limnological processes contributed to the suppression of primary productivity to oligotrophic levels (Hobbie, 1976; Bush and Bonde, 1977; 

Woods, 1979, 1981). The essence of the reports by these 

authors is that the weak thermal structure of Lake
22 NUTRIENT LOADINGS, PRODUCTIVITY, AND TROPHIC STATE OF LAKE KOOCANUSA, 1972-80

30 i i i m

10

EUTROPHIC 

ZONE

I I I I I I

Q73

Q75° 74 

THEORETICAL

D72

Q76

79^80 

77

1.0

0.5

0.3

O.I

0.05

0.03

OLIGOTROPHIC 

ZONE

72Q

LOWER BOUNDARY FOR 

EXCESSIVE LOADING

UPPER BOUNDARY FOR 

PERMISSIBLE LOADING

LAKE KOOCANUSA DATA 

AND ABBREVIATED YEAR

0.01 I I I I i 1 i i i i

O.I 0.3 0.5 1.0 3 5 10 30 50 100 

MEAN DEPTH DIVIDED BY HYDRAULIC-RESIDENCE TIME, IN METERS PER YEAR

300 5OO 1000

FIGURE 8. Application of Lake Koocanusa data in table 17 to Vollenweider total phosphorus loading and mean depth/hydraulic-residence

time diagram. Illustration modified from Rast and Lee (1978).

Koocanusa allowed circulation of phytoplankton out of 

an often shallow euphotic zone. Circulation into poorlight conditions reduced phytoplankton photosynthesis 

and, hence, annual primary productivity.

One can conclude that physical processes within Lake 

Koocanusa were more important in determining its 

trophic state, based on primary productivity, than was 

determination of its trophic state based on influent nutrient loadings. This conclusion supports a generalization expressed by Carlson (1979) that the use of nutrient loadings to characterize trophic state fails to consider the actual response of the lake; the nutrient loading

method only considers the lake's expected response to 

external stimuli.

CONCLUSIONS

Preimpoundment and postimpoundment studies of 

Lake Koocanusa have generated an extensive data base 

that has provided insight into the physical, chemical, 

and biological characteristics of the reservoir and its 

drainage basin. In these studies, the relationship of nutrient loadings, primary productivity, and trophic state 

has received much attention, largely because the preimpoundment studies reported large loadings of nitrogen
CONCLUSIONS 23

100

50

30

10

U

£ « 

K 10

U. K

o ui «.°-

5 0.5

0.3

O.I

O 72

D 73

DT4

79^77

THEORETICAL

O 

O

O

 EUTROPHIC

A MESOTROPHIC

O OLIGOTROPHIC

,, LAKE KOOCANUSA DATA

7Z U AND ABBREVIATED YEAR

O

I i I i I i I

O.I 0.3 0.5 1.0 3 5 10 30 50 IOO

MEAN DEPTH DIVIDED BY HYDRAULIC-RESIDENCE TIME, IN METERS PER YEAR

3OO 5OO IOOO

FIGURE 9. Application of Lake Koocanusa data in table 17 to Vollenweider total nitrogen loading and mean depth/hydraulic-residence time diagram. Illustration modified from Rast and Lee (1978).

and phosphorus that might have caused eutrophication 

of Lake Koocanusa. During 1972-80, the reservoir was 

classifiable as eutrophic, based on the relationship of 

annual areal loadings of total nitrogen or total phosphorus to the ratio of mean depth and hydraulic-residence time. The eutrophic classification was not 

changed by the substantial reductions in total phosphorus loading that commenced in 1976 as a result of 

an upstream fertilizer plant instituting improved treatment of its effluent. However, the average daily areal 

primary productivity in Lake Koocanusa during 1972-80 

characterized its trophic state as oligotrophic in conflict with the eutrophic classification based on nutrientloading data. This discrepency of classification in 

trophic state is attributed to the failure of nutrient-loading models to adequately account for limnological processes within Lake Koocanusa, which affected the availability of influent nutrient loadings to phytoplankton.

Previous studies of limnological processes in Lake 

Koocanusa reported a weak thermal structure and, 

therefore, hypothesized that phytoplankton was circulated out of the euphotic zone. Such circulation was verified by a study of chlorophyll a distribution, which revealed that more than one-half of the reservoir's 

phytoplankton was located beneath the euphotic zone. 

Multiple regression analysis demonstrated that mean 

annual euphotic zone depth was an important predictor 

of annual areal primary productivity.

The relationship of daily values of primary productivity and concurrent values for physical, chemical, and 

biological variables was not studied in this project. Such 

an evaluation likely would yield valuable insight into 

the reasons why Lake Koocanusa was oligotrophic during 1972-80.
24 NUTRIENT LOADINGS, PRODUCTIVITY, AND TROPHIC STATE OF LAKE KOOCANUSA, 1972-80

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U.S. GOVERNMENT PRINTING OFFICE: 1982 576-034/7 Region No. 8