Estimated Responses of Lake Ontario Phytoplankton Biomass to Varying Nutrient Levels

J. Great Lakes Res., October 1977. Internat. Assoc. Great Lakes Res. 3(1-2): 123-131.

ESTIMATED RESPONSES OF LAKE ONTARIO PHYTOPLANKTON BIOMASS TO VARYING NUTRIENT LEVELS

Robert V. Thomann Richard P. Winfield Environmental Engineering & Science Program Manhattan College Bronx, New York and Daniel S. Szumski Hydroscience, Inc. Westwood, New Jersey

ABSTRACT. A series of simulations of the response of the open lake region of Lake Ontario to various levels of nutrient input is described using a simplified dynamic model of phytoplankton-nutrient interactions in a vertically segmented lake. The analysis of the simulations indicates the importance of the overall loss rates of nutrient. Under an hypothesized, but reasonable, set of model parameters, the simulations indicate that the present observed open lake phytoplankton biomass of Lake Ontario does not appear to be in equilibrium with the present input nutrient load. For an assumed equilibrium condition, the simulations indicate that reductions in phosphorus load will be accompanied by reductions in biomass. A "pastoral" simulation using load estimates consistent with the conditions prior to man's intensive activity indicates that spring phytoplankton levels were 30% 70% ofpresent levels depending upon the kinetic assumptions. Analysis of lake response to the u.S.-Canada Water Quality Agreement loads using three kinetic assumptions (optimistic, reasonable, pessimistic) indicates a range from 25% decrease to 80% increase in peak phytoplankton over present levels. For an implementation period of 1 0 years, a load reduction rate ofabout 1-1.5 metric tons phosphorus! day per year appears to be a sound objective to maintain or reduce present phytoplankton levels.

INTRODUCTION The International Joint Commission, as part of its broad based responsibility for water resource management on the Great Lakes, has adopted programs for the control of eutrophication processes in the lower Great Lakes. The Water Quality Agreement (WQA) between the United States and Canada constitutes one of the major elements of these programs by providing for the systematic reduction of nutrient loadings to the lower Lakes. The foremost question raised by the WQA load reduction program is: What response, in terms of phytoplankton biomass levels, can be expected as a result of ongoing nutrient removal programs? This paper describes the use of a simplified model of phytoplankton dynamics of Lake Ontario (the Lake 1 model) to estimate the lake-wide response to various levels of nutrient loading. The Lake 1 model (Thomann et al. 1975) is a preliminary framework for estimating whole lake phytoplankton response and to provide some input

into the ongoing decision making process on Lake Ontario. A range of external nitrogen and phosphorus loading is examined as well as the sensitivity of the results to various model parameters. All of the work is aimed at providing estimates of phytoplankton biomass to a first approximation. The results presented herein serve only as indicators of the general direction to be expected under remedial nutrient control programs. The details of the model (including initial calibration studies) and the full range of loads and simulation results are given by Thomann et al. (1975, 1976) and Hydroscience (1976). Figure 1 shows the principal components of the assessment framework. The nutrient input components are: a) Lake Erie input b) Inputs to Niagara River c) Inputs to Lake Ontario 1) Municipal - directly to the lake and to tributaries to the lake

123

124

THOMANN, WINFIELD and SZUMSKI

BOO

4.°

4.°

44°

44°

43°

43°

o

I 74°

LAKE I MODEL:

\j]

o

'7m.

13m

.

LAKE INPUTS

}<:===o

DIRECT TO LAKE MUNICIPAL l~ TO TRIBUTARY TO LAKE NON -POINT TRIBUTARY ~ - ­ INDUSTRIAL ATMOSPHERIC

INPUTS

42° SCALE

,

i

i

10

20

SO

F9 40

50

M'LES

SEDIMENT

( NOT ALL SYSTEMS AND INTERACTIONS ARE SHOWN)

FIG. 1. Principal components ofassessment framework

2) Non-point tributary inputs 3) Industrial input 4) Atmospheric and sediment sources. The geographical scope is lake-wide and the simulations are, therefore, for the whole lake only. Near shore problems are not considered. The measure of eutrophication is taken as the phytoplankton chlorophyll a concentration. The emphasis is on the response of the open lake total biomass to a range of nutrient loading and the sensitivity of the response to varying estimates of model parameters and coefficients. Figure 1 also shows the geometry of the Lake 1 model. The principal features included in the model are: a) a two-layer system with a sediment layer, the mixing and stratification being accomplished by vertical exchange b) phytoplankton settling c) external environmental inputs of nutrients d) external environmental inputs of solar radiation, water temperature and other system parameters. A simplified system diagram showing the inter-

action of the key variables is indicated in Figure 1. Actually, ten dependent variables are included and incorporate the major features of the interactions of phytoplankton, zooplankton and nutrients. Table 1 gives the basic physical data used for the Lake 1 model. Extensive analyses and summary data from 1967-1970 formed the basis for calibration of the model (Thomann et al. 1975). The results of the calibration indicated that the model provides a reasonable comparison to observed lake-wide average values of cWorophyll, zooplankton carbon, and various forms of nitrogen and phosphorus. The analyses indicate that the spring growth phase and peak phytoplankton biomass are primarily controlled by increasing light and temperature and phosphorus limitation. The mid-summer minimum in phytoplankton is estimated to be due primarily to zooplankton grazing and nitrogen limitation. The broad fall peak in phytoplankton is a complex interaction of nutrient regeneration (up to five times the external nutrient inputs), subsequent nutrient limitation and then the fall overturn. Both nitrogen and phosphorus are important

125

PHYTOPLANKTON RESPONSES TO NUTRIENTS TABLE 1. Basic physical data of the lake 1 model.

Segment No.

Segment Interface

Volume (m 3 x10 6 )

%

297,000

19

Depth (m)

Surface Area (m 2 )

17

%

43,500

1232

19

188,500

5323

81

1.64x101 0

1-2 2

Flow m 3 /sec

cfs

1,373,000

81

73.3 0.89x101 0

2-3 3 (sediment)

0.15*

Note: Vertical dispersion coefficient between segments No.1 and 2 varied from ~.86 cm 2 /sec (~9.3 m 2 /day) *Segment #3 depth is arbitrary

nutrients in this dynamic succession. Although the model parameters used in the calibration are all considered reasonable and within reported literature ranges, no claim is made as to the uniqueness of the particular parameter set that was finally derived. Nevertheless, in comparison to the existing empirical techniques in use for Lake Ontario, the model development indicated that a sufficient base had been established to explore the behavior of the lake under different nutrient inputs. THE SIMULATION PROBLEM IN LARGE LAKES The estimation of future levels of water quality on large lake systems, such as Lake Ontario, is complicated by the long retention time of the lake and the usual, relatively short period of observed data on the state of the lake. Changes in lake water quality are therefore difficult to perceive on a year-to-year basis. For example, for Lake Ontario, which has an eight year hydraulic detention time, the relative time scale of interest is on the order of tens of years; i.e. it may take 10-20 years for the lake to respond to changes in external inputs. The observation time for Lake Ontario of about 5 years is short compared to the response time. A difficult problem in prediction is therefore presented; namely, the estimation of long term system response based on a short observational period. The problem is somewhat analogous to attempting to estimate the frequency of occurrence of a one in ten year drought flow based on one or two years of record. As shown previously (Thomann et al. 1975), the time variable responses computed for a period of one year are responsive primarily to the specified initial conditions rather than the external inputs. This, of course, can be seen from a nutrient balance equation for a well-mixed lake.

Thus, c-

W (1.e-O/t +K)t) + c e-(l/t o+K)t Q+KV 0 0

(1)

where c = the whole lake nutrient concentration (mg/l), W is the external source of nutrients (kg/day), V is the lake volume, (m 3 ), Q is the flow (m 3 /sec) through the lake, to is the hydraulic detention time (=V/Q), K(l/day) is the overall decay of the nutrient due to settling or chemical reactions, and t is time. In analyzing one year of data or even several years of data, two estimates must be made. First, the external load, W, must be estimated and then the overall loss rate of the nutrient, K, must be determined. The external nutrient load is estimated from a variety of data sources but usually for a given year or group of years close to the sampling interval of the receiving water nutrient concentrations. The overall decay rate is either estimated as part of the settling of the phytoplankton, as in the dynamic phytoplankton model, or is estimated from the nutrient data itself. But, the latter course of action assumes the lake to be in equilibrium with the present load, an assumption that cannot be checked until the lake has actually been observed for a period of at least one-two detention times (8-16 years for Lake Ontario). The dilemma is made clear by an example from Lake Ontario. Present total phosphorus load to the lake is about 34 tonnes P/day (75,000 Ibs P/day). Using Eq. (1), if total phosphorus were completely conserved (K=0), the total within-lake phosphorus concentration at equilibrium is equal to 60 J.1g/1 for a flow of 6570 m3 /sec (232,000 ft3/ sec). Now, the average observed total phosphorus concentrations in Lake Ontario during the period 1967-1970 and 1972-1973 is about 20 J.1g P/l. For this initial condition, the concentration at

126

mOMANN, WINFIELD and SZUMSKI

E

6' 0.06 f=

~ ~

~0.04 ~

.....-

~002

/'

: /'

/'

INCREASE DUE TO

EXTERNAL SOURCE

( / /

~

:

I

I-

I:

I

I

I

~

/

/ :

EOUILlBRIUM

CONCENTRA nON

~

TOTA:--

u

~

---:::::::~

/~

INITIAL CONDIT/ON

----~ECREASE __

--~

0.00 L--O"---L-..L....L-L-L-..L....L-L-L_,L-l,-L-L-L-.l..'5--l I,I\ ,---_-----l O

for constructing a long term record of the state of the lake. The difficulty at the present time is that it is not yet possible to deal in a reasonable way with the interaction of the sediment interstitial water chemistry and the chemistry of the solid phase. In the second course of action, reasonable hypotheses on the phosphorus and nitrogen components (e.g. phytoplankton and detrital settling) can be formulated and tested using the dynamic model. Simulations based on those results can then be prepared but with full recognition that the long term behavior has only been grossly approximated.

TIME, years

FIG. 2. R1ustration of dominant effect of initial condition during a one-year analysis. 0.06

~

ATENOOF2YEAR$

f-

a

f-

0.00 L 0.0

-'--

0.5

--l.._ _--J

1.0

OVERALL LOSS RATE-K, 0.00' Iday

FIG. 3. The insensitivity of a one-year calculation to the overall loss rate.

the end of the first year would be about 25 J1.g/1, close to the observed value. Figure 2 indicates these results and shows the dominant effect of the initial condition during the first year. For a lake with a large volume, such as Lake Ontario, (1.67.10 12 m 3 ), a value ofK = O.OOI/day, gives a value of total P at equilibrium of about 15 J1.g/1, which is within 25% of present levels. Is a value of K = O.OOI/day reasonable? Unfortunately, a one-year data analysis does not necessarily provide the needed information. Figure 3 shows that for Lake Ontario the difference at the end of one year of analysis with or without a value of K is too slight to determine a reasonable estimate. Only two courses qf action appear open for Lake Ontario. "Long term" data (e.g. 5-10 years) on some aspect of the lake biomass behavior could be analyzed to provide information on the decay rate. The sediments also seem to contain the data

EXTERNAL NUTRIENT INPUTS FOR SIMULATIONS The principal sources of nutrients to Lake Ontario are: 1) Niagara River, including input from Lake Erie and waste discharges on the Niagara River itself; 2) other tributary inputs in the Lake Ontario basin; 3) direct discharges of municipal and industrial wastes; 4) local drainage to the Lake and; 5) atmospheric inputs. The major contributions are from the first three categories although atmospheric inputs may also prove to be significant. A complete review of input nutrient loads is given by Hydroscience (1976). A range of conditions on the external nutrient inputs has been examined. In order to place the present loads into an historical perspective, a preliminary analysis was made of the nutrient inputs that might have existed at some distant time in the past. These loads were termed the Pastoral Loads. In addi,tion, a review was made of nutrient reductions suggested by Vollenweider (1968) and those suggested by the Great Lakes Water Quality Agreement (1972). The simulations were therefore prepared using a wide range of input nutrient loads with the Pastoral Loads as a baseline condition. The focus of this paper is on the effect of the phosphorus loading since this was the nutrient singled out in the WQA and is the primary nutrient limiting the spring peak. Present Inputs The estimated 1974 loading to the lake is shown in Table 2. As shown, the major input is from the Niagara River which includes discharges on the Niagara River itself as well as the output from Lake Erie. Direct municipal and industrial discharges to the lake account for about 12-17%, the remainder of the load enters from tributaries to the lake. Estimates of earlier loading conditions have been

PHYTOPLANKTON RESPONSES TO NUTRIENTS

127

TABLE 2. Estimated 1974 nutrient inputs to Lake Ontario. Total Nitrogen

Total Phosphorus Source

%of Pounds/day (metric tons/day)a total

Niagara Tributaries Municipalb Industrial

46,200 16,300 11,800 700

Total

75,000

21.0 7.3 5.4 0.3

-34.0

pounds/ton

62 21 16 1

545,200 326,500 102,000 20,200

100

1,043,900

-

%of (metric tons/day)a total

270 148 46 9

-473

57 31 10 2

-100

aMetric ton/day = 2205 lbs/day bDirect discharges to lake

made by the IJC for 1967 (International Lake Erie and International Lake Ontario - St. Lawrence River Water Pollution Control Board 1969), Casey and Salbach (1974) for 1972-1973 and supplemented by other estimates for intervening years. The total phosphorus load for 1967 was estimated at 34.1 tonnes/day, identical to that of the 1974 estimate although the distribution among sources is somewhat different. It is estimated (Hydroscience 1976) that the total phosphorus load increased to a peak value of about 43 tonnes/day in 1969 and then has decreased to the 1974 level of 34 tonnes/day partly due to reduction in phosphorus loading by municipal sources. Total nitrogen loading in 1967 was estimated at 400 tonnes/day and the general trend over the ensuing seven years to 1974 has been upward, due only in part to the high flow in the Niagara River. Pastoral and Water Quality Agreement Loads It is difficult to evaluate whether phytoplankton biomass observed at present in the open lake represents a "serious" condition or a condition only marginally different from an unstressed situation. If water quality objectives are set without considering what the state of the water body would be if man had not happened on the stage, unrealistically high expectations might occur. One might ask then, what was a reasonable level of phytoplankton biomass in Lake Ontario at some time in the past? A "pastoral" input condition was therefore estimated assuming the tributary basins to be principally rural areas, with no major population centers, and hence no significant human waste water entering the lake. Loehr (1972) summarizes characteristics of rural runoff and reports about 0.3 kg P/ha-year (0.25 Ibs/acre-year) and total nitrogen loading of 1.5 to 3.2 kg N/ha-year (1.3-2.9 Ibs/acre-year) in runoff from rural areas containing no signifi-

cant human wastewater contributions to the streams. Loading rates of 2.24 kg N/ha-year and 0.3 kg P/ha-year were therefore used for the basin contribution to the nutrient loads. The Niagara River load was calculated using southern Lake Huron mean concentration data (Great Lakes Water Quality Board 1973). With these loading rates, a total of 9.3 tonnes/day of total phosphorus and 184 tonnes/day of total nitrogen are estimated as the pastoral loads. The loads promulgated under the u.S.-Canada Water Quality Agreement on phosphorus control have also been used in the simulations. Under the Agreement, a total mass loading of 24.9 tonnes/day is specified. This total includes 11.9 tonnes/day allowable from Lake Erie to the Niagara River. The effluent concentration for the resulting municipal mass load to Lake Ontario is approximately 2.2 mg P/l at 1974 population levels and 1.3 mg P/1 at estimated 2015 population. The Agreement also stipulates that the phosphorus concentration in municipal effluents should not exceed 1 mg P/!. This results in a total mass load of 20.8 tonnes/day. This is similar to the 21.3 tonnes/day determined from use of the loadingdepth plots of Vollenweider (1968). If 0.1 mg P/1 in the effluent is considered as a technologically feasible limit, then the total mass loading is 17.8 tonnes/day. Table 3 summarizes the levels used in the simulations and indicates that maximum feasible control of Lake Ontario discharges will reduce phosphorus loads by about 50%. Therefore, half of the present load to Lake Ontario is due to uncontrollable sources and the phosphorus leaving Lake Erie. Present loads represent an approximate three-fold increase over the pastoral condition. Figure 4 shows a summary of phosphorus loading to the Lake for the first four conditions of Table 3 and includes the distribution of sources of phosphorus.

THOMANN, WINFIELD and SZUMSKI

128

TABLE 3. Summary of phosphorus loads for simulation.

Simulation Condition Continuation of Present Load Agreement Mass Load Agreement-1 mgP/1 (Vollenweider Reduction) Agreement-0.1 mgP/1 (Tech. feasible) Pastoral Loads

~=3

1bs/day

Metric tons day

%of present

75,000

34.0

100

54,800

24.8

73

46,900

20.8

61

39,200

17.8

52

20,500

9.3

27

CTI

RESULTS OF SIMULATIONS In each simulation, the Lake 1 model kinetic structure was used. The nutrient loads were inputted; first as an instantaneous drop from present levels and secondly, as a linear decrease in load to the new level over a la-year implementation period. The model was then run until a new dynamic equilibrium was obtained. No attempt was made to estimate an actual future load time history; rather, a range of external conditions was imposed to illustrate the nature of the lake response. The calibration analysis of Lake 1 provides only an estimate of the loss of phytoplankton nitrogen and phosphorus to the sediments (Thomann et aI. 1975). From the short observation period available it is not possible to estimate the long term decay of dissolved inorganic forms. The following range of conditions was therefore used in the simulations: 1) A "reasonable" kinetic condition of nonliving organic nitrogen and phosphorus decay of O.OOI/day and phytoplankton settling rate of 0.1 m/day. 2) A "pessimistic" kinetic condition of loss of nutrient only through phytoplankton settling. 3) An "optimistic" kinetic condition which assumes the lake to be in equilibrium with the present loads and which implies a loss of inorganic as well as organic forms of nutrients. Figure 5 shows the result from a typical simulation, in this case, a maintenance of present loads and the reasonable kinetic condition. The spring peak of phytoplankton reaches a new dynamic equilibrium after 8-10 years or about equal to the detention time of the lake.

PIIECIf'ITATION AND stOIMENT "[lEAS(, llIl)NIC'f'ALIUIDINOIlSHIIAl

~T"'~UTA"'ES

_L"'~EE"'E

1974

WQA LOAD

("~:~;::;,,,)

LOADS

~" IfIIETII'CTON·2105LBS/OAY

TECHNOLC'GICALLY

FCE~;i:~~J

FIG. 4. Summary of phosphorus loading to Lake Ontario. (14.9)

(153)

{15.n

(15.8)

(15.8)

NOTE' VALUES IN PARENTHESES ARE COMPUTED PEAK SPRING VALUES

19

20 TIME, years

FIG. 5. Dynamic behavior of phytoplankton biomass in epilimnion. continuation of present inputs.

Figure 6 is a summary of the simulations under four loading conditions and the three kinetic cases. For each load reduction, the decrease has been assumed to be instantaneous. The values shown are the computed spring peak chlorophyll levels. It is seen that the determination of the overall loss rate of the nutrients is critical to the simulation. If the lake is assumed to be in equilibrium

129

PHYTOPLANKTON RESPONSES TO NUTRIENTS

PRESENT

25 20

z ...J ...J

15

a..

10

>I

0

PESSIMISTIC

;?

WQA

25

REASONABLE

OPTIMISTIC

20 15 10

r

PESSIMISTIC REASONABLE

a:= 0---

...JC)

5

OPTIMISTIC

5

I2-

U

z zo 0-

4

12

8

16

24

20

4

8

12

16

20

24

~z ~~

Z::J

«...Ja.. a..

w

0

VOLLENWEIDER

25 20

~

>I

a..

15

~

«w

10

a..

PASTORAL

25 20

~

PESSIMISTIC REASONABLE

15

OPTIMISTIC

5 4

12

8

PESSIMISTIC

10

16

20

5 4

24

REASONABLE

OPTIMISTIC

8

12

16

20

24

YEARS

YEARS

FIG. 6. Sensitivity of phytoplankton response to kinetic assumptions. 16....-----------------..,

I

~ 14 "C

~ 12

O'#'?'~

S 10 •

<.:> ... PRESENT ~ 8

~

e:

------19

-.lfl~~

<'>

",~'I-'''~

.'1-'

~

17

~,"

,,,,,

15 13

<'>"'' ':...''

~

9 6 z: g .....

'

I~,A

O'l-'f'''''''~' '---- ~----21 ""~:...."'o

l1

,," \~'r'-~';;,.....---------9

PASTORAL ....J

_

~

Z

~

fl~-O

7

w ~

>-

~ 10

II

12

FIG. 7. Peak phytoplankton chlorophyll 0-]7 m, as a function of nitrogen and phosphorus input, K org = 0.001/ day, K inorganic = 0.0.

with present loads, then reductions of load result in reductions of biomass which continue for a period of about 5-10 years to a new level. If the reasonable or pessimistic kinetic condition is used, then reductions in nutrient input do not necessarily result in reductions in biomass. This is a

consequence of the non-equilibrium condition implied by these kinetics. The pastoral case is interesting since it shows that peak phytoplankton levels in the past may have been as low as 3 fJ.g chlorophyll/lor about 30% of present levels. The wide range in response under any given loading condition illustrates the need to explore the long tenn behavior of large lakes and also graphically shows the problem of implementing eutrophication control programs· under uncertainty of expected responses. The calculated responses also indicated that for various kinetic assumptions either nitrogen or phosphorus would be the more limiting nutrient controlling phytoplankton growth. This is shown in Figure 7 which is a plot of the peak chlorophyll levels at equilibrium for the reasonable kinetic case and for a wide range of phosphorus and nitrogen loadings. The simulations divide rather sharply into two regions that are primarily phosphorus and nitrogen limiting. The earlier work (Thomann et al. 1975) indicated that both phosphorus and nitrogen limit phytoplankton growth.

130

THOMANN, WINFIELD and SZUMSKI

Simulations Under a 10-Year Implementation Program The simulation results presented in Figures 6 and 7 reflect long term trends in phytoplankton biomass that may result from various nutrient input conditions. While the results are not intended to be predictions of future conditions, they do provide insight into the behavior of the system under alternative loading scenarios. However, the reduction of nutrient inputs on a scale as large as Lake Ontario requires some time for actual implementation. Thus, a series of simulations have also been carried out using implementation periods of 10 and 20 years and various levels of Niagara River input (Hydroscience 1976). Recognizing that environmental conditions may vary from year to year and that long term simulations of greater than 20 years are largely descriptive, the simulations have been summarized for a 10-year period. That is, the phytoplankton response 10 years from an initial starting point is used as a decision variable. Additional variables are the rate of phosphorus load reduction and the assumed range of kinetic conditions. Figure 8 presents a summary of these results. The computed chlorophyll concentration is presented for the 10th year into an implementation program as a function of the phosphorus load reduction rate. The figure shows the range under the three kinetic assumptions and also shows the present range of peak annual chlorophyll concentrations observed in Lake Ontario. The load reduction scale on the abscissa is parallelled by two other scales describing the total load reduction after 10 years and the residual loading at the end of 10 years. Finally, the five reference loading conditions given in Table 3 are shown. Figure 8 shows that there is some uncertainty that present peak chlorophyll levels will be maintained under the Agreement after a 10-year period. Under the reasonable kinetic assumption, chlorophyll concentrations would increase above present levels even with load reductions. If optimistic kinetic conditions are assumed, however, the results indicate about a 40% decrease from present levels in peak biomass if the mass loading of the Agreement is reached in 10 years, Le. approximately 0.9 tonnes per year (2000 lbsjday per year). If one uses the range between optimistic and reasonable kinetics as a guideline, the results in Figure 8 indicate that substantial reductions must be accomplished over the 10-year period to maintain present conditions. It can also be noted that the technologically feasible level of 0.1 mg Pjl in

, - - - - - - - - - - - - - , FIGURE REPRESENTS CONDITIONS TEN YEARS FROM THE PRESENT

TIME

--]>

-' -'

>-

if o

OPTIMISTIC KINETIC

a:

--

CONDITIONS

o

i! , '-' ~

........

-

....

;'l Q.

_ _I--_---JL....-_---l_ _---l

o~

o

o.~

1.0

1.5

z.o

RATE OF PHOSPHORUS LOAD REDUCTION (METRIC TONS/DAY YEAR)

IIfDTE: IIIIITIUC TQffoUO'LU/OilY

FIG. 8. Effect of phosphorus load reduction rate.

the effluent provides a greater degree of assurance that the present biomass level will be maintained or reduced. Thus, the rate of implementation appears to critically impact the degree to which the goals of the Water Quality Agreement are realized in terms of phytoplankton biomass in Lake Ontario. It appears that a load reduction rate of 1-1.5 tonnesj day (2000-3000 Ibsjday) per year for a 10-year period is a sound objective to maintain or reduce present phytoplankton biomass levels. DISCUSSION It should be strongly noted that these simulations are indicative of general trends in open lake behavior and do not reflect near-shore responses which may be quite different. Also, because of the sensitivity of the simulations to the long term kinetics in the model, the results should only be interpreted as general guidelines and directions. The results highlight the uncertainty that accompanies the understanding of predicting limnological processes in large lakes. The determination of overall loss rates of various nutrient species requires further research into phosphorus and nitrogen chemistry under assumptions of long detention times (e.g. decades). Research into modeling the dynamic behavior of phytoplankton

PHYTOPLANKTON RESPONSES TO NUTRIENTS in the Great Lakes is still very much in its infancy. As such, the generation of simulations, as reported herein, is based on an imperfect understanding of potentially critical mechanisms. Nevertheless, policies and decisions will still have to be made even though future research may suggest adjustments and corrections. The simulations shown in this paper, therefore, are intended to provide additional insight into the behavior of phytoplankton biomass in Lake Ontario and to aid in the development of future policies on nutrient control for the lake.

ACKNOWLEDGMENTS Special thanks are due to Messrs. William Richardson and Nelson Thomas of EPA, Grosse Ile, for their help and support during this investigation and to Dr. Wu-Seng Lung of Hydroscience, Inc., who diligently followed through on the final simulations. This work was supported in part by EPA Research Grant R 800610 to the Environmental Engineering and Science Program of Manhattan College. The load assessments and summary simulations were part of a contractual arrangement between the IJC and Hydroscience, Inc. REFERENCES Bannennan, R. T., Armstrong, D. E., Harris, R. F. and Holdren, G. C. 1975. Phosphorus Uptake and Release by Lake Ontario Sediments. Environmental Protection Agency, Corvallis, Oregon. 660/3·75-006. Casey, D. J. and Salbach, S. E. 1974. IFYGL Stream Material Balance Study. Environmental Protection Agency, Rochester, New York and Ontario Ministry

131

of the Environment, Toronto, Canada (Unpublished manuscript.) Great Lakes Water Quality Agreement with Annexes and Texts and Tenns of Reference Between the United States and Canada. Signed at Ottawa. 1972. Great Lakes Water Quality Board 1973. Great Lakes Water Quality Annual Report to the International Joint Commission. Hydroscience, Inc. 1976. Assessment of the Effects of Nutrient Loadings on Lake Ontario Using a Mathematical Model of the Phytoplankton. Prepared for International Joint Commission, Windsor, Ontario, Canada. Westwood, New Jersey. Loehr, R. C. 1972. Agricultural Runoff· Characteristics and Control. J. San. Eng. Div. ASCE. 98:909·925, No. SAG, Proc. Paper 9406. St. Lawrence River Water Pollution Control Boards. 1969. Report to the International Joint Commission on the Pollution of Lake Erie, Lake Ontario and the International Section of the St. Lawrence River, Vol. 3. International Lake Erie and International Lake Ontario. Thomann, R. V., Di Toro, D. M., Winfield, R. P. and O'Connor, D. J. 1975. Mathematical Modeling of Phytoplankton in Lake Ontario: 1. Model Development and Verification. Environmental Protection Agency, Corvallis, Oregon, 660/3-75-005. _=---=--_' Winfield, R. P., Di Toro, D. M., and O'Connor, D. J. 1976. Mathematical Modeling of Phytoplankton in Lake Ontario: 2. Simulations Using Lake 1 Model. Environmental Protection Agency, Duluth, Minnesota, 660/3-76-065. Vollenweider, R. A. 1968. Scientific Fundamentals of the Eutrophication of Lakes and Flowing Waters, with Particular Reference to Nitrogen and Phosphorus as Factors in Eutrophication. Organization for Economic Cooperation and Development, Director for Scientific Affairs.