Mathematical Modeling of Phytoplankton in Lake Ontario,Part 2 Simulations Using Lake 1 Model
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Date
1976-08
Journal Title
Journal ISSN
Volume Title
Publisher
Enviromental Protection Agency (EPA)
Abstract
The results of a series of simulations of the response of the open lake region of Lake Ontario to various levels of nutrient input are described in this report. The simulations use a simplified dynamic model of phytoplankton nutrient interactions in a vertically segmented structure.
The lake is assumed to be well-mixed in the horizontal direction. The problem of long term simulations (10-20 years) that draw on short term observation and verification periods (5 years) is discussed and it is indicated that the overall
loss rates of nutrient are of particular importance. Under a 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.
Therefore, if the present load is continued, it is estimated that spring peak phytoplankton chlorophyll in the epilimnion will continue to increase to a new level about 45% higher
than present levels. The interaction of nitrogen and phosphorus is also described by the simulations and the results indicate a tendency for nitrogen limitation to be an increasing dominant factor in controlling the spring
bloom.
A pastoral" simulation using load estimates, indicative of conditions prior to man's intensive activity provides an approximation of an early state of the lake. This "hindcast" indicates that spring phytoplankton levels were some 40% less than present levels and average
annual epilimnion biomass under equilibrium with present loads is about twice that under pastoral conditions. A series of analyses is also conducted comparing simulations
from the dynamic model to estimates made from simplified plots of loading versus lake geometry. The results from the dynamic model indicate that a reduction in external nutrient load does not result in an accompanying decrease in
phytoplankton biomass, due to the hypothesized non-equilibrium condition of Lake Ontario. The dynamic model results are therefore in contrast to the results one would obtain from
using "admissable" loading concept which indicates an improvement in lake trophic status.
Analysis of lake response to the U.S.-Canada Water Quality Agreement (WQA) loads using the hypothesized parameters indicates about a 6% reduction in peak phytoplankton at
equilibrium. The implications of the results appear to be of some importance since the analyses indicate that it may difficult to achieve measurable reductions below present
levels of phytoplankton biomass in the open lake. From a decision and policy making viewpoint then, the simulations tend to indicate that maximum point source nutrient control for Lake Ontario will, at best, be a "ho1ding" action rather than a significant improvement in the status of the
be open lake. This report was submitted in partial fulfillment of Grant Number R 800610 to the Environmental Engineering and Science Program, Manhattan College, Bronx, New York
by the U.S. Environmental Protection Agency. Work was completed as of April, 1975.
Description
Keywords
phytoplankton, Lake Ontario, nutrients inputs, simulations, mathematical modeling, pastoral responses, water quality, vollenweider reduction