Estimated Responses of Lake Ontario Phytoplankton Biomass to Varying Nutrient Levels

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% of present 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 10 years, a load reduction rate of about 1-1.5 metric tons phosphorus/day per year appears to be a sound objective to maintain or reduce present phytoplankton levels.