Predicting the effect of toxic substances on pelagic ecosystems

Abstract

Recent advances in the description of pelagic ecosystems using the biomass size spectrum can be adapted to predict the effect of toxic substances on whole ecosystems. The method is demonstrated using Lake Ontario as an example. First, the shape of the biomass size spectrum is predicted. Using a previously published estimate of particle-size conversion efficiency (—log(biomass conversion efficiency)/log(predator-prey size ratio)), and an estimated exponent of the allometric relationship between body size and turnover rate of 0.24, the biomass over logarithmically equal size classes in Lake Ontario is estimated to be proportional to body size raised to the exponent −0.02. The biomass within various size classes is then estimated from data on zooplankton and phytoplankton biomass. Total biomass, from algae to fish, is estimated to be 7–16 g m −2 dry weight. The change in biomass and production over various size classes after addition of a toxic substance can then be predicted if the total nutrient load remains relatively constant and if the effect of the substance on biomass conversion efficiency is known. A toxicity sufficient to decrease growth and conversion efficiency by 50% is predicted to reduce production of large fish by 91%, but phytoplankton production by only 12% in the lake. Fish biomass is predicted to drop substantially, phytoplankton biomass should rise, and zooplankton biomass should remain relatively unaffected.