Modeling the effect of algal biomass on multispecies aquatic microcosms response to copper toxicity

A model was developed to simulate the effects of copper on an aquatic microcosm consisting of 10 phytoplankton species and 5 zooplankton species grown in a defined medium. The copper toxicity equations in the model were based on the results of single species toxicity tests at different initial cell densities. Model output was evaluated by comparing predictions with results of replicated [3] 35 day microcosm experiments having copper added at 3 different times (days 7, 14 or 21) at 500 ppb concentration. These experiments offered a wide range of plankton and nutrient conditions at the time of copper addition. The baseline for these experiments was set by a control experiment, with no copper addition and a model, MICMOD, developed to simulate microcosm behavior. Two toxicity models were tested; the ‘neutral’ model defined copper toxicity to plankton as depending only on the dissolved copper concentration in the medium, while the ‘biomass’ model also decreased toxicity in proportion to algal biomass. Graphical and statistical comparisons showed the toxicity model including algal biomass to better fit the data. Both the experimental results and the model suggest that the effects of copper may be strongly influenced by the density and species composition of the biota and the related differences in water chemistry at the time of copper addition. The apparent importance of algal biomass to copper toxicity may be due to (1) changing copper availability either through direct absorption or adsorption, (2) production of chelates by the algae which complex copper in less toxic forms or (3) changing the pH which affects copper ionization and particularly the concentration of the cupric ion (Cu 2+ ) form, which is highly toxic.