Effect of Cadmium on a Stable, Large Volume, Laboratory Ecosystem Containing Daphnia and Phytoplankton

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Abstract
Stable 3400-L indoor Daphnia-phytoplankton ecosystems were exposed to cadmium to test the hypothesis that sublethal concentrations of toxicants cause an increase in the biomass of lower trophic levels and a decrease in the biomass of higher trophic levels. At 5 .mu.g .cntdot. L-1 the Daphnia population collapsed after approximately 9 wk and chlorophyll levels increased to levels substantially greater than those observed in the control. At 15 .mu.g .cntdot. L-1 the Daphnia population declined after approximately 5 wk. Chlorophyll levels remained low even after the Daphnia collapse. At concentrations of 1 .mu.g .cntdot. L-1 and below, the ecosystem appeared to remain stable. The nature of the response of the ecosystem to 5 .mu.g Cd .cntdot. L-1 was predicted correctly, but the magnitude of the response was not. A synergistic effect between cadmium toxicity and Daphina population growth inhibition by high algal concentrations resulted in a much more dramatic response than predicted. A standard benchtop Daphina bioassay could be used to accurately predict the concentration of cadmium below which no detrimental ecosystem effects are likely to occur; however, the degree of response cannot be predicted.