Four in situ experiments with small-volume (28-L) enclosures were conducted in Experimental Lakes Area Lake 382 to assess the effects of inorganic mercury and cadmium on the plankton community. Additions of 0.9 × 10−8 mol Cd/L caused significant reductions of crustacean zooplankton density, zooplankton community similarity, zooplankton species diversity, and dissolved oxygen concentration over 3-wk periods. Additions of 1.0 × 10−8 mol Hg/L caused reductions similar to those of 0.9 × 10−8 mol Cd/L. Phytoplankton primary productivity was reduced by additions of 2.0 × 10−8 mol Hg/L but not by 1.0 × 10−8 mol Hg/L. On a molar concentration basis, the toxicities of inorganic mercury and cadmium were quite similar. Significant changes in lake plankton communities may occur at molar concentrations above 0.2 × 10−8 mol/L. Potentially toxic concentrations of cadmium may occur in many contaminated lakes, but toxic concentrations of mercury probably occur in few lakes.
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Marshall et al. (1981) studied this question.