Combined Effects of Cadmium and Zinc on a Lake Mighican Zooplankton Community

Two 3-week experiments with small enclosures were conducted in situ in northeastern Green Bay, Lake Michigan, to compare the effects of separate and combined additions of cadmium and zinc on the zooplankton community. The radioisotopes 109Cd and 65Zn were used as tracers to determine the sorption of cadmium and zinc by different particle size fractions. Separate additions of 2 μg Cd/L or 100 μg Zn/L, as well as combined additions of 2 μg Cd/L + 100 μg Zn/Lor 1 μg Cd/L + 50 μg Zn/L, caused significant reductions of total crustacean density, species diversity, two community similarity indices, and final dissolved oxygen concentration. The effects of separate additions of 100 μg Zn/L were generally more pronounced than those of 2 μg Cd/L, but were similar to those of ∼5 μg Cd/L. The effects of combined additions of 2 μg Cd/L + 100 μg Zn/L were not significantly different from those of 100 μg Zn/Lalone and were primarily due to zinc because it reduced cadmium uptake by the plankton (10–85 μm and >85 μm fractions). The effects of combined additions of l μg Cd/L + 50 μg Zn/L probably were also mainly due to zinc. The overall results of this study suggest that less than a ten-fold increase in the concentration of zinc in Lake Michigan (presently ∼5 μg/L) could have pronounced effects on the plankton community.