Fluctuations in zooplankton abundance and species composition are described for two copper experiments conducted in Controlled Experimental Ecosystems during 1974. Copper concentrations of 5, 10, and 50 μg/l were tested. The major phenomenon in both experiments was a severe
reduction (> 80%) in the abundance of zooplankton in all CEBs, control as well as copper polluted. A portion of the population decline was attributed to grazing by carnivorous ctenophores and medusae, making it difficult to quantitatively assess the effects of copper. The abundance of ctenophores
and medusae remained higher in control than in copper perturbated CEEs, indicating that these organisms are adversely affected by the copper concentrations used in the experiments. At a nominal concentration of 50 μg/l copper, abundances of Pseudocalanus sp. and Acartia
longiremis were reduced to 50% of their original levels 3-3.5 times more quickly than in the controls. Results in 5 and 10 μg/l CEEs were more variable and less significant. It is not known whether these effects were the direct result of copper on the organisms, or an indirect
result of alterations at lower trophic levels.
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