The Influence of Fish–Zooplankton–Phytoplankton Interactions on the Results of Selenium Toxicity Experiments Within Large Enclosures

The response of zooplankton to sodium selenite at concentrations of 1,10, and 100 μg Se∙L−1was studied using large (100 m3) enclosures in mercury-contaminated Clay Lake, Ontario (50°03′N, 90°30′W). Crustacean community development in treated enclosures exhibited no obvious acute or chronic effects from the range of selenium concentrations applied. Seasonal changes in zooplankton composition, abundance, and reproduction were similar among controls and the 1 and 10μg Se∙L−1enclosures. Differences in species dynamics observed in the 100 μg Se∙L−1enclosure and in a sediment control enclosure were attributable to biotic interactions rather than selenium. Predation appeared to be the dominant factor governing zooplankton community structure in all enclosures. Differences in the number of planktivorous fish among enclosures led to variations in the abundance of predatory Leptodora kindtii, which in turn affected abundances of the small cladoceran Bosmina longirostris.