Alterations in zooplankton community structure after selenium-induced replacement of a fish community: a natural whole-lake experiment

Abstract

Most of the fish species in Belews Lake, a cooling reservoir in the Piedmont area of North Carolina (USA), were eliminated in 1976–77 because of selenium poisoning. Zooplankton samples collected in 1974–75, prior to the fish kill, when the lake's fauna included a regionally-typical assemblage of piscivores and planktivores at a density of 14000 ha−1, were compared to those collected in 1984–86, when the piscine community was dominated by three planktivorous fish (Notropis lutrensis, Pimephales promelas and Gambusia affinis) at a density of 28000 ha−1. Zooplankton diversity was similar in 1984–86 compared to that in 1974–75. However, densities of the predominant cladocerans (Bosmina longirostris, Pseudosida bidentata, Ceriodaphnia pulchella, Holopedium amazonicum, Daphnia parvula, D. ambigua) and copepods (Diacyclops thomasi, Mesocyclops edax, Tropocyclops prasinus, Skistodiaptomus pallidus, S. reighardi) declined, though the magnitude of the reduction was greatest for the cladocerans and S. reighardi. Densities of copepod nauplii and rotifers also decreased markedly. No colonizations or extinctions of crustacean species occurred between 1975 and 1986. Thus, while crustacean species richness (n = 18 species) remained the same, the relative proportions of individual species changed dramatically, most notably among the Cladocera. The observed changes in crustacean species densities and dominance are attributed to the replacement of the regionally-typical fish fauna by one consisting primarily of planktivores.