Cladoceran community dynamics reflect temperature gradients in a cooling water reservoir

Cladoceran community dynamics in limnetic systems often are correlated with temperature patterns. We sought to relate cladoceran temporal patterns and community composition to spatial and temporal temperature patterns in a cooling water reservoir (Newton Lake, Jasper County, Illinois). Effluent released into Newton Lake creates a temperature gradient where portions of the reservoir experience summer maxima in excess of 35 C, while other portions resemble a typical temperate system. We measured temperature and collected zooplankton at four locations arranged at increasing distances from the power plant cooling water outfalls. Cladoceran community density was higher near the warm effluent during winter, although no statistically significant differences amongst sites were observed during summer. Bray-Curtis dissimilarity values and nonmetric multi-dimensional scaling suggest cladoceran communities in the reservoir respond to this altered temperature with respect to abundance and seasonality of species. Daphnia lumholtzi, an exotic cladoceran, also is present in Newton Lake and sometimes is the dominant cladoceran taxon. High abundance of D. lumholtzi seems to be unrelated to warm effluents as it occurs at all sampling locations. Although morphological features of D. lumholtzi are thought to deter depredation, temporal and spatial distance likely occurs between this species and gape-limited vertebrate planktivores, thereby reducing potential negative impacts of this species in Newton Lake.