Individual Species and Community Level Consequences of Thermal Effluent on Midwestern Reservoirs

Few studies have addressed the consequences of thermal discharge in comparison to other undisturbed control systems. We examined multiple reservoirs in Illinois receiving thermal effluent from local power plants, and compared them with control lakes of similar size and location. We assessed the long-term effects of a warmer environment on a number of biotic and abiotic variables, temporal patterns, and fish and zooplankton community structure. Over a 14 year period, we found differences in a number of variables measured, including fish growth, zooplankton communities, and the timing of temporal processes. Specifically, we found growth rates for largemouth bass and bluegill to be higher in the warmed systems, and found that spawning began earlier for most fish species in the power plant lakes. To evaluate the effect of thermal discharge on individual species, we examined physiological condition of resident largemouth bass and again compared them to control fish. Fish from a a subset of study lakes were used to evaluate their response to a six-hour heat shock from an ambient temperature of 25° C to 34° C. Although fish from heated lakes were similar to controls in the physiological variables measured, including glucose, cortisol, and packed cell volume, fish from the warmer systems survived longer in thermal maxima experiments. Our results suggest that thermal effluent from power plants has
system-wide effects, and these have implications for our understanding of global climate change and the effects of warmer thermal regimes.