No abstract

Zooplankton mortalities resulting from passage through the Donald C. Cook Nuclear Plant (southeastern Lake Michigan) were studied over an 8-year (1975-1982) period. The power plant operated at a low AT (< 12°C) and discharge water temperatures did not exceed 35°C (except September 1978). While zooplankton mortalities were significantly greater in discharge than intake waters, differences were small, averaging < 3%. There was no evidence of additional delayed effects on zooplankton mortality following plant passage. Calanoid copepods (Diaptomus spp, Eurytemora affinis and Limnocalanus macrurus) were most sensitive to plant passage, cyclopoid copepods least sensitive, while cladocerans (Daphnia spp, Eubosmina coregoni) were intermediate in sensitivity. There was no relationship between zooplankton mortalities and temperature (AT, discharge water temperature), suggesting that thermal stresses were not the major source of mortality. The single exception was September 1978 when discharge water temperatures exceeded 35°C and there were large differences between intake and discharge water zooplankton mortalities (net mortality differences of 14-22% for the two units). Mechanical stresses appeared to be the major cause of zooplankton mortality. However, there was only a weak relationship between mortality as a result of plant passage and zooplankton size. Based on our long-term preoperational (1970-1974) and operational (1975-1982) ecological studies in the vicinity of the power plant, we hypothesize that fish predation, rather than power plant operation, probably was the major source of zooplankton mortality in inshore waters during much of the year.