Abstract Florida Power Corporation (FPC) owns and operates the Anclote Power Plant at the mouth of the Anclote River on the Gulf of Mexico near Tarpon Springs, Florida. The circulating water system for the plant consists of six circulating water pumps, four cooling tower pumps serving two sidestream mechanical draft cooling towers, and four dilution pumps. The dilution pumps, which transfer water directly from the intake canal to the discharge canal, are capable of achieving FPC’s thermal discharge limitations when the ambient temperature is favorable. However, allowable dilution pump usage has been severely limited in past renewals of the NPDES Permit due to the conservative agency assumption that no zooplankton survived entrainment through these pumps. However, during a recent renewal of the NPDES Permit for Anclote, FPC proposed to the Florida Department of Environmental Protection (FDEP) an increase in dilution pump operation. To facilitate the review of this proposal, FPC agreed to perform a study of entrainment survival through the plant during September–November 1995. The results of this study demonstrated that overall entrainment survival for selected life stages and species of zooplankton, including ichthyoplankton, was significantly greater than had been previously hypothesized. Interestingly, the data demonstrated that zooplankton survived entrainment through the unit condensers at a mean overall rate of 68.2%, despite experiencing a temperature differential of up to 7°C. Mean overall survival through the dilution pumps was calculated to be 87.0%. This information was used by the FDEP as the basis for a permit revision allowing increased usage of the dilution pumps for control of thermal discharges. This allows FPC to greatly reduce operation of the cooling towers, which cause near total mortality of entrained zooplankton due to mechanical and chemical stressors.
