The phytoplankton of the reservoir Embalse Rio Tercero presented three to four annual pulses depending on the hydrological conditions and the water column stability during stratification. Since the start of operation of the nuclear plant many modifications have been observed, among them, worth mentioning being the decrease in phytoplankton density and less heterogenous space distribution, besides an increase in the volume of water held back by the dam and a decrease in water level fluctuation.