During a winter–spring season, physicochemical variables and plankton dynamics were studied in Lough Derg (Ireland). Samples were collected monthly from three water layers (surface, middle and bottom) at a deep central sampling point on the lake. As expected, phytoand zooplankton densities and chlorophyll a concentrations were low during the study period, probably reflecting low temperatures and wind-driven turbulence typical of the winter-spring period. Diatoms and rotifers were the most abundant phyto- and zooplankton groups, respectively.