Winter dynamics of abiotic and biotic parameters in eutrophic temperate lakes

Winter conditions in lakes of temperate climatic zones, especially the duration and thickness of ice and snow cover, are strongly influenced by climate changes. The aim of this study was to follow the dynamics of abiotic (organic carbon, total phosphorus, total nitrogen) and biotic (chlorophyll a, cyanobacteria, algae, nanoflagellates, ciliates, rotifers, crustaceans) parameters during two consecutive winters in three eutrophic lakes of different mixing regime (northeastern Poland). Our results showed that dissolved organic carbon was highly stable during both winters and practically did not differ among the studied lakes. Total phosphorus and chlorophyll a concentrations differed significantly between the dimictic and meromictic lakes. The concentrations of nutrients and chlorophyll a and the biomass of phytoplankton, protists, rotifers and crustaceans reached higher values in late winter than in early winter, depending on the lake type (morphometry, mixing regime) and the year of the study. The composition of phytoplankton and ciliates was more or less similar throughout the study in all lakes during both years, but the share of individual groups in the total biomass differed between early and late winter. A high cyanobacterial biomass in January under the ice in the meromictic lake was probably a legacy of the summer/autumn community. Small prostomatid ciliates were the most important group in late winter in the shallow lake. Cladocerans (Bosmina coregoni and B. longispina) were an important component of winter crustacean community on some occasions, especially in the shallow lake. Phosphorus was the major factor determining phytoplankton biomass. Heterotrophic nanoflagellates and rotifers were negatively related to ice cover, while ciliates were positively related to the water temperature. Our study demonstrated that the dynamics, abundance, and structure of the phytoplankton and zooplankton communities in eutrophic lakes changed from one winter to the next despite similar environmental conditions.