Under-ice environmental conditions, planktonic communities and ichthyofauna in dystrophic lakes

Abstract

High content of humic substances and low pH values are factors limiting the species richness and abundance of some organisms in dystrophic lakes. Unfavourable winter conditions (i.e. low water temperature, poor light conditions, ice/snow cover) may additionally restrict their development. The aim of this study was to compare ice-on (winter) and ice-off (summer) abiotic (organic carbon, total phosphorus and nitrogen) and biotic (chlorophyll a, phytoplankton, ciliates, rotifers, crustaceans, fish) parameters in five dystrophic lakes of the Wigry National Park (north-east Poland). We tested the hypothesis that the abundance and diversity of planktonic organisms in dystrophic lakes could be lower in winter than in summer. Our results showed that the winter period, with ice and snow cover, was characterised by clearly higher oxygen concentrations, pH values and conductivity, but lower total phosphorus concentrations, species richness of phytoplankton, ciliates, rotifers and crustaceans, and phytoplankton to zooplankton biomass ratios in comparison to the summer. All of the studied groups of organisms, except rotifers, reached relatively high abundances and biomasses in both seasons and, in some lakes, they were higher in winter than in summer. Our results suggest that fish composition and abundance did not play an important role in structuring plankton communities. The small dystrophic lakes, although located close to one another, differed in terms of abiotic parameters and had specific species compositions of phyto- and zooplankton. Warmer winters, which are the result of climate change, may favour the intensive development of planktonic communities under the ice in dystrophic lakes of temperate climatic zones.

Keywords

• ice cover
• phytoplankton
• ciliates
• rotifers
• crustaceans