Vertical variability of pelagic zooplankton fauna in deep Latvian lakes, with some notes on changes in ecological conditions

Abstract

Vertical structure of zooplankton communities from five deep Latvian lakes during the late summer and early autumn season was analysed. Changes in species composition, abundance, and dominant species assemblages were assessed in relation to temperature, oxygen, pH, chlorophyll a, oxidation reduction potential, and conductivity. Where possible, comparison with historical data was made. No significant changes of ecological conditions and annual dynamics of zooplankton diversity and abundance during the last 50 years were observed. However, a decrease of oxygen concentration occurred in deep lakes, caused probably by the pressure of human induced eutrophication. Also, slight differences were observed in the species composition, which most likely mark changes in the trophic status. A total of 24 to 38 Rotifera and Cladocera species were found during investigations from all lakes. The dominant species were Conochilus hippocrepis, Keratella cochlearis, Polyarthra vulgaris, Filinia longiseta, Kellicottia longispina, Daphnia (Daphnia) cucullata, Diaphanosoma brachyurum, Bosmina (Eubosmina) crassicornis, and B. (Eubosmina) longispina. Zooplankton communities can be defined as distinct groups both at species and higher levels on the basis of seasonal stratification in deep lakes. Temperature, oxidation reduction potential, and chlorophyll a concentration were found to be statistically the most influential environmental factors in the vertical distribution of zooplankton. Also sampling date and depth showed strong trends concerning zooplankton abundance and species composition. Vertical segregation between samples occurred within water layers of 0-10 m, 10-20 m, and below 20 m depth. The samples collected at the beginning of the sampling season assembled into a separate group.