Long term changes of crustacean zooplankton communities in Lake Wigry as a response to changing trophic conditions - 100 years of observations

Abstract

Wigry Lake has one of the longest zooplankton data sets in Poland and has been investigated for over 100 years. Therefore, our goal was to present long-term changes in zooplankton structures against the background of changing trophic conditions. The species composition of crustacean zooplankton in the first half of the 20ᵗʰ century was typical for a large mesotrophic lake, with a high share of Oaphnia cucullata and stenotherm species: Eurytemora lacustris, Heterocope appendiculata, Cyclops lacustris, Bythotrephes brevimanus, Oaphnia longiremis. The results of the research from the years 1965-1985 were very pessimistic, because the lake was exposed to intense eutrophication and there were important changes in zooplankton communities. The glacial relicts and species characteristic for low trophic condtions declined, while the share of species typical for high trophic conditions (Mesocyclops leuckarti, Thermocyclops oithonoides, Oiaphanosoma brachyurum) increased. As a result, species characteristic for eutrophic conditions made up to 70% of crustacean zooplankton, when in 1921 their share did not exceed 25%. The results of hydrochemical and biological studies of the last 30 years indicate a gradual tendency to improve the trophic state of Lake Wigry, due to biomanipulation activities carried out in the lake and its catchment area. We also have observed a marked increase in the occurrence of glacial relicts and the dominance of large Oaphnia individuals, which indicates an improvement in water quality.