The aim of this study was to determine factors affecting the vertical distribution of Eurytemora lacustris in mesotrophic lakes (Wigry, Hańcza, Szurpiły; north-eastern Poland) during the summer stagnation. Eurytemora lacustris was found in all of the studied lakes, with the highest abundance (8 ind. L-1) in Lake Wigry. In Lake Szurpiły, E. lacustris has never been recorded before. The results of this study revealed that E. lacustris was most numerous in thermocline zones, suggesting that this species could temporarily tolerate warmer water and lower oxygen concentrations due to better food resources. During the study, it was found that a large part of the E. lacustris population had epibiont ciliates, in contrast to other species of zooplankton that did not have any epibionts. The improvement in the water quality of many deep lakes could lead to an increase in the abundance of E. lacustris. However, epibiont ciliates may be a threat for this species and may play a substantial role in determining the production, distribution, and community dynamics of E. lacustris.
Keywords: Environmental factors; Epibionts; Eurytemora lacustris; Food resources; Vertical distribution.