Species diversity and spatial structure of pelagic zooplankton in a lake of glacial origin during summer stratification

Lake South Volos (55 degrees 37' N, 27 degrees 08' E, area 1.2 km(2), max. depth 40.4 m) is a stratificated mesotrophic lake with some features of oligotrophy. From May to September 2000, 54 Rotifera, 11 Copepoda and 17 Cladocera species were recorded in the pelagic zone. A substantional increase in the depth of the thermocline took place from May to July. It was accompanied by a rise in species richness of the zooplankton (Rotifera and Crustacea). Increase in the number of non-overlapping niches was observed with the increase in rotifer species richness. It was shown, that maximum species density was related to temperature, and ecological groups of the species developed in water layers of certain temperatures, though this may not always be indicative of thermopreference. Species dominance in a particular water layer in the column was caused by a combination of factors ("sliced functioning") influencing the zooplankton community. No active diurnal movements between the layers (epi-, meta- and, hypolimnion, and the upper hypolimnion/clinolimnion) were demonstrated by the majority of crustacean species.