Vertical and seasonal distributions of micro-organisms, zooplankton and phytoplankton in a eutrophic lake

Abstract

The vertical distributions of bacteria, picoalgae, protozoan and metazoan zooplankton, and phytoplankton in the highly eutrophie Lake Köyliönjärvi (SW Finland) were studied monthly during the period of ice-cover in January-April 1996. For comparison, we also provide some data on the distributions of the plankton during the summer. The whole water column remained oxic during the ice-covered period, although the near-bottom oxygen concentrations were always very low. The heterotrophic nanoflagellates were more abundant in winter than in summer, but ciliates, picoalgae and bacteria were more numerous in summer. In general both zooplankton and phytoplankton had low biomass during the ice-covered period. However, the biomass of the diatom Aulacoseira islandica ssp. islandica was high under the ice in April. The calanoid copepod Eudiaptomus graciloides was the dominant zooplankton species from January to March, but had almost disappeared by the beginning of April and did not increase again until in June. The dominant rotifer species in winter were Keratella cochlearis, Filinia terminalis, and Filinia longiseta in the surface water and Rotaria neptunia near the bottom.