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Seasonal population dynamics and community composition of planktonic ciliates were studied for over a decade in a naturally eutrophic shallow lake, Lake Vortsjarv, which has a large open surface area (270 km 2 on average) and highly turbid water (Secchi depth 300 µm) gymnostomes. There was a positive correlation between ciliates and phyto- and bacterioplankton, implying that the ciliates were clearly bottom-up or food-controlled. This was further confi rmed by the positive correlation between ciliates and metazooplankton. As both bacteria and phytoplankton are coupled with resuspension of lake sediments, the large fl uctuations in water level infl uenced the ciliate community biomass annually.

The aim of the present study was to describe the role of ciliates and heterotrophic nanoflagellates in macrophyte and phytoplankton dominated shallow lakes and to compare the grazing on pico- and nano-sized particles by these protozoans. We sampled three shallow lakes, seasonally, one dominated by macrophytes, one by phytoplankton, and one with both conditions (two basins of this lake were sampled separately). The role of ciliates as consumers of bacteria and small algae was important in both lake types. In plankton dominated lakes the grazing rate of ciliates on bacteria was higher than in macrophyte dominated ones. But when ciliates were grazing on small algae, the highest rates were found in macrophyte lakes. Our results suggest that the microbial loop is weaker in macrophyte dominated lakes and grows stronger, when a lake becomes more turbid. The dominance of herbivorous ciliates in macrophyte lakes in summer and their high grazing rates on phytoplankton may have contributed to the decrease in phytoplankton abundance in these lakes. Heterotrophic nanoflagellates were relatively less abundant and, due to their low specific filtering rates, grazed only a minor fraction of the bacterioplankton.

Quick changes in community composition and population dynamics of planktonic ciliates were studied in the ponds of two fish farms obtaining water from different sources (from a fast flowing river or a stillwater reservoir). Samples were taken at two-day intervals during a short period after springtime filling of the ponds. At the beginning of the investigation period the community in the lotic ponds was dominated by species that favour periphyton or benthos as their habitat. They were gradually replaced by euplanktonic species. In the lentic ponds euplanktonic species dominated throughout our study. The similarity index ISj between lotic ponds and their inflow was 0.67 at the start of our study but decreased to 0.2 during 14 days. If the inflow was from a stillwater reservoir, the similarity index decreased only moderately. In the lotic ponds ciliate community shifted from bacterivory to algivory while in the case of lentic ponds the situation was the opposite - algivorous species were gradually replaced by bacterivores. Abiotic and biotic factors that determine ciliate community structure were separated. The abiotic features are ruling when the system goes through major changes (in our study the lotic pond system where water flow ceased dramatically). The biotic factors are crucial in more stable systems (in our study the lentic pond system) and can change the community structure in same degree as the abiotic ones.