Abiotic and biotic factors regulating dynamics of bacterioplankton in a large shallow lake.

Lake Võrtsjärv (270 km2, mean depth 2.8 m, Estonia) is an eutrophic and turbid lake (Secchi depth 0.5-1 m) with a high nutrient load (total nitrogen 1-2 mg N l(-1), total phosphorus approximately 50 microg P l(-1)) leading to a highly productive phytoplankton population (average chlorophyll a concentration 24 microg l(-1)). Seasonal dynamics of the main members of pelagic microbial loop - phyto-, bacterio- and protozooplankton - has been studied for several years (1993-1998) in lake. The most prominent characteristic of this naturally eutrophic, turbid shallow lake is an inter-annual water level fluctuation (3.2 m) which is in the range of its average depth. Bacterioplankton growth was favored by high water level whereas phytoplankton growth was favored by low water level. In autumn and sometimes in winter bacterioplankton production (BP) was unbalanced with respect to primary production (PP) (gross BP 20-90% higher than PP) suggesting to additional sources of organic carbon. The main grazers of bacteria were probably various ciliates. In contrast to most eutrophic and stratified lakes, the relative prevalence of the microbial loop over the linear phagotrophic food chain was obvious in this eutrophic shallow lake. The bacterioplankton showed a pronounced seasonal succession with low diversity at high production periods in summer and with high diversity at low biomass and production periods in winter and spring.