Seasonal Dynamics of Bacterio‐ and Phytoplankton in Large and Shallow Eutrophic Lake Võrtsjärv, Estonia

Abstract

AbstractThe seasonal dynamics of bacterioplankton abundance, biomass and production was measured in the large (surface area 270 km2) and shallow (mean depth 2.8 m) Lake Võrtsjärv during one year. Bacterial biomass and heterotrophic activity (production) was measured using thymidine and leucine incorporation methods. The bacterial biomass production was estimated by leucine incorporation, the cell number production by thymidine incorporation. Primary production and photosynthetic extracellular release was measured simultaneously to evaluate their role on heterotrophic bacterial activity. Relationships between released fraction of primary production and heterotrophic bacteria, as well as between biomass of bacteria and biomass/composition of phytoplankton are discussed. In terms of the carbon balance, the total primary production could support carbon need of heterotrophic bacteria in Lake Võrtsjärv. Neither biomass nor activity of bacteria showed tight coupling to primary production over the whole investigation period. In spring and summer the relationship between bacterial production and primary production was weak. The development of cyanobacterial bloom in summer provided more substrates for heterotrophic bacteria than diatoms in spring. In autumn both activity of bacteria and algae decreased rapidly in a coupled manner.