SEASONAL DYNAMICS OF BACTERIOPLANKTON AND PHYTOPLANKTON IN LARGE AND SHALLOW EUTROPHIC LAKE-VORTSJARV, ESTONIA

The seasonal dynamics of bacterioplankton abundance, biomass and produ ction was measured in the large (surface area 270 km(2)) and shallow ( mean depth 2.8 m) Lake Vortsjarv during one year. Bacterial biomass an d heterotrophic activity (production) was measured using thymidine and leucine incorporation methods. The bacterial biomass production was e stimated by leucine incorporation, the cell number production by thymi dine incorporation. Primary production and photosynthetic extracellula r release was measured simultaneously to evaluate their role on hetero trophic bacterial activity. Relationships between released fraction of primary production and heterotrophic bacteria, as well as between bio mass of bacteria and biomass/composition of phytoplankton are discusse d. In terms of the carbon balance, the total primary production could support carbon need of heterotrophic bacteria in Lake Vortsjarv. Neith er biomass nor activity of bacteria showed tight coupling to primary p roduction over the whole investigation period. In spring and summer th e 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 autu mn both activity of bacteria and algae decreased rapidly in a coupled manner.