MICROBIAL FOOD-WEB IN A LARGE SHALLOW LAKE (LAKE-BALATON, HUNGARY)

Seasonal variations of phyto-, bacterio- and colourless flagellate pla nkton were followed across a year in the large shallow Lake Balaton (H ungary). Yearly average chlorophyll-a concentration was 11 mu g l(-1), while the corresponding values of bacterioplankton and heterotrophic nanoflagellate (HNF) plankton biomass (fresh weight) were 0.24 mg l(-1 ) and 0.35 mg l(-1), respectively. About half of planktonic primary pr oduction was channelled through bacterioplankton on the yearly basis. However, there was no significant correlation between phytoplankton bi omass and bacterial abundance. Bacterial specific growth rates were in the range of 0.009 and 0.09 h(-1), and ended to follow the seasonal c hanges in water temperature. In some periods of the year, predator-pre y relationships between the HNF and bacterial abundance were obvious. The estimated HNF grazing on bacteria varied between 3% and 227% of th e daily bacterial production. On an annual basis, 87% of bacterial cel l production was grazed by HNF plankton.