TROPHIC INTERACTIONS WITHIN THE MICROBIAL FOOD-WEB IN PIBURGER-SEE (AUSTRIA)

The structure, seasonal dynamics and trophic interactions of the micro bial food web were investigated in the oligo-mesotrophic Piburger See. Abundance and biomass of the microbial food web components as well as bacterial production and predation by protists were quantified at wee kly intervals in the euphotic zone. Phytoplankton biomass showed a max imum under ice and in summer. Abundance of plastidic flagellates was c orrelated with chlorophyll-a (r = 0.48; p < 0.02). High abundance of n anociliates at the surface (ca. 1 x 10(5) ind.l-1), mainly Balanion pl anctonicum, were found to be important for the start of the clear-wate r phase. Mixotrophic ciliates were present under ice and during summer . Among them Pelagohalteria viridis reached abundances of up to 3 x 10 (3) ind.l-1. Bacterivory exceeded bacterial production on several occa sions although, on average, they were balanced. Aplasticid flagellates were the main controllers of bacterial production, ciliates had a ver y low impact (< 3.1% of water column cleared per day). Autotrophic pic oplankton represented a significant alternative C pool for protists ma king up to 43% of total picoplankton biomass. During most time of the year the microbial food web (MFW) seems to act as a sink for carbon, e xcept during the clear-water phase when cladocerans have a strong graz ing impact on the whole MFW. Using a multiple regression model we foun d that 35% of the variability of bacterial production was explained by the abundance of bacteria and temperature. This value increased to 50 % when the number of active (INT) bacteria was used instead of total b acteria. Bacterial abundances showed a low predictive value on aplasti dic flagellate abundances, suggesting that trophic pathways in this la ke are strongly fluctuating and that a coupling between both populatio ns does not always exist.