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

The structure, seasonal dynamics and trophic interactions of the microbial 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 weekly intervals in the euphotic zone. Phytoplankton biomass showed a maximum under ice and in summer. Abundance of plastidic flagellates was correlated with chlorophyll-a (r = 0.48; p < 0.02). High abundance of nanociliates at the surface (ca. 1 x 10(5) ind.l-1), mainly Balanion planctonicum, were found to be important for the start of the clear-water 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 occasions although, on average, they were balanced. Aplasticid flagellates were the main controllers of bacterial production, ciliates had a very low impact (< 3.1% of water column cleared per day). Autotrophic picoplankton represented a significant alternative C pool for protists making 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, except during the clear-water phase when cladocerans have a strong grazing impact on the whole MFW. Using a multiple regression model we found 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 bacteria. Bacterial abundances showed a low predictive value on aplastidic flagellate abundances, suggesting that trophic pathways in this lake are strongly fluctuating and that a coupling between both populations does not always exist.