Spatio-temporal patterns of bacterioplankton production and community composition related to phytoplankton composition and protistan bacterivory in a dam reservoir

Seasonal changes in the abundance and production of epilimnetic bacterioplankton, protistan abundance and bacterivory, and extracellular phytoplankton production (EPP) were studied at 3 sampling stations (DAM, MIDDLE and RIVER) located along the longitudinal axis of the canyon-shaped, meso-eutrophic Rimov reservoir (Czech Republic). We found that at the river inflow (RIVER) compared to lacustrine parts of the reservoir (MIDDLE and DAM), different sources of organic carbon and of bacterial mortality control bacterioplankton dynamics and community composition. At the RIVER site, EPP accounted for a negligible part of bacterial carbon demand, thus indicating the prominent role of allochthonous sources of organic substrates in the river inflow. In addition, protistan bacterivory removed there, on average, only 9% of bacterial production. In contrast, at the lacustrine MIDDLE and DAM stations, protistan bacterivory accounted for 47 and 78% of bacterial production, respectively. Moreover, at these stations EPP was an autochthonous source of organic carbon sufficient to meet bacterial carbon demand and EPP was tightly correlated with bacterial carbon demand (DAM, r super(2) = 0.589, p < 0.005; MIDDLE, r super(2) = 0.716, p < 0.001). At the DAM site, we analyzed changes in EPP in relationship to phytoplankton community dynamics and found that cryptophytes were associated with EPP. Only 2 algal groups, cryptophytes in a spring-early-summer period and diatoms in a summer- fall period, clearly dominated the phytoplankton. Changes in phytoplankton composition were related to changes in bacterial community composition studied by means of group-specific rRNA-targeted oligonucleotide probes. A trend of increased proportions of certain bacterial groups, mainly of the genus-like R-BT065 subcluster of Betaproteobacteria, was detected for the periods of high EPP levels, dominated by cryptophytes. More than 52% of the seasonal variability in the abundance of the R-BT065 cluster was explained by changing EPP levels that indicated a tight taxon-specific algal-bacterial relationship.