Virioplankton and microbial communities in two Czech rivers (Svratka and Morava River)

Viruses are identified as the most abundant and dynamic component of aquatic communities in marine and freshwater environments. However, there is still limited information about viral abundance and ecology in the rivers. We studied virioplankton abundance in relation to bacterial abundance, chlorophyll a (Chl a; expressed as total Chl a, Chl a in green algae, and in cyanobacteria), and environmental parameters (inorganic nutrient concentration, temperature) in two Czech rivers throughout a 6-month period. Viral abundances reached similar values in both rivers and ranged from 1.85 to 58.1 x 10(6) viruses/ mL and from 1.68 to 47.4 x 10(6) viruses/mL in the Svratka and the Morava River, respectively. The temporal variations in viral and bacterial abundances were large in both rivers, as they varied by more than one order of magnitude during the study. We did not observe a spatial variability in viruses or other biotic parameters in spite of differences in some environmental parameters within each river and also between the two rivers. Significant cross-correlations among viruses, bacteria, Chl a, and orthophosphate concentration were found, suggesting that these parameters may influence viral abundance and dynamics in the rivers.