Picocyanobacteria in eutrophic reservoirs of the Middle Volga: abundance, production, viral infection]

During the summer season of 2010, abundance and productivity of picocyanobacteria in Gorky and Cheboksary Reservoirs have being examined. It is found out that in the eutrophic reservoirs of the Middle Volga abundance and biomass of picocyanobacteria, averaged over the water column, varied within the range of (34-322) x 103 cells/ml and 38-455 mg/m3 respectively. In more productive Cheboksary Reservoir, the contribution of picocyanobacteria in total biomass and production of phytoplankton (4.7 0.7 and 8.3 1.3% respectively) was lower than in less productive Gorky Reservoir (10.6 2.1 and 19.2 3.0% respectively). In both reservoirs, high level of picocyanobacteria infection by viruses was detected. The frequency of visible infected cells and virus-induced mortality of picocyanobacteria in Cheboksary Reservoir were substantially higher (3.2 0.4% of total abundance and 21.8 2.9% of daily production) than in Gorky Reservoir (1.7 0.2% of total abundance and 11.0 1.7% of daily production). The results obtained indicate that in eutrophic reservoirs during summer bloom of large cyanobacteria their abundance is regulated to a great extent by viruses.