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The seasonal and vertical distribution of freshwater viruses and the related induced bacterial mortality were determined in the water column down to the surface sediments of the thermally stratified reservoir Lake Grangent, France, between March and November 2007 in relation to environmental parameters. Viral abundance (VA, range = 0.9 to 3.5 x 1010 l―1), bacterial abundance (BA), and the frequency of infected cells were significantly higher in the oxic than in the anoxic waters, which was partly related to forcing from the thermal stratification together with vertical changes in dissolved oxygen, ammonia and phytoplankton biomass. BA was the best predictor (R2 = 0.78) for VA in the oxygenated waters only. The virus-to-bacteria ratio (VBR) yielded less variability and did not differ significantly along the water column (mean ± SD value = 5.5 ± 1.3). In the sediments, both VA and BA were one order of magnitude higher than in the water column with a higher variable VBR ratio (10.6 ± 8.9). Paradoxically, the frequency of visibly infected bacterial cells (determined from transmission electron microscopy) in the sediments was significantly lower (on average 3- to 7-fold) compared with the water column, which is in agreement with recent benthic reports in geographically contrasting lake environments. On average, viruses destroyed 23 % of bacterial production in the oxygenated surface waters, but only 9 % in deep anoxic waters and 3 % in the sediments. Overall, there was a remarkably low viral influence on heterotrophic bacteria in the anoxic waters and overlying sediments of Lake Grangent, where recurrent shift from summer planktonic to winter benthic phases in cyanobacterial blooms is typical.

Low effect of viruses on bacteria in deep anoxic water and sediment of a productive freshwater reservoir