Viral abundance in aquatic systems: a comparison between marine and fresh waters

In order to investigate the factors controlling viral abundance, 22 lakes In Quebec were surveyed. We measured viral and bacterial abundance, bacterial production, chlorophyll a, total phosphorus and DOC (dissolved organic carbon) concentrations. Regression models built with these data were compared to models based on literature data, which to date have been collected largely from marine sites Positive empirical relationships were found between viral abundance and (1) chlorophyll a concentrations, (2) bacterial abundances, (3) bacterial production, and (4 ) total phosphorus concentration. There was little to no trend in the virus-to-bacteria ratio with increasing trophy. Analysis of covariance revealed significant differences between relations in marine and freshwater systems. The virusto-bacteria ratio was significantly higher in freshwater (mode = 22.5) than marine environments (mode = 2.51, and there were significantly more bacteria per unit chlorophyll in our freshwater samples. We suggest that this difference is related to the increased dependence of freshwater bacteria on allochthonous material relative to marine systems, as well as the increased relative importance of photosynthetic cyanobacteria in lakes.