Variable importance of viral-induced bacterial mortality along gradients of trophic status and humic content in lakes

1. The goals of this study were to assess which environmental gradients can be important in predicting viral abundance (VA), the frequency of virus-infected cells (FVIC) and burst size (BS), and to assess which lake characters favour viral-induced bacterial mortality (VBM) as opposed to potential grazer-induced bacterial mortality (PGBM). 2. The epilimnion and hypolimnion of 21 lakes differing in trophic status and humic content were investigated. Samples were obtained for viral and bacterial abundance, the FVIC and burst size, flagellate and ciliate abundances and water chemistry. The potential importance of VBM in relation to PGBM was calculated based on data on FVIC and flagellate and ciliate abundance. 3. Partial least squares regression (PLS) analysis showed that VA as well as the ratio between viral and bacterial abundance were positively related to inorganic nutrient concentrations, dissolved organic carbon concentration, chlorophyll-a concentration and bacterial production. 4. The PLS model also revealed that FVIC decreased with increasing trophic status whereas BS increased. Furthermore, potential grazing was positively related to increasing trophic status and decreasing humic content. PGBM appeared to be dominating over VBM in the majority of lakes sampled, especially in the epilimnion. 5. Thus, the relative importance of viruses for bacterial mortality was potentially highest in humic lakes of medium trophic status and it was also greater in the hypolimnion than in the epilimnion.