Comparison of virus- and bacterivory-induced bacterial mortality in the eutrophic Masan Bay, Korea

Characteristics of viral ecology and the relative importance of virus and bacterivory-induced bacterial mortality were investigated in surface and hypoxic layers of the eutrophic Masan Bay, Korea, in summer 1999. During the study, environmental and microbial variables changed strongly in the bay. However, the viral to bacterial abundance ratio (VBR) mostly varied less than 3-fold even in a declining phase of viruses and bacteria, indicating a tight coupling between bacterial and viral assemblages. Comparisons of viral lysis and heterotrophic nanoflagellate (HNF) grazing rates showed that viruses exerted a small (average 9.4 %, range 1.6 to 34.4 %, n = 29) but relatively constant control on bacterial production in the bay, whereas HNF exerted a highly variable (average 41 %, range 0.3 to 183.4 %, n = 17) effect on the mortality of bacteria. However, the relative ratios of virus- to HNF-induced bacterial mortality varied from 0.03 to 2.0, comparable to a range reported for open ocean waters, except for 2 data of 17.9 and 24.5 from low-salinity conditions. Overall, virus-bacteria-HNF interactions in the bay seemed to be generally regulated in a steady-state situation.