Since viral lysis and nanoflagellate grazing differ in their impact on the aquatic food web, it is important to assess the relative importance of both bacterial mortality factors. In this study, an adapted version of the modified dilution method was applied to simultaneously estimate the impact of both virus and nanoflagellate grazing on the mortality of heterotrophic bacteria. A series of experiments was conducted monthly from April to December 2011 and April to October 2012. The growth rates of bacteria we measured ranged from 0.078 h⁻¹ (April 2011) to 0.42 h⁻¹ (September 2011), indicating that temperature can be important in controlling the seasonal variations of bacterial growth. Furthermore, it appeared that seasonal changes in nanoflagellate grazing and viral lysis could account for 34% to 68% and 13% to 138% of the daily removal of bacterial production, respectively. We suggest that nanoflagellate grazing might play a key role in controlling bacterial biomass and might exceed the impact of viral lysis during the summer period (July to August) because of the higher abundance of nanoflagellates at that time. Viral lysis, on the other hand, was identified as the main cause of bacterial mortality between September and December. Based on these findings in this study, the seasonal variations in bacterial abundance we observed can be explained by a scenario in which both growth rates and loss rates (grazing + viral lysis) influence the dynamics of the bacteria community.

This study investigated the relative effect of nanoflagellate grazing and lytic mortality on bacteria. Growth and removal rates for bacteria in the inner and outer regions of the Changjiang River plume were characterized by a series of dilution experiments at four sites, two inner sites (I-1 and I-2) and two outer sites (O-1 and O-2) in the summers of 2011 and 2012. Bacterial growth rates varied between 0.058 and 0.157 h⁻¹, with higher growth rates detected in the inner plume. Grazing mortality rates ranged from 0.042 to 0.126 h−1, with highest grazing rates detected in the inner plume. Viral lysis of bacteria was not significant at three of eight stations, and accounted for >50% of bacterial mortality only once. These findings suggest that grazing nanoflagellates may play a key role in controlling bacterial biomass, and the impact of the nanoflagellates exceeds that of viral lysis during the summer period, especially in the inner region of the Changjiang River plume. It is speculated that the weakening correlation between viruses and bacteria in the inner plume, as well as the increasing virus to bacteria ratio from offshore oligotrophic waters to estuarine waters, may be a consequence of the increasing relative abundance of non-bacteriophage viruses with increasing environmental productivity.