AME prepress abstract  -  doi: 10.3354/ame01180

ABSTRACT: Lake Erie is the most productive of the North American Great Lakes and experiences annual periods of hypolimnetic hypoxia with unknown consequences for the microbial food web. For this study, we established the abundance and mortality rates of microbes in Lake Erie during thermal stratification and determined how these abundances and rates varied with changes in bottom-water dissolved oxygen. The microbial plankton community (heterotrophic bacteria, cyanobacteria, eukaryotic phytoplankton, nanozooplankton, microzooplankton) was quantified in surface and bottom waters along with rates of herbivory and bacterivory of eukaryotic and prokaryotic picoplankton and viral lysis of bacteria. High rates of grazing mortality of prokaryotic picoplankton (1.4 ± 0.6 d^-1) and eukaryotic algae (0.66 ± 0.27 d^-1) and significant correlations between microzooplankton abundances and all picoplankton populations quantified (p<0.05) during this study evidenced the strong impact of grazing on picoplanktonic communities in Lake Erie. Microbial herbivory accounted for half of total phytoplankton mortality per day. Bacterivory combined with viral lysis turned over 85% of the heterotrophic bacterial community daily. During the onset of hypolimnetic hypoxia, abundances of ciliates and rotifers decreased significantly (p<0.05) and herbivory was undetectable. Concurrently, bacterivory persisted at rates equal to those found in higher oxygen waters and abundances of heterotrophic nanoflagellates did not change significantly. These results suggest that during hypoxia in Lake Erie, herbivory by microzooplankton is disrupted, but bacterivory by heterotrophic nanoflagellates persists. Finally, rates of viral lysis of heterotrophic bacteria were higher in the hypolimnion compared to surface waters (p<0.05), suggesting increased viral lysis may enhance the regeneration of organic matter in bottom waters during hypoxic events.