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Aquatic Microbial Ecology

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AME 51:117-128 (2008)  -  DOI:

Grazing and virus-induced mortality of microbial populations before and during the onset of annual hypoxia in Lake Erie

Christopher J. Gobler1,*, Timothy W. Davis1, Sarah N. Deonarine1, Matthew A. Saxton2, Peter J. Lavrentyev3, Frank J. Jochem4, Steven W. Wilhelm2

1School of Marine and Atmospheric Sciences, Stony Brook University, Southampton, New York 11968, USA
2Department of Microbiology, The University of Tennessee, Knoxville, Tennessee 37996, USA
3Department of Biology, The University of Akron, Akron, Ohio 44325, USA
4Marine Biology Program, Florida International University, North Miami, Florida 33181, USA

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. We established the abundances and mortality rates of microbes in Lake Erie during thermal stratification and determined how they varied with changes in bottom-water dissolved oxygen concentrations. The microbial plankton community (heterotrophic bacteria, Cyanobacteria, eukaryotic phytoplankton, nanozooplankton, microzooplankton) was quantified in surface and bottom waters along with measurements of herbivory and bacterivory rates on eukaryotic and prokaryotic picoplankton and rates of 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 demonstrated the strong impacts of grazing on Lake Erie picoplanktonic communities. Microbial herbivory accounted for half of total phytoplankton mortality per day. Bacterivory and viral lysis turned over 85% of the heterotrophic bacterial community each day. During the onset of hypolimnetic hypoxia, abundances of ciliates and rotifers decreased significantly and herbivory was undetectable. Concurrently, bacterivory persisted at rates equal to those found in shallower oxygenated waters, and abundances of heterotrophic nanoflagellates did not change significantly. These results suggest that, during hypoxia events 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 than in surface waters, suggesting that increased viral lysis may enhance regeneration of organic matter in bottom waters during hypoxic events.

KEY WORDS: Microbial mortality · Great Lakes · Grazing · Viruses · Hypoxia

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Cite this article as: Gobler CJ, Davis TW, Deonarine SN, Saxton MA, Lavrentyev PJ, Jochem FJ, Wilhelm SW (2008) Grazing and virus-induced mortality of microbial populations before and during the onset of annual hypoxia in Lake Erie. Aquat Microb Ecol 51:117-128.

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