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 ) 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.

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Harmful brown tides caused by the pelagophyte Aureococcus anophagefferens have occurred in mid-Atlantic estuaries for 2 decades. Low grazing rates by microzooplankton have been implicated as a possible cause of these events, but no study to date has concurrently quantified zooplankton population densities and zooplankton grazing rates of A. anophagefferens cells. We conducted field studies from 2002 to 2004 to quantify grazing on the brown tide alga A. anophagefferens by meso-, micro-, and nanozooplankton, while concurrently establishing the composition of the plankton community. Research sites included an estuary that experienced an intense brown tide (Chincoteague Bay, Maryland [MD]; 2004: 2 × 10 6 cells ml ). The MD site was dominated by small autotrophs (< 5 µm), such as A. anophagefferens and other picoeukaryotes, and small heterotrophs, such as Paulinella ovalis, while the NY site hosted a range of large and small autotrophs and zooplankton. Experiments indicated that small zooplankton (3 to 5 µm) were consumers of A. anophagefferens at bloom and non-bloom locations. However, dilution experiments documented active grazing on most picoplankton except A. anophagefferens in MD, while grazing rates on the brown tide alga in NY were comparable to grazing rates on the total phytoplankton community and other picoplankton. Experimental enrichment of estuarine waters with mesozooplankton indicated a preferential avoidance of A. anophagefferens by grazers during intense blooms, but active consumption during non-bloom conditions. Differences in the effect of grazing between sites suggest that zooplankton may be controlling brown tides in NY, but allowing blooms to form due to low grazing pressure in MD. These findings further suggest that the zooplankton community in NY has changed from one which formerly avoided the consumption of A. anophagefferens to one which currently contributes to top-down control of brown tides.KEY WORDS: Harmful algal blooms · Brown tide · Aureococcus anophagefferens · Zooplankton · Grazing · Trophic cascade · Paulinella ovalis · Synechococcus sp. Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 44: [181][182][183][184][185][186][187][188][189][190][191][192][193][194][195] 2006 have negatively affected the hard clam Mercenaria mercenaria fishery as well (reviewed in Gobler et al. 2005).Aureococcus anophagefferens may adversely affect the growth and survival of micro-and mesozooplankton (Caron et al. 1989, Lonsdale et al. 1996, Sieracki et al. 2004. Microzooplankton (20 to 200 µm;Sieburth et al. 1978, Omori & Ikeda 1984 are particularly important in marine food webs as a trophic link between pico-(< 2 µm) and nanoplankton (2 to 20 µm) and mesozooplankton (> 200 µm;Lonsdale et al. 1996, Kiørboe 1998, Calbet & Landry 1999, Klein Breteler et al. 1999). During brown tides, feeding selectivity by microzooplankton may result in lower grazing rates on A. anophagefferens relative to grazing rates on co-occurring phytoplankton, thereby prom...

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