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 × 10⁶ cells ml⁻¹) and one that experienced sporadic blooms (Quantuck Bay, New York [NY]; 2002: 8 × 10⁵ cells ml⁻¹; 2003 and 2004: × 10⁴ cells ml⁻¹). The MD site was dominated by small autotrophs (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.

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