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We hypothesized that the grazing on phytoplankton by the microzooplankton community is size-dependent and, therefore, the top-down control on phytoplankton by microzooplankton community could be one possible mechanism explaining why small phytoplankton become less abundant than large phytoplankton in eutrophic waters. We tested this hypothesis using the dilution method to measure microzooplankton grazing rates and phytoplankton growth rates in the eutrophic waters of the Barataria estuary, southeastern Louisiana. Microzooplankton grazing rates on the slower growing, small phytoplankton (\5 lm) were higher than on the large phytoplankton ([20 lm) which had relatively faster growth rates. The proportional loss of the small, medium, large phytoplankton, and total phytoplankton community by microzooplankton grazing was 44, 53, 0, and 29%, respectively. The relative weakness of top-down grazing control on large phytoplankton by microzooplankton, and the relatively fast growth of large phytoplankton, may be why the average size of phytoplankton, whether isolated cells or colonies, tend to increase in these eutrophic waters and elsewhere.

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