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Phytoplankton pigment specific growth and losses due to microzooplankton grazing in a northern Gulf of Mexico estuary during winter/fall

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Microzooplankton dilution grazing experiments were carried out on 6 dates, over a 3 month period at 2 locations in the Bay of St. Louis, MS (BSL) to determine phytoplankton pigment specific growth rates under natural (µ0) and replete (µn) nutrient conditions and microzooplankton grazing. We hypothesized that diatoms would be the largest portion of the phytoplankton composition due to the winter/fall season and that these organisms would have the highest growth/grazing rates. We suspected that river flow from the Jourdan River would adversely affect growth and grazing rates of all phytoplankton classes. Growth rates of 5 phytoplankton accessory pigments (peridinin, fucoxanthin, alloxanthin, zeaxanthin, chlorophyll b) were identified. Intrinsic growth rates (µ0) were often zero or negative (range: -0.46–0.56/d) at the location nearest the Jourdan River, particularly for alloxanthin (e.g. cryptophytes) and peridinin (e.g. dinoflagellates). Significant grazing of chlorophyll a was observed on 3 of 6 dates while grazing on marker pigments was variable. The phytoplankton community appeared nutrient limited during all but one experiment (µ0n). Intrinsic growth and grazing rates were correlated (p < 0.05, Spearman Rank Order correlation). Peridinin- and alloxanthin-based growth and grazing rates were correlated positively with salinity suggesting a river influence on these 2 phytoplankton pigment classes. We conclude that in the BSL microzooplankton preferentially grazed on the phytoplankton class which had the highest intrinsic growth rate. We show that this is greatly affected by riverine input into the estuary and nutrient limitation.

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