Aquatic Sciences Meeting, Albuquerque 2001

PC09 River and Margin Biogeochemistry

Date: Wednesday, February 14, 2001

Location: Southwest Hall

Dagg, M, , Louisiana Universities Marine Consortium, Chauvin, USA, mdagg@lumcon.edu

Liu, H, , Louisiana Universities Marine Consortium, Chauvin, USA, hliu@lumcon.edu

MICROZOOPLANKTON GRAZING OF PHYTOPLANKTON IN THE MISSISSIPPI RIVER PLUME

Phytoplankton growth rate and microzooplankton grazing rate were studied in the Mississippi River plume during three cruises (April, July and September) using the dilution technique. High phytoplankton growth rates of more than 2 doublings per day were found in waters with intermediate salinity during all cruises, with highest growth rates (> 4 doublings per day) occuring in summer. Growth was limited by depleted silicate concentration at the edge of the plume during summer. Microzooplankton grazing rate ranged from near zero to as high as 1.5 day-1. Grazing to growth ratio increased linearly with salinity, from less than 0.3 at a salinity of 10 to > 0.7 at salinities > 30. Grazing rates were higher during the day than at night. Our results suggest that microzooplankton herbivory is an important trophic process that mediates phytoplankton blooms in regions of river-ocean interaction. . High phytoplankton growth in plume waters of intermediate salinity results from high nutrient concentrations in river water and increased water column stability and light availability. Microzooplankton grazing rates are initially low in the low and intermediate salinity waters because of the time-lag in establishing high stocks. As the plume extends and mixes with sea water, microzooplankton grazing increases and eventually surpasses phytoplankton growth.

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Aquatic Sciences Meeting, Albuquerque 2001

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