Structure and Dynamics of Plankton Communities in the Antarctic Frontal Zone: Interactions of Physical Forcing, Iron Limitation and Microzooplankton Grazing

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96-34053 Landry This research project is part of the US Joint Global Ocean Flux Study (JGOFS) Southern Ocean Program aimed at (1 ) a better understanding of the fluxes of carbon, both organic and inorganic, in the Southern Ocean, (2) identifying the physical, ecological and biogeochemical favors and processes which regulate the magnitude and variability of these fluxes, and (3) placing these fluxes into the context of the contemporary global carbon cycle. The Southern Ocean is a vast and variable environment with a potentially large role in global carbon cycling. The JGOFS Southern Ocean Program seeks to advance the understanding of this region by investigating seasonal and spatial dynamics in two important subsystems -- the Ross Sea shelf and the Antarctic Polar Front Zone. This project will contribute to this effort by determining microbial community structure and by assessing rates of phytoplankton growth and microzooplankton grazing on four cruises covering the spring, summer, and autumn seasons in the open-ocean frontal zone. The overall goal of this research is to understand how physical processes (advection, mixing, and light), iron-limitation, and grazing interact to determine plankton community structure and production in the open oceans. Population abundances and biomasses of the plankton community will be assessed by analytical flow cytometry (bacteria and phytoplankton) and microscopy (nano- and microplankton) to determine their temporal and spatial patterns relative to physical features of the polar front. Growth rates of phytoplankton and bacteria, and microzooplankton herbivory and bacterivory will be measured with a suite of complementary methods including dilution, fluorescently labeled prey, size fractionation, and a Iysozme assay for bacterivory. Results of this project will contribute to the general understanding of phytoplankton control mechanisms and trophic interactions in high-nutrient, low-chlorophyll regions of the world's ocea ns. They will also help to identify how potential global change affects water-column stability and how physical forcing in the Southern Ocean may alter food web structure, carbon storage and export from the euphotic zone.