Does Copepod Grazing Control Large Phytoplankton in the HNLC Region of the Southern Ocean?

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95-30819 DAGG 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 factors 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. Large phytoplankton, especially diatoms, typically grow slowly in the permanently ice-free regions of the Southern Ocean, including the Antarctic Polar Front Zone (APFZ), because of micronutrient (Fe) limitation or light limitation or limitation associated with deep mixing. It is hypothesized that under these conditions the growth of large phytoplankton is approximately balanced by grazing from the copepod community. When growth of large phytoplankton is enhanced by Fe inputs or improved light conditions, it is hypothesized that the additional growth is mostly absorbed by the functional response of the copepod community. Larger enhancements of phytoplankton growth however, will saturate the grazing on diatoms increases the Si:C ratio of sinking matter because approximately 70% of ingested carbon is assimilated across the copepod gut wall but Si is digestively inert. These relationships will be addressed by shipboard experiments designed to measure the feeding rates of large copepods on large phytoplankton of the APFZ, and by measurements of the ratio of carbon to biogenic silica in copepod fecal pellets and in sediment trap materials.