Nitrogen Cycling in the Upwelling Waters of the Equatorial Pacific

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The upwelling of nutrients in Equatorial Pacific waters enhances plankton biomass and primary production over a large area and may account for a significant fraction of the global new production. Nitrate persists in the euphotic zone in an area a few hundred kilometers north and south of the upwelling axis and several hypotheses have been advanced to account for the apparent inability of the phytoplankton to fully utilize it. This project hypothesizes that grazing controls phytoplankton biomass and that recycling of nutrients provides much of the phytoplankton nitrogen requirement. This study will utilize new techniques which have been applied over the last few years in the Sargasso Sea and the JGOFS North Atlantic Bloom Experiment to test the grazing/regeneration hypothesis as part of the U.S JGOFS Equatorial Pacific program. This hypothesis implies specific patterns and distributions of nitrogenous nutrients, their utilization and regeneration, and natural N isotope abundance in nutrients and suspended and sinking particulate material. This project is collaborative with Chris Garside (9022726) and consists of two major components: 1) nutrient concentration measurements of high precision (nanomolar sensitivity where necessary) and high spatial resolution; 2) low level and high sensitivity tracer studies of their biological uptake and transformations. The results of this study will constitute an assessment of the relative importance of "new" and "regenerated" nitrogen in the Equatorial Pacific region. The principal mechanisms regulating production will be identified, and the extent to which "new" production contributes to particle flux and carbon sequestration will be assessed.