Nitrate - Ammonium Interactions and Nitrate Supply in the Regulation of Arabian Sea Regional New Production

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9312383 Garside Quantitative elucidation of the processes that regulate biological production in the sea is fundamental to an understanding of the role the oceans play in the earth's climate system. Temporal and spatial variability in the processes that govern the upward rate of nutrient flux from the deep-sea, and the drift of currents in the upper ocean, determine the distribution of marine plankton and set limits for their production. Pursuit of JGOFS objective requires investigation into these physical-biological linkages. They influence the global rates of carbon sequestration and release by the sea. How balance between these processes was regulated in the past, and how it is regulated today is an important research component of US national and international endeavors to understand and predict responses to global change. The monsoonal system of the Arabian Sea affords unique opportunity for the pursuit of these studies. Its intense seasonal cycle in air-sea interaction allows for the development and testing of models relating to the physical climate-driven process linkages to biological responses and consequent feedback via air-sea exchange of gases. In addition to the intensity of these processes, and hence their strong seasonal signal in the Arabian Sea, the record in marine fossil assemblages and isotopes in ocean sediments, is valuable in the development and testing of time varying models treating coupled physical and biological aspects of the ocean climate system. This study relates to one particular link in the system: the regulation of primary planktonic production by the supply rate of the limiting nutrient, nitrogen. The coarse seasonal pattern in Arabian Sea production is one of the most intense, production being associated with the summer SW monsoon. Upwelling delivers a pulse of nutrients to surface waters, which in turn stimulates a plankton remain generally higher than those typical of oceanic waters elsewher e. In this study the availability of nitrogen, both in the upwelled form of nitrate and in the recycled form of ammonium, will be assessed regionally during the low production NE winter monsoon and the two inter-monsoon transition periods. The interplay between dependence on the "new" and "regenerated" forms of nitrogen determine what fraction of total primary production is at any time available for export to the deep-sea. The techniques applied will permit the quantification of new production and its regulation throughout the low nutrient portion of the year. Nutrient analyses sensitive in the nanomolar range will be employed for quantifying the abundance, and with 15N labeled substrates, the rates of supply and utilization of nitrite, nitrate and ammonium in the upper ocean. This study will be conducted in collaboration with investigators who are addressing other aspects of the nitrogen cycle and biological production. New understanding resulting from these complementary projects will contribute to the formulation and testing of regional models that use remotely sensed information on biological and physical features and processes to better predict how production processes in the Arabian Sea will respond to future climate change.