U.S. JGOFS: Microbial Communities in the Arabian Sea: Composition, Size, Spectra, and Growth Rates from Individual Cell Properties

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9311113 Olson A central mission of the JGOFS project in the Arabian Sea is to determine how efficiently the carbon pump operates in this region and how it responds to changes in physical forcing. To approach these problems we need to analyze the structure of the food web and determine how it varies with geographical and temporal gradients in nutrients supply and water column mixing. The juxtaposition of productive and oligotrophic oceanic systems in the Arabian Sea, and the regular oscillation between very high and low rates of nutrient supply under relatively constant conditions of solar illumination, offers opportunities for us to gain insights into food web regulation processes. In this project, Dr. Olson will (1) Characterize the distribution, abundance, and light scatter properties of the picoplankton and nanoplankton at the four intensive stations and at selected intermediate stations, using dual beam flow cytometry and DNA specific staining to distinguish heterotrophic as well as autotrophic organisms; (2) Estimate in situ growth rates of specific groups of phytoplankton, and grazing pressure on these groups, from diel patterns in cell number, light scatter properties and DNA content of individual cells from depth profiles; and (3) Test the hypothesis that the phytoplankton are physiologically N-limited during non-monsoon periods by measuring the C:N ratios of phytoplankton ( in particular, coccolithophores and picoplankton) purified by cell sorting. Relative responses of different groups of phytoplankton to N enrichment will be determined from 14C bicarbonate incubation experiments followed by cell sorting to obtain cell-specific growth rates.