SIZE-FRACTIONATED PRIMARY PRODUCTION AND NITROGEN ASSIMILATION IN THE NORTHWESTERN INDIAN-OCEAN

Rates of phytoplankton production and nitrogen assimilation were measured at various stations along a transect in the northwestern Indian Ocean, from near the equator, northwards into the upwelling system off the Arabian peninsula, during September-October 1986. The measurements were made using in situ incubation techniques with the simultaneous use of C-14 and N-15 isotopes. Samples were fractionated after the incubations into three size classes: 0.2-0.8 mum, 0.8-5.0 mum, and >5.0 mum for the C-14 incubations; and <5.0 mum and >5.0 mum for the 15 N incubations. The assimilation of nitrate and ammonium was measured. These measurements were supported by a detailed description of the horizontal and vertical distributions of chlorophyll, temperature and underwater light field, by the deployment of a towed undulating oceanographic recorder. Rates of primary production ranged from approximately 0.5 g C m-2 day-1 at the equator, reducing to <0.3 in the oligotrophic gyre in the central northern Indian Ocean, and to >2.5 in the upwelling region off the coast of Oman; total nitrogen assimilation followed a similar pattern. Very significant variations in the size distribution of the activity of the plankton were observed. Over 75% of the carbon and nitrogen assimilation was in the <5 mum fraction at the south of the transect; this decreased to approximately 10% in the upwelling region. New production (f ratios) varied between approximately 0. 1 in the oligotrophic regions and 0.9 in the upwelling region.