SIZE-FRACTIONATED PRIMARY PRODUCTION AND NITROGEN ASSIMILATION IN THENORTHWESTERN INDIAN-OCEAN

Rates of phytoplankton production and nitrogen assimilation were measu red at various stations along a transect in the northwestern Indian Oc ean, from near the equator, northwards into the upwelling system off t he Arabian peninsula, during September-October 1986. The measurements were made using in situ incubation techniques with the simultaneous us e of C-14 and N-15 isotopes. Samples were fractionated after the incub ations 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 incu bations. The assimilation of nitrate and ammonium was measured. These measurements were supported by a detailed description of the horizonta l and vertical distributions of chlorophyll, temperature and underwate r light field, by the deployment of a towed undulating oceanographic r ecorder. 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 i n the central northern Indian Ocean, and to >2.5 in the upwelling regi on off the coast of Oman; total nitrogen assimilation followed a simil ar pattern. Very significant variations in the size distribution of th e activity of the plankton were observed. Over 75% of the carbon and n itrogen assimilation was in the <5 mum fraction at the south of the tr ansect; this decreased to approximately 10% in the upwelling region. N ew production (f ratios) varied between approximately 0. 1 in the olig otrophic regions and 0.9 in the upwelling region.