Size-fractionated primary production and nitrogen assimilation in the northwestern Indian Ocean

N. J.P. Owens, P. H. Burkill, R. F.C. Mantoura, E. M.S. Woodward, I. E. Bellan, J. Aiken, R. J.M. Howland, C. A. Llewellyn

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Abstract

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 14C and 15N isotopes. Samples were fractionated after the incubations into three size classes: 0.2-0.8μm, 0.8-5.0 μm, and >5.0μm for the 14C incubations; and <5.0μm and >5μm for the 15N 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 μm fraction at the south of the transect; this decreased to approximately 10% in the upwelling region. New production (φratios) varied between approximately 0.1 in the oligotrophic regions and 0.9 in the upwelling region.

Original languageEnglish
Pages (from-to)697-709
Number of pages13
JournalDeep-Sea Research Part II
Volume40
Issue number3
DOIs
Publication statusPublished - 1993

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