PRIMARY ORGANIC PRODUCTION IN RELATION TO THE CHEMISTRY AND HYDROGRAPHY OF THE WESTERN INDIAN OCEAN1

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Abstract
Primary organic production was measured at 231 stations in the western Indian Ocean. Low levels were measured in the eastern part of the region, both north and south of the Equator. Intermediate levels were encountered in the equatorial area, in the Mozambique Channel, and in the southern part of the region. High productivity was observed in the northern and western portions, particularly in the western Arabian Sea.The high productivity of the Arabian Sea is attributed to the presence of unusually high concentrations of plant nutrients that extend nearly to the sea surface, from which there is ready transport into the euphotic zone by local advective processes. The distributional pattern of the nutrients, in turn, is believed to result from a major system of upwelling, whereby water from intermediate depths is transported north across the Equator and towards the surface, compensating for the southerly flow of surface water and its loss from the Arabian Sea. The fertility of the region is further enhanced by the mechanism of a nutrient trap which is inherent in all countercurrent upwelling systems.Annual organic production for the 23 × 106 km2 area studied was 3 × 1012 kg, averaging about twice that of the oceans as a whole.