Biological indications of active upwelling in the Northwestern Indian Ocean in 1964 and 1979, and a comparison with Peru and Northwest Africa

Abstract

When surface properties of the northwestern Indian Ocean observed during the southwest monsoon of 1964 (R.R.S. ‘DISCOVERY’) are compared with those of 1979 (R.V. ‘ISELIN’), the area of upwelling around Ras Hafun (near 10°N) showed the clearest similarity in the two years. The highest surface concentration of chlorophyll a between Mombasa (4°S) and Cabo Guardafui (12°N) occurred near Ras Hafun in both years and was between 4 and 5 mg m −3. Sea-surface temperatures around Ras Hafun were lower than elsewhere along the coast, and surface nitrate was higher. The abundance of the copepod Calanoides carinatus was >100 m −3 there and near 5°N in both years. The age-structure of this dominant copepod suggests active reproduction in both years. Major dissimilarities between 1964 and 1979 were observed in the vertical distribution of temperature and nitrate near 5°N, which suggested that upwelling there in August 1964 was less intense than in July 1979. The intensity of upwelling near 5°N probably reached a peak in July both years, implying a relatively stable annual cycle for the upwellings off Somalia. The relationship of primary productivity and chlorophyll a observed off Somalia is very similar to that observed off Peru, and standing stocks of net zooplankton are also similar, but both are quite different from northwestern Africa. Perhaps the most striking difference, however, is the areal of extent of upwelling off Somalia which is considerably larger than that of the other two upwelling areas.