Phytoplankton and foraminiferal frequencies in northern Indian Ocean and Red Sea surface waters

Abstract

This paper describes the distribution patterns of living planktonic foraminifers, coccolithophorids and picocyanobacteria along an east-west traverse in northern Indian Ocean and Red Sea surface waters during the southwestern monsoon. The absolute frequencies of coccolithophorids and foraminifers more or less correlated with temperature in the western Arabian Sea and South of India, where deeper waters well up into the photic zone. The increased frequencies in these upwelling zones were especially accounted for by the coccolithophorids Emiliania huxleyi (Lohmann) Hay & Mohler, Gephyrocapsa oceanica Kamptner and the foraminifer Globigerina bulloides d'Orbigny. The coccolithophorids Umbellosphaera irregularis Paasche and U. tenuis (Kamptner) Paasche showed a preference for warm, oligotrophic waters, and were absent in the upwelling areas. The small, red-pigmented picocyanobacteria in surface waters (depth 0 to 5 m) formed an important part of the primary production. Their relative frequency was more or less constant, on an average of 2·10 7 cells·dm −3 along the entire transect, so their relative contribution to biomass was higher in nutrient-poor environments than in nutrient-rich upwelling waters. Both carbonate producing foraminifers and coccolithoporids increase in number with improved nutrient availability in the upwelling areas. Their relative frequencies in sediments underlying the upwelling zones will reflect the varying upwelling conditions, caused by fluctuations in monsoonal atmospheric circulation, and thus allow for the reconstruction of past climatic change in this area.