Phytoplankton community structure in the Arabian Sea during and after the SW monsoon, 1994

Diatoms, dinoflagellates, coccolithophores, nanoflagellates, picophytoplank ton and procaryote algae (Synechococcus spp. and prochlorophytes) were quan tified by microscopy and flow cytometry, and their biomass determined, at 1 2 stations along a 1600 km transect across the Arabian Sea at the end of th e SW monsoon in September, and during the inter-monsoon period of November/ December 1994. The transect spanned contrasting oceanic conditions that var ied from seasonally eutrophic, upwelling waters through mesotrophic, downwe lling waters to permanently oligotrophic, stratified waters. The overall di versity of diatoms, dinoflagellates and coccolithophores along the transect was not significantly different between the SW monsoon and inter-monsoon. However, diatoms showed greatest diversity during the SW monsoon and coccol ithophores were most diverse during the inter-monsoon. Integrated phytoplan kton standing stocks during the SW monsoon ranged from 3 to 9g Cm-2 in the upwelling eutrophic waters, from 3 to 5g Cm-2 in downwelling waters, and fr om 1 to 2g Cm-2 in oligotrophic waters. Similar phytoplankton standing stoc ks were found in oligotrophic waters during the inter-monsoon, but were ca. 40% lower compared to the SW monsoon in the more physically dynamic waters . Phytoplankton abundance and biomass was dominated by procaryote taxa. Syn echococcus spp. were abundant(often > 10(8) cells l(-1))during both the SW monsoon and inter-monsoon, where the nitrate concentration was greater than or equal to 0.1 mu moll(-1), and often dominated the phytoplankton standin g stocks. Prochlorophytes were restricted to oligotrophic stratified waters during the SW monsoon period but were found at all stations along the tran sect during the inter-monsoon, dominating the phytoplankton standing stocks (> 40%) in the oligotrophic region during this period. Of the nano- and mi cro-phytoplankton, only diatoms contributed significantly to phytoplankton standing stocks, and then only in near-shore upwelling waters during the SW monsoon. There were significant changes in the temporal composition of the phytoplankton community. In nearshore waters a mixed community of diatoms an Synechococcus spp. dominated during the SW monsoon. This gave way to a c ommunity dominated by Synechococcus spp. in the inter-monsoon. In the downw elling zone, a Synechococcus spp. dominated community was replaced by a mix ed procaryote community of Synechococcus spp. and prochlorophytes. In the o ligotrophic stratified waters, the mix of procaryote algae was replaced by one dominated by prochlorophytes alone. (C) 1999 Elsevier Science Ltd. All rights reserved.