SEASONAL AND SPATIAL VARIABILITY IN PHYTOPLANKTON BIOMASS, PRODUCTIVITY AND GROWTH IN THE NORTHWESTERN INDIAN-OCEAN - THE SOUTHWEST AND NORTHEAST MONSOON, 1992-1993

Phytoplankton abundance, composition, primary production and growth ra te were measured in the Somali Basin, the Gulf of Aden, and the southe rn part of the Red Sea during the SW monsoon (May-Aug 1992) and NE mon soon (Jan-Feb 1993). Strong upwelling (SST<20 degrees C) occurred betw een 7 degrees and 11 degrees N along the Somali coast during July. Dia toms dominated in patchy blooms of phytoplankton north of Ras Hafun at the border of the Somali Current Picophytoplankton (mainly Synechococ cus and pico-eukaryotes) were present, but their contribution to the t otal chlorophyll a content was less than 30%. Nutrient concentrations in the main upwelling wedge south of Ras Hafun were high (nitrate over 15 mu M), but chlorophyll a concentrations remained relatively low (l ess than or equal to 0.6 mg.m(-3) both in the freshly upwelled water o f the Somali Current and downstream around Socotra Island and in the G reat Whirl. In the latter areas, nutrients were not depleted (nitrate concentration varied between 3 and 8 mu M) but primary production did not exceed 1 g C.m(-2).day(-1) as mixed layer depth largely exceeded t he euphotic zone depth. Primary production for the whole northern Soma li Basin showed a mean of 1.25 g C.m(-2).day(-1) (range 0.8-2.8 g C.m( -2).day(-1)). During the NE monsoon, phytoplankton was dominated by pi cophytoplankton (up to 80% of total chlorophyll a content; dominated b y Synechococcus, pico-eukaryotes and Prochlorococcus). Strong winds re sulted in nutrient entrainment from deeper water. Surface values of ch lorophyll a were ca 0.3 mg.m(-3) and mean primary production of the So mali Basin was 0.8 g C.m(-2).day(-1) (range 0.5-1.0 g C.m(-2).day(-1)) . In the southern Red Sea and Gulf of Aden a reverse seasonal pattern occurred. During the SW monsoon, both areas were oligotrophic, includi ng a deep chlorophyll maximum. Phytoplankton was dominated by picoplan kton, namely Synechococcus. Primary production was low (0.5-0.6 g C.m( -2).day(-1)). During the NE monsoon winter cooling resulted in deep ve rtical mixing and surface water replete with nutrients. Chlorophyll a values varied between 0.5 and 1.0 mg.m(-3), with the highest values in diatom blooms in the Red Sea. Mean primary production was 1.2 g C.m(- 2).day(-1) (range 0.5-2.2 g C.m(-2).day(-1)). A detailed study, applyi ng flow-cytometry, of the daily net growth rates of individual phytopl ankton species/groups during the NE monsoon showed a large spatial var iation indicative of active dynamics in the algal composition even on a daily basis. For picophytoplankton the highest growth rates exceeded 1.5 day(-1), but larger phytoplankton showed lower maxima for growth rates (ca 1 day(-1)). Net growth rates decreased often to negative val ues at the bottom of the euphotic zone, implying a high grazing pressu re. Net growth rates were converted to daily variations in plant carbo n and compared with C-14-based primary production data (only during th e NE monsoon). In nutrient replete regions, the two methods showed com parable values for carbon production, indicating that all newly fixed carbon resulted in an increase in plant biomass. Higher nutrient level s in particular favoured growth of largerphytoplankton (> 3 mu m). In more oligotrophic (low surface nitrate) areas, <35% of the primary pro duction resulted in a net increase in plant carbon. At some stations g razing pressure balanced phytoplankton growth, as plant carbon did not change over a full day incubation period. (C) 1997 Elsevier Science L td.