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 rate were measured in the Somali Basin, the Gulf of Aden, and the southern part of the Red Sea during the SW monsoon (May-Aug 1992) and NE monsoon (Jan-Feb 1993). Strong upwelling (SST <20°C) occurred between 7° and 11°N along the Somali coast during July. Diatoms dominated in patchy blooms of phytoplankton north of Ras Hafun at the border of the Somali Current. Picophytoplankton (mainly Synechococcus and pico-eukaryotes) were present, but their contribution to the total chlorophyll a content was less than 30%. Nutrient concentrations in the main upwelling wedge south of Ras Hafun were high (nitrate over 15 μM), but chlorophyll a concentrations remained relatively low (≤0.6 mg.m ^-3) both in the freshly upwelled water of the Somali Current and downstream around Socotra Island and in the Great Whirl. In the latter areas, nutrients were not depleted (nitrate concentration varied between 3 and 8 μM) but primary production did not exceed 1 g C.m ^-2.day ^-1 as mixed layer depth largely exceeded the euphotic zone depth. Primary production for the whole northern Somali 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 picophytoplankton (up to 80% of total chlorophyll a content; dominated by Synechococcus, pico-eukaryotes and Prochlorococcus). Strong winds resulted in nutrient entrainment from deeper water. Surface values of chlorophyll a were ca 0.3 mg.m ^-3 and mean primary production of the Somali 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, including a deep chlorophyll maximum. Phytoplankton was dominated by picoplankton, namely Synechococcus. Primary production was low (0.5-0.6 g C.m ^-2. day ^-1 h). During the NE monsoon winter cooling resulted in deep vertical 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, applying flow-cytometry, of the daily net growth rates of individual phytoplankton species/groups during the NE monsoon showed a large spatial variation 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 values at the bottom of the euphotic zone, implying a high grazing pressure. Net growth rates were converted to daily variations in plant carbon and compared with ¹⁴C-based primary production data (only during the NE monsoon). In nutrient replete regions, the two methods showed comparable values for carbon production, indicating that all newly fixed carbon resulted in an increase in plant biomass. Higher nutrient levels in particular favoured growth of larger phytoplankton (> 3 μm). In more oligotrophic (low surface nitrate) areas, < 35% of the primary production resulted in a net increase in plant carbon. At some stations grazing pressure balanced phytoplankton growth, as plant carbon did not change over a full day incubation period.