Microbial community dynamics and taxon-specific phytoplankton production in the Arabian Sea during the 1995 monsoon seasons

As part of the US JGOFS Arabian Sea Process Study in 1995, we investigated temporal and spatial patterns in microbial dynamics and production during the late Southwest (SW) Monsoon (August–September 1995) and the early Northeast (NE) Monsoon (November–December 1995) seasons using the seawater-dilution technique. Experiments were coupled with population assessments from high-performance liquid chromatography, flow cytometry, and microscopy to estimate further taxon-specific phytoplankton growth, grazing and production. Dilution estimates of total primary production varied substantially, from 7 to 423 μg C l−1 d−1, and were generally in good agreement with rate estimates from 14C-uptake incubations. Both primary production and secondary bacterial production were, on average, 2.5× higher during the SW Monsoon than the NE Monsoon. Relative to the total community, photosynthetic prokaryotes contributed 23% and 53% of production during the SW and NE Monsoons, respectively. Prochlorococcus spp. production was well balanced by grazing losses, while >50% of Synechococcus spp. production during the SW Monsoon appeared to escape grazing by protists. Diatoms comprised >30% of primary production at a high biomass station during the SW Monsoon but <30% at all stations during the NE Monsoon. Growth rates of Synechococcus spp. and diatoms appeared to be limited by inorganic nitrogen concentrations, while Prochlorococcus spp., dinoflagellates and Phaeocystis spp. were not. Losses to protistan grazing were strongly correlated with phytoplankton biomass and production. Despite sufficient prey levels, protistan biomass was modest and constant across the region during both seasons. Of the larger taxa, diatoms were grazed the least effectively with only 50% of daily production accounted for by protistan grazing. Combined estimates of protistan and mesozooplankton grazing at upwelling stations during the SW Monsoon leave ∼10% of primary production unaccounted for and available for sinking and/or lateral advection. Similarly high rates of net production at northern coastal stations during the NE Monsoon suggest that this area also may contribute to regional export flux.