Nano- and microplankton in the northern Arabian Sea during the Southwest Monsoon, August-September 1995 - A US-JGOFS study

As part of the US Joint Global Ocean Flux Studies (JGOFS) Arabian Sea Progr am, we determined the abundance and biomass of autotrophic and heterotrophi c nano- and microplankton in the upper 100 m at 10 stations in the northern Indian Ocean during the late Southwest Monsoon from 17 August through 15 S eptember 1995. Autotrophic nano- and microplankton biomass ranged from 0.2 to 68.0 mu g Cl-1, with most of the biomass in the upper 20-60 m. Phytoplan kton assemblages varied markedly in composition along a transect from onsho re to about 1500 km offshore. Larger forms, such as diatoms and colonies of the prymnesiophyte Phaeocystis, dominated stations inshore of about 1000 k m, whereas picoplankton dominated offshore. Heterotrophic nano- and micropl ankton biomass varied from similar to 1 to 12 mu g Cl-1, and nanoflagellate s, dinoflagellates, and ciliates reached maximum biomass at different locat ions and depths. Heterotrophs comprised 18-27% of the biomass over most of the transect. Biomass of all groups of organisms was strongly negatively co rrelated with depth and positively correlated with each other, suggesting a dynamic food web. Size structure of organisms among stations suggested tha t larger consumers occurred where phytoplankton cells were large. Sediment trap data indicate high organic carbon and biogenic silica flux at the time of our study. Our findings of abundant diatoms over much of the study area and their apparent transition from healthy-looking cells nearshore to sene scent ones offshore suggest that populations could have sunk as a bloom ter minated, in addition to being available for mesozooplankton grazers. (C) 19 98 Elsevier Science Ltd. All rights reserved.