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 Program, we determined the abundance and biomass of autotrophic and heterotrophic 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 September 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. Phytoplankton assemblages varied markedly in composition along a transect from onshore to about 1500 km offshore. Larger forms, such as diatoms and colonies of the prymnesiophyte Phaeocystis, dominated stations inshore of about 1000 km, whereas picoplankton dominated offshore. Heterotrophic nano- and microplankton biomass varied from similar to 1 to 12 mu g Cl-1, and nanoflagellates, dinoflagellates, and ciliates reached maximum biomass at different locations and depths. Heterotrophs comprised 18-27% of the biomass over most of the transect. Biomass of all groups of organisms was strongly negatively correlated with depth and positively correlated with each other, suggesting a dynamic food web. Size structure of organisms among stations suggested that 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 senescent ones offshore suggest that populations could have sunk as a bloom terminated, in addition to being available for mesozooplankton grazers. (C) 1998 Elsevier Science Ltd. All rights reserved.