Picophytoplankton dynamics and production in the Arabian Sea during the 1995 Southwest Monsoon

Phytoplankton community structure is expected to shift to larger cells (e.g ., diatoms) with monsoonal forcing in the Arabian Sea, but recent studies s uggest that small primary producers remain active and important, even in ar eas strongly influenced by coastal upwelling. To better understand the role of smaller phytoplankton in such systems, we investigated growth and grazi ng rates of picophytoplankton populations and their contributions to phytop lankton community biomass and primary productivity during the 1995 Southwes t Monsoon (August-September). Environmental conditions at six study station s varied broadly from open-ocean oligotrophic to coastal eutrophic, with mi xed-layer nitrate and chlorophyll concentrations ranging from 0.01 to 11.5 mu M NO3 and 0.16 to 1.5 mu g Chl a. Picophytoplankton comprised up to 92% of phytoplankton carbon at the oceanic stations, 35% in the diatom-dominate d coastal zone, and 26% in a declining Phaeocystis bloom. Concurrent in sit u dilution and C-14-uptake experiments gave comparable ranges of community growth rates (0.53-1.05 d(-1) and 0.44-1.17 d(-1), to the 1% light level), but uncertainties in C:Chl a confounded agreement at individual stations. M icrozooplankton grazing utilized 81% of community phytoplankton growth at t he oligotrophic stations and 54% at high-nutrient coastal stations, Prochlo rococcus (PRO) was present at two oligotrophic stations, where its maximum growth approached 1.4 d(-1) (two doublings per day) and depth-integrated gr owth varied from 0.2 to 0.8 d(-1). Synechococcus (SYN) growth ranged from 0 .5 to 1.1 d(-1) at offshore stations and 0.6 to 0.7 d(-1) at coastal sites. Except for the most oligotrophic stations, growth rates of picoeukaryotic algae (PEUK) exceeded PRO and SYN. reaching 1.3 d(-1) offshore and decreasi ng to 0.8 d(-1) at the most coastal station. Microzooplankton grazing impac t averaged 90, 70, and 86% of growth for PRO, SYN, and PEUK, respectively. Picoplankton as a group accounted for 64% of estimated gross carbon product ion for all stations. and 50% at high-nutrient. upwelling stations. Prokary otes (PRO and SYN) contributed disproportionately to production relative to biomass at the most oligotrophic station, while PEUK were more important a t the coastal stations. Even during intense monsoonal forcing in the Arabia n Sea, picoeukaryotic algae appear to account for a large portion of primar y production in the coastal upwelling regions, supporting an active communi ty of protistan grazers and a high rate of carbon cycling in these areas. ( C) 1999 Elsevier Science Ltd. All rights reserved.