Deep-Sea Research II 46 (1999) 1745 }1768 Picophytoplankton dynamics and production in the Arabian Sea during the 1995 Southwest Monsoon Susan L. Brown*, Michael R. Landry, Richard T. Barber, Lisa Campbell, David L. Garrison, Marcia M. Gowing Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Rd., Honolulu, HI 96822, USA NSOE, Duke University, Beaufort, NC 28516, USA Department of Oceanography, Texas A&M University, College Station, TX 77843, USA Division of Ocean Sciences, Biological Oceanography Program, National Science Foundation, 4201 Wilson Blvd. Arlington, VA 22230, USA Institute of Marine Sciences, University of California at Santa Cruz, Santa Cruz, CA 95064, USA Received 5 September 1997; received in revised form 1 December 1998; accepted 10 December 1998 Abstract Phytoplankton community structure is expected to shift to larger cells (e.g., diatoms) with monsoonal forcing in the Arabian Sea, but recent studies suggest that small primary producers remain active and important, even in areas strongly in#uenced by coastal upwelling. To better understand the role of smaller phytoplankton in such systems, we investigated growth and grazing rates of picophytoplankton populations and their contributions to phytoplankton community biomass and primary productivity during the 1995 Southwest Monsoon (August}September). Environmental conditions at six study stations varied broadly from open- ocean oligotrophic to coastal eutrophic, with mixed-layer nitrate and chlorophyll concentra- tions ranging from 0.01 to 11.5 M NO  and 0.16 to 1.5 g Chl a. Picophytoplankton comprised up to 92% of phytoplankton carbon at the oceanic stations, 35% in the diatom- dominated coastal zone, and 26% in a declining Phaeocystis bloom. Concurrent in situ dilution and C-uptake experiments gave comparable ranges of community growth rates (0.53}1.05 d and 0.44}1.17 d, to the 1% light level), but uncertainties in C:Chl a con- founded agreement at individual stations. Microzooplankton grazing utilized 81% of commun- ity phytoplankton growth at the oligotrophic stations and 54% at high-nutrient coastal stations. Prochlorococcus (PRO) was present at two oligotrophic stations, where its maximum growth approached 1.4 d (two doublings per day) and depth-integrated growth varied from * Corresponding author. E-mail address: sbrown@soest.hawaii.edu (S.L. Brown) 0967-0645/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 9 ) 0 0 0 4 2 - 9