Phytoplankton growth and mortality during the 1995 Northeast Monsoon and Spring Intermonsoon in the Arabian Sea

Phytoplankton growth rates and mortality rates were experimentally examined at eight stations in the Arabian Sea along the U.S. JGOFS cruise track dur ing the 1995 Northeast Monsoon (January) and Spring Intermonsoon (March-Apr il). Instantaneous growth rates averaged over an entire cruise were approxi mately twice as high during the NE Monsoon than during the Spring Intermons oon period (overall averages of 0.84 +/- 0.29 (s.d.) versus 0.44 +/- 0.19 d (-1)). Average herbivore grazing (mortality) rates, however, were quite sim ilar for the two seasons (overall averages of 0.35 +/- 0.18 and 0.30 +/- 0. 17 d(-1) for the NE Monsoon and Spring Intermonsoon, respectively). The abs olute amounts of phytoplankton biomass consumed during each season also wer e similar (29 and 25% of standing stock consumed d(-1) for the January and March-April cruises, respectively), as were the geographical trends of this removal. These seasonal trends in growth and removal rates resulted in net phytoplankton growth rates that were considerably higher during the Januar y cruise (0.48 d(-1)) than during the March-April cruise (0.14 d(-1)). That is, phytoplankton production was more closely balanced during the Spring I ntermonsoon season (87% of daily primary production consumed) relative to t he NE Monsoon season (49% of daily primary production consumed). Station-to -station variability was high for rate measurements during either cruise. N evertheless, there was a clear onshore-offshore trend in the absolute rate of removal of phytoplankton biomass (mu g chlorophyll consumed l(-1) d(-1)) during both cruises. Coastal stations had removal rates that were typicall y 2-4 times higher than removal rates at oceanic stations. (C) 1999 Elsevie r Science Ltd. All rights reserved.