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

Abstract

Phytoplankton growth rates and mortality rates were experimentally examined at eight stations in the Arabian Sea along the U.S. JGOFS cruise track during the 1995 Northeast Monsoon (January) and Spring Intermonsoon (March–April). Instantaneous growth rates averaged over an entire cruise were approximately twice as high during the NE Monsoon than during the Spring Intermonsoon 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 similar 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 absolute amounts of phytoplankton biomass consumed during each season also were 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 January 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 Intermonsoon season (87% of daily primary production consumed) relative to the NE Monsoon season (49% of daily primary production consumed). Station-to-station variability was high for rate measurements during either cruise. Nevertheless, there was a clear onshore–offshore trend in the absolute rate of removal of phytoplankton biomass (μg chlorophyll consumed l −1 d −1) during both cruises. Coastal stations had removal rates that were typically 2–4 times higher than removal rates at oceanic stations.