Seasonal response of zooplankton to monsoonal reversals in the Arabian Sea

The US JGOFS Arabian Sea Process Study was designed to provide a seasonally and spatially resolved carbon budget for a basin exhibiting some of the hi ghest and lowest concentrations of plant biomass in the world's ocean. Duri ng the US JGOFS Process Study in the Arabian Sea (September 1994-January 19 96), the absolute maximum in biomass of epipelagic zooplankton in the entir e study was observed during the Southwest Monsoon season inshore of the Fin dlater Jet in the area of upwelling. The greatest contrast between high and low biomass in the study area also was observed during the Southwest Monso on, as was the strongest onshore-offshore gradient in biomass. Lowest bioma ss throughout the study was observed at the most offshore station (S15), ou tside the direct influence of the monsoon forcing. The greatest day/night c ontrasts in biomass were observed nearshore in all seasons, with nighttime biomass exceeding daytime in the Northeast Monsoon season, but daytime exce eding nighttime in the Southwest Monsoon season. The diel vertical migratio n patterns in general reversed between the monsoons at all stations in the southern part of the study area. Virtually, no diel vertical migration of z ooplankton took place in any season at the station with strong, persistent subsurface suboxic conditions (N7), suggesting that these conditions suppre ss migration. Based on the distribution of biomass, we hypothesize that ins hore of the Findlater Jet, zooplankton grazing on phytoplankton is the domi nant pathway of carbon transformation during both monsoon seasons, whereas offshore the zooplankton feed primarily on microplankton or are carnivorous , conditions that result in reduction of the carbon flux mediated by the zo oplankton. Predation by mesopelagic fish, primarily myctophids, may equal d aily growth of zooplankton inshore of the Findlater Jet during all seasons. This suggests that the food web inshore of the Findlater Jet is well integ rated, may have evolved during past periods of intensified upwelling, and h as a distinctly annual cycle. (C) 1998 Elsevier Science Ltd. All rights res erved.