Mesozooplankton biomass, composition and distribution in the Arabian Sea during the Fall Intermonsoon: implications of oxygen gradients

Mesozooplankton biomass and distribution of calanoid copepods were studied for the upper 500m between 3 and 21 degreesN along an open-ocean transect i n the Arabian Sea during the Fall Intermonsoon (September-October 1992, 199 3). The region studied has strong gradients in the oxygen minimum zone (OMZ ), being acutely deficient in oxygen in the north and with increasing conce ntrations towards the south. In the upper layers, mesozooplankton biomass w as higher in the northern latitudes, above the thermocline, apparently avoi ding the OMZ. In the deeper strata, relatively higher biomass was observed in the more oxygenated southern latitudes. Highest mesozooplankton biomass in open waters of the Arabian Sea was observed during the intermonsoon seas ons. Calanoid copepods occurred at all depths, and surface-living species d id not undertake conspicuous diel migrations, even when they occurred in th e deeper waters of the more southern latitudes. A few species belonging to the families Metridinidae and Augaptilidae appeared to be characteristic of the poorly oxygenated mid-depths of the more northern latitudes. Seasonal variations in the composition of the copepod community were negligible. The evolution of the OMZ in the Arabian Sea and its implications are discussed . (C) 2001 Elsevier Science Ltd. All rights reserved.