The structure of zooplankton communities, in the 2 to 2000 mu m size range, in the Arabian Sea during and after the SW monsoon, 1994

Zooplankton communities, studied in the surface mixed layer on a 1600 m tra nsect across the Arabian Sea, were found to differ in their temporal and sp atial response to seasonal forcing. The transect studied, spanned seasonall y eutrophic upwelling, mesotrophic downwelling and aseasonal oligotrophic w aters. The nano- and microzooplankton communities constituted a relatively constant compartment in the tropical monsoon ecosystem, whilst the mesozoop lankton showed a clear response to both upwelling and season. The heterotro phic nanoflagellates were concentrated in the surface mixed layer, except i n the eutrophic upwelling waters of the SW monsoon. They reached maximum ce ll concentrations of 855 ml(-1) during the SW monsoon and a maximum biomass of 8.4 mg Cm-3 during the intermonsoon. Nanozooplankton standing stocks, i n the surface mixed layer, ranged between 7 and 333 mg Cm-2, with highest s tocks found during the intermonsoon. The microzooplankton community was dom inated by Protozoa, particularly aloricate ciliates and heterotrophic dinof lagellates, which accounted for up to 99% in terms of numbers and up to 71% of the biomass. Sarcodines and metazoan nauplii were recorded in lower num bers (< 400 l(-1)). The microzooplankton were also concentrated in the surf ace mixed layer during both periods, except in the eutrophic coastal waters during the SW monsoon, when relatively high biomass values were found belo w the mixed layer depth. Their standing stocks, in the surface mixed layer, ranged between 50 and 182 mg Cm-2, with the highest concentration found in the mesotrophic offshore waters during the late monsoon period. Total meso zooplankton standing stocks, in the surface 100 m, decreased with distance from the coastal to offshore waters and between seasons, decreasing from 12 48 to 238 mg Cm-2 during the late SW monsoon and 656-89 mg Cm-2 during the following intermonsoon. The largest size class, of 1000-2000 mu m sized org anisms, dominated throughout except at the oligotrophic station during the intermonsoon period, when the smallest class, of 200-500 mu m, were more im portant. The shift in size structure from large to small zooplankton occurr ed in response to a shift in dominance from large to small phytoplankton ce lls both spatially, along a eutrophic-oligotrophic gradient, and seasonally . These responses are a result of the physical forcing associated with the monsoon seasons in the Arabian Sea. (C) 1999 Elsevier Science Ltd. All righ ts reserved.