Zooplankton herbivory in the Arabian Sea during and after the SW monsoon, 1994

Microzooplankton herbivory in the Arabian Sea was measured using dilution e xperiments towards the end of the SW monsoon in September and during the in termonsoon to NE monsoon period in November-December 1994, Microzooplankton grazing resulted in a turnover of phytoplankton stocks that ranged from 11 to 49% per day. This was equivalent to grazing fluxes of between 1 and 17 mg Cm-3 d(-1). Depth-integrated microzooplankton herbivory ranged between 1 61 and 415 mg Cm-2 d(-1) during the SW monsoon cruise, and between 110 and 407mg Cm-2 d(-1) during the intermonsoon period. Microzooplankton grazed be tween 4 and 60% of daily primary production, with higher percentages found during the intermonsoon season. Phytoplankton growth rates during the SW mo nsoon ranged from 0.3 to 1.8 d(-1), with lower values in upwelling waters a nd higher values in downwelling and oligotrophic areas. During the intermon soon period, phytoplankton growth was more uniform across the basin and ave raged 0.68 +/- 0.15 d(-1). Microzooplankton abundance in experimental sampl es varied between 2800 and 16 162 cells l(-1), equivalent to a biomass of b etween 1.1 and 7.2 mg Cm-3. The mean cell carbon content of microzooplankto n was similar in both periods and ranged from 0.33 to 0.55 ng C cell(-1). M icrozooplankton were smallest in downwelling waters and largest in oligotro phic waters. Average clearance rates in those taxa that took up fluorescent ly-labelled algae ranged from 0.2 to 14 mu lind(-1) hr(-1). Average mesozoo plankton grazing rates, derived from biomass data, varied from 19 to 92 mg Cm-2 d(-1); these rates accounted for removal of between 4 and 12% of the d aily primary production. Mesozooplankton herbivory was most pronounced in u pwelling and downwelling waters and reduced in stratified oligotrophic wate rs during the SW monsoon period. Microzooplankton herbivory was greater tha n the average mesozooplankton herbivory at all stations, during both the SW monsoon and intermonsoon periods. (C) 1999 Elsevier Science Ltd. All right s reserved.