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

Microzooplankton herbivory in the Arabian Sea was measured using dilution experiments towards the end of the SW monsoon in September and during the intermonsoon 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 C m-3 d-1. Depth-integrated microzooplankton herbivory ranged between 161 and 415 mg C m-2 d-1 during the SW monsoon cruise, and between 110 and 407 mg C m-2 d-1 during the intermonsoon period. Microzooplankton grazed between 4 and 60% of daily primary production, with higher percentages found during the intermonsoon season. Phytoplankton growth rates during the SW monsoon ranged from 0.3 to 1.8 d-1, with lower values in upwelling waters and higher values in downwelling and oligotrophic areas. During the intermonsoon period, phytoplankton growth was more uniform across the basin and averaged 0.68±0.15 d-1. Microzooplankton abundance in experimental samples varied between 2800 and 16 162 cells l-1, equivalent to a biomass of between 1.1 and 7.2 mg C m-3. The mean cell carbon content of microzooplankton was similar in both periods and ranged from 0.33 to 0.55 ng C cell-1. Microzooplankton were smallest in downwelling waters and largest in oligotrophic waters. Average clearance rates in those taxa that took up fluorescently-labelled algae ranged from 0.2 to 14 μl ind-1 hr-1. Average mesozooplankton grazing rates, derived from biomass data, varied from 19 to 92 mg C m-2 d-1; these rates accounted for removal of between 4 and 12% of the daily primary production. Mesozooplankton herbivory was most pronounced in upwelling and downwelling waters and reduced in stratified oligotrophic waters during the SW monsoon period. Microzooplankton herbivory was greater than the average mesozooplankton herbivory at all stations, during both the SW monsoon and intermonsoon periods.