Mesozooplankton production and grazing in the Arabian Sea

Growth rates, grazing and fecal pellet production by mesozooplankton size c lasses in the surface 200 m are compared over two inshore/offshore transect s in the Northern Arabian Sea during different monsoon seasons. We derived these rate parameters from measured biomass and several empirical models th at estimate copepod production from temperature, body weight and fond avail ability. The multivariate regression published by Hirst and Sheader (1997, Marine Ecology Progress Series, 154, 155-165) gave the most reasonable rate estimates when compared to direct grazing measurements as well as publishe d data on copepod ingestion and growth rates. In general, zooplankton rate estimates were highest at the inshore stations where phytoplankton producti on and zooplankton biomass were maximum. Overall cruise means of zooplankto n biomass and rate estimates during the early and late NE Monsoon, Spring I nter-monsoon and SW Monsoon were not significantly different. The estimated zooplankton community (all size fractions) growth rate averaged 0.12d(-1) over all stations during the different monsoon seasons. Although smaller zo oplankton size fractions grew faster, slower growing > 2 mm zooplankton dom inated the zooplankton biomass of the Arabian Sea and this resulted in a lo wer overall community growth rate. Estimated total carbon (phytoplankton, p rotozoa and detritus) ingestion averaged 44 mM C m(-2) d(-1), which was app roximately 40% of primary production, Expressed as a percentage of biomass, we found that zooplankton ingested approximately 40% of their body carbon d(-1). Zooplankton fecal pellet production averaged 13 mM C m(-2) d(-1) or roughly 12% of primary production. This estimated fecal pellet production w as greater than measurements of the export flux during the NE Monsoon and S pring Intermonsoon. However, estimated fecal pellet production was less tha n measured export flux during the SW Monsoon when sinking phytoplankton lik ely contributed directly to the export flux. Our data suggest that relative to other ocean basins, the grazing impact of mesozooplankton is si,signifi cant in the Arabian Sea as a consequence of the high zooplankton biomass, a bundance of diatoms, and warm temperatures, which result in high zooplankto n metabolic demands and growth rates. The observation that the average zoop lankton biomass (110 mM C m(-2)) did not change significantly over the mons oon seasons suggests that zooplankton production (13 mM C m(-2) d(-1)) and zooplankton mortality were similar over the study interval. (C) 2000 Elsevi er Science Ltd. All rights reserved.