Mesozooplankton production and grazing in the Arabian Sea

Abstract

Growth rates, grazing and fecal pellet production by mesozooplankton size classes in the surface 200 m are compared over two inshore/offshore transects in the Northern Arabian Sea during different monsoon seasons. We derived these rate parameters from measured biomass and several empirical models that estimate copepod production from temperature, body weight and food availability. 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 published data on copepod ingestion and growth rates. In general, zooplankton rate estimates were highest at the inshore stations where phytoplankton production and zooplankton biomass were maximum. Overall cruise means of zooplankton biomass and rate estimates during the early and late NE Monsoon, Spring Inter-monsoon and SW Monsoon were not significantly different. The estimated zooplankton community (all size fractions) growth rate averaged 0.12 d −1 over all stations during the different monsoon seasons. Although smaller zooplankton size fractions grew faster, slower growing >2 mm zooplankton dominated the zooplankton biomass of the Arabian Sea and this resulted in a lower overall community growth rate. Estimated total carbon (phytoplankton, protozoa and detritus) ingestion averaged 44 mM C m −2 d −1, which was approximately 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 was greater than measurements of the export flux during the NE Monsoon and Spring Intermonsoon. However, estimated fecal pellet production was less than measured export flux during the SW Monsoon when sinking phytoplankton likely contributed directly to the export flux. Our data suggest that relative to other ocean basins, the grazing impact of mesozooplankton is significant in the Arabian Sea as a consequence of the high zooplankton biomass, abundance of diatoms, and warm temperatures, which result in high zooplankton metabolic demands and growth rates. The observation that the average zooplankton biomass (110 mM C m −2) did not change significantly over the monsoon seasons suggests that zooplankton production (13 mM C m −2 d −1) and zooplankton mortality were similar over the study interval.