Mesozooplankton community structure in the upper 1,000 m along the western Bay of Bengal during the 2002 fall intermonsoon

Abstract

Stratification, no upwelling, sediment load, and heavy cloud cover are known to limit primary production in the western Bay of Bengal. Studies on primary consumers in this area are few. Recent studies in the Bay have shown the role of cold-core eddies in enhancing the biological production. This study was carried out to provide a detailed account of variation in mesozooplankton biomass, abundance, and copepod assemblages between cold-core eddy and non-eddy regions in the western Bay. In this study carried out in the western Bay during fall 2002 intermonsoon, we observed a very high zooplankton biovolume of 2.2 ml/m3 in the mixed layer at station WB3 located within a shallow cold-core eddy. Zooplankton from 29 groups were observed during this study. Calanoid and poecilostomatoid copepods substantially contributed to the total zooplankton abundance and carbon biomass. Below the 200 m depth, there were fewer groups but higher proportion of copepods. Copepodites of calanoids were more abundant between 500 and 1,000 m at the WB3 eddy station. Invertebrate eggs made up a staggering 65% of the total collection in the 200- to 300-m stratum at WB1, a location in the other cold-core eddy. Copepod species diversity (3.39 to 4.77) and richness (2.32 to 4.84) were lower at WB3. Among 147 copepod species in 69 genera found, Oncaea venusta (17% of the total copepod abundance), Paracalanus indicus (5.4%), Lucicutia flavicornis (5.1%), and Pleuromamma indica (4.5%) were the four most dominant ones. High copepod diversity throughout upper 1,000 m of the western Bay is attributed to the moderate oligotrophy. We reported 93 copepod species for the first time from this region, from which 7 are first records for the Indian Ocean. Cold-core eddies seem to play a pivotal role in sustaining zooplankton in nutrient-limiting regions such as the western Bay of Bengal.