The effect of semi-permanent eddies on the distribution of mesozooplankton in the central Bay of Bengal

Abstract

Five locations in the central Bay of Bengal (BoB) along a longitudinal transect at 88E were sampled during the Fall Intermonsoon, 2002 for mesozooplankton biomass, abundance, groups and copepod species composition. Stations at 9N and 18N had signatures of cold-core eddies. Despite the notion that biological productivity in the BoB is poor, the mesozooplankton biomass (57-185 mM C m sup(-2)) and abundance (33-171 [sup(-10 sup(3)] individuals m sup(-2)) in the upper 1000 m were high and are close to the annual averages reported from the central Arabian Sea. The cold-core eddy at 9N led to elevated chlorophyll a (maximum 0.4 mg m sup(-3)) and mesozooplankton biomass up to 36 mg C m sup(-3) in the mixed layer. The biomass at the other cold-core eddy station at 18N was not as high as that at 9N, due to lower chl a levels apparently due to light limitation caused by turbidity. Based on the spatial distribution of copepods (>70% of the total mesozooplankton abundance), the 9N station and the 18N and 20N stations differed completely from the in-between stations at 12N and 15N. While the number of copepod species was the highest at 9N, diversity and evenness were the lowest due to the predominance of Oncaea venusta (26%) and Paracalanus indicus (25%). The lowest species richness with highest evenness was observed at 18N probably due to the oxygen depletion at shallower depths (5 mu M between 80 and 500 m). Species richness and diversity were the highest at the noneddy location 12N, where biomass (60 mew M C m sup(-2)) and abundance (70 x 10 sup(3) individuals m sup(-2)) were lower. Of the eight most abundant species, O. venusta (21%), P. indicus (19%), P. aculeatus, Oithona similis, Corycaeus danae and Eucalanus monachus were abundant in the upper 200 m of the two cold-core eddy stations, whereas Pleuromamma indica and Lucicutia flavicornis were abundant throughout the upper 1000 m north of 15N. Though many groups (26) and as many as 170 copepod species were recorded during this study, copepod species richness diminished with the dwindling dissolved oxygen concentrations north of 15N. It is inferred that in the intensely stratified surface waters of the central BoB, cold-core eddies are important for promoting considerable biological production