Response of micro- and mesozooplankton in the southwestern Bay of Bengal to a cyclonic eddy during the winter monsoon, 2005

A cyclonic eddy (CE) in the southwestern Bay of Bengal (SWBoB; 10–15° N; 81–87° E) during winter monsoon 2005 and associated changes in the open ocean hydrography and productivity patterns were studied using satellite observations and in situ measurements. Analysis of the satellite-derived sea surface height anomaly (SSHA) indicated the existence of a large eddy (10–15° N; 81–87° E) from November to January, with its core centered at 13° N and 82° E. The large positive wind stress curl (~1.8 × 10 −7 N m −2 ) and resultant Ekman pumping (~3 × 10 −2 cm s −1 ) were identified as the prominent forcing mechanisms. In addition, the cyclonic storms and depressions experienced in the region during the study period seem to have served to maintain the strength of the CE through associated Ekman pumping. The cold (~26.6 °C), nutrient-enriched (NO 3 > 2 μM, PO 4 > 0.73 μM and SiO 4 > 3 μM) upwelled waters in the upper layers of the CE enhanced the biological production (chl. a > 0.56 mg m −3 ). Dissolved oxygen in the surface waters was > 200 μM. The phytoplankton and zooplankton biomass recorded during the season was comparable or perhaps higher than the peak values reported from the northeastern Arabian Sea (chlorophyll a concentrations of 0.2 to 0.4 mg m −3 and zooplankton biovolume 0.6 ml m −3 ) during winter. Occurrence of a higher mesozooplankton biovolume (0.8 ml m −3 ) and relatively low abundance of microzooplankton indicates the prevalence of a short food chain. In conclusion, high biological production, both at primary as well as secondary level, suggests the prevalence of an efficient food web as a result of physical forcing and subsequent ecological interactions evident up to tertiary level in an oligotrophic basin like BoB.