Role of Mesoscale Eddies in the Distribution Pattern of Zooplankton Standing Stock of Western Bay of Bengal During Spring Transition

Hydrography and spatial variability of zooplankton community during spring transition associated with Anti Cyclonic Eddies (ACE) and Cyclonic Eddies (CE) in the south western Bay of Bengal is investigated. In situ data collected on board FORV Sagar Sampada and satellite derived products were used for the study. Physical parameters such as temperature, salinity, and sigma-t in the surface waters showed typical spring condition with almost uniform distribution in the region. However, MLD, ILD and BLT showed noticeable variation between ACE and CE region. The satellite derived SSHA overlaid geostrophic current pattern indicates the piling up of surface water (20cm) in the region as the result of ACE centred at 13.25°N, 83°E and negative anomaly of 24cm associated with Cyclonic eddy at 15°N, 84°E. Observed cyclonic eddies (15°N, 84°E) are characterised by depressing temperature by about 7°C at 100m depth. The forcing mechanism of ACE identified as negative wind stress curl and associated westward propagating Rossby waves and formation of CE might be due to baroclinic instability developed by equator ward flow of fresh plume water from the north during the winter monsoon period. The biological response in the primary and secondary level to these phenomena explained based on chlorophyll-a distribution and mesozooplankton standing stock. Satellite imagery showed a uniform distribution of surface chlorophyll (< 0.1 mg mË—3) in the study region, except a slight increase in the CE. Subsurface Chlorophyll Maxima (SCM) observed at the base of the mixed layer showed the preference of phytoplankton community to the optimum environmental conditions. Mesozooplankton standing stock observed was relatively higher at CE and the centre of ACE. Enhanced phytoplankton standing stock due to the nutrient enrichment at CE favoured the abundance of zooplankton community. In contrast to the general trend, core of ACE supported higher zooplankton standing with dominance of copepods, chaetognaths, ostracods etc. Relatively higher biological production in the primary as well as secondary level during the present study was associated either with the extended winter production to the early months of spring or the characteristics like intensity, age, spatial variation of prevailing physical processes.