Implications of stratification on the heterotrophic plankton community (micro- & meso zooplankton) in the South Eastern Arabian Sea

The response of the plankton community (micro- herein after MZP and mesozooplankton) to the seasonal intrusion of Bay of Bengal (BoB) water into the Arabian Sea from November (Initial Phase-IP) to January (Peak Phase-PP) and resultant stratification was tracked in the present study. During the winter monsoon (November – February) as part of seasonally reversing monsoonal circulation, low saline BoB waters intrude into the Arabian Sea which initiates stratification. The peak phase of salinity stratification in the Eastern Arabian Sea is in January during which a thick barrier layer develops in the region. The study showed that during the initial phase of the BoB water intrusion, fluctuation was observed in the MZP and mesozooplankton community with noticeable coast-oceanic variations while such variation was not found during the peak phase when the entire SEAS was influenced by the intruded low saline BoB waters. This variation in the interaction between MZP and mesozooplankton during the initial phase can be attributed to the seasonal transition which was more pronounced in the offshore during the initial phase. A statistically significant correlation was observed between barrier layer thickness (BLT), an indicator of stratification with plankton diversity. The insignificant coast-oceanic variations (homogeneity) in planktonic components during the peak phase can be attributed to higher abundance of grazers. Among the MZP community, the ciliates (CTS) formed the predominant group and its contribution varied between 50% & 59% during the initial and peak phase respectively with the highest dissimilarity observed along the Trivandrum transect. The study points to the fact that interactions between trophic levels during the seasonal transition phase when stratification sets in, is subtle which is evident as coast-oceanic variation, while in established stratification the interaction between plankton at different trophic level, gets stabilized due to balance between the Bottom-up and Top-down controls.