Impact of 2015-2016 ENSO on the winter bloom and associated phytoplankton community shift in the northeastern Arabian Sea

The present study explores the impact of 2015-16 El-Niño Southern Oscillation (ENSO) on the winter phytoplankton bloom in the northeastern Arabian Sea (NEAS). In situ Chlorophyll a (Chla) from the bio-Argo float (2013-2016) and phytoplankton pigment data collected onboard R.V. Sindhu Sankalp (February 2013 and 2016) have been used to decipher the 2015-2016 ENSO modulated variability. During a normal winter, convective mixing deepens the mixed layer (ML) and increases the availability of nutrients in the euphotic zone. During the 2015-16 ENSO, the intensity of the convection (evaporation) was weaker than normal and this, in turn, weakened the convective mixing in the following winter. However, during the 2015-16 ENSO event, a warm core eddy deepened the ML further by pushing the nutracline downwards, thereby reducing the availability of nutrients. Changes in the physical condition during the 2015-16 ENSO altered the nutrient availability which in turn affected the Chla production and associated phytoplankton community structure. The Bio-Argo observations show 50% reduction in Chla during February 2016 compared to a normal year, which we attribute to the ENSO event of 2015-2016. Additionally, pigment data shows a reduction in the abundance of microplankton (diatoms and dinoflagellates) and an increase in abundance of picoplankton in the NEAS. Our results clearly indicate that 2015 ENSO played a dominant role in altering the phytoplankton community composition and in turn the productivity of the Arabian Sea.