Seasonal and inter-annual variability of chemotaxonomic marker pigments in the north-eastern Arabian Sea

Abstract

Using in situ data, the present study explored the seasonal and inter-annual variations of phytoplankton characteristics in the north-eastern Arabian Sea (NEAS). Phytoplankton pigment assessment was carried out at the Arabian Sea Time Series (ASTS) site in the NEAS (17°N 68°E) over 2011-2017 during the late south-west monsoon (SWM) and north-east monsoon (NEM). The site is largely influenced by open ocean upwelling during the SWM and winter convective mixing during the NEM, enhancing the productivity of the region. Winter convective mixing led to high biomass in the euphotic zone during the NEM, while late SWM showed oligotrophic conditions with subsurface chlorophyll maxima (Chlmax). Our measurements showed that the NEM supports larger phytoplankton, whereas the late SWM supports smaller groups, highlighting the crucial role of picoplankton in the productivity of the NEAS. Microscopic observation (cells >10 µm in size) also supported the presence of a higher number of diatoms and dinoflagellates during the NEM as compared to those during the late SWM. Among the picoplankton, Prochlorococcus counts were high during the late SWM than the NEM, which is also reflected in marker pigments.