Response of surface chlorophyll to aerosol dust input in the Central Arabian Sea

The decadal trends in satellite-derived surface chlorophyll in conjunction with Aerosol Optical Depth (AOD) are explored in a unique area in the central Arabian Sea, known to mimic High Nutrient and Low Chlorophyll (HNLC) like conditions during late summer monsoon. The analysis indicates two recurring seasonal blooms, possibly associated with distinct biogeochemical processes in the studied region. Furthermore, the mineral dust deposition in July every year coincides with one such increase in surface chlorophyll followed by a lag period until winter monsoon. This rapid increase in the phytoplankton biomass just after the aeolian input is possibly due to an enhancement in soluble iron within the mixed layer, as suggested by AOD dust data. Most likely, this rapid increase in biomass may induce further depletion of soluble iron leading to HNLC-like conditions during the late Summer Monsoon, as reported earlier. This hypothesis is consistent with the satellite observation, which shows a decrease in surface chlorophyll during subsequent months until the convective mixing between December - January (winter monsoon). The study reveals that the presence of the HNLC region in the central Arabian Sea during the summer monsoon is not perennial like the Southern Ocean. Instead, it is a transient phenomenon primarily controlled by aerosol deposition and rapid uptake of soluble iron, which facilitate the diatom blooms as suggested by the recent output from the NASA Ocean Biogeochemical Model (NOBM).