Impact of ocean warming on net primary production in the northern Indian Ocean: role of aerosols and freshening of surface ocean

Warming due to climate change stratifies the upper ocean and reduces nutrient input to the photic zone resulting in a decline in net primary production (NPP). On the other hand, climate change increases both anthropogenic aerosol input into the atmosphere and the river discharge due to the melting of glaciers on land resulting in enhanced nutrient inputs to the surface ocean and NPP. To examine the balance between these two processes, spatial and temporal variations in the rate of warming, NPP, aerosol optical depth (AOD), and sea surface salinity (SSS) were studied between 2001 and 2020 in the northern Indian Ocean. Strong heterogeneity in the warming of the sea surface was observed in the northern Indian Ocean with significant warming in the south of 12°N. Insignificant trends in warming were observed in the northern Arabian Sea (AS), north of 12°N, during winter and fall, and western Bay of Bengal (BoB) during winter, spring, and fall associated with higher levels of anthropogenic AOD (AAOD) due to a reduction in incoming solar radiation. The decline in NPP was observed in the south of 12°N in both AS and BoB and correlated inversely with SST suggesting that a weak supply of nutrients due to upper ocean stratification controlled NPP. Despite warming, the weak trends in NPP in the north of 12°N were associated with higher AAOD levels and their rate of increase suggesting that the deposition of nutrients from the aerosols seems to be compensating for declining trends due to warming. The decrease in sea surface salinity confirmed an increase in river discharge, and nutrient supply led to weak NPP trends in the northern BoB. This study suggests that the enhanced atmospheric aerosols and river discharge played a significant role in warming and changes in NPP in the northern Indian Ocean, and these parameters must be included in the ocean biogeochemical models for accurate prediction of possible changes in the upper ocean biogeochemistry in the future due to climate change.