Response of oceanic subsurface chlorophyll maxima to environmental drivers in the Northern Indian Ocean

Subsurface Chlorophyll Maxima (SCM) contributes a significant proportion to depth-integrated ocean primary production, making it important to understand its spatiotemporal variability in changing environmental conditions. Based on field observations and in situ data, we studied SCM characteristics in four distinct environmental settings across Northern Indian Ocean: SEAS-south eastern Arabian Sea (coastal upwelling zone), SAS-Southern Arabian Sea (Arabian Sea mini warm pool-ASMWP), SBOB-Southern Bay of Bengal (presence of mesoscale eddies) and ANS-Andaman Sea (region of active volcanoes). SCM displayed significant spatial variability: Z_SCM (SCM depth) ranged between 25 and 88 m (mean = 59.5 m), Chl_max (SCM magnitude) ranged between 0.07 and 0.2 mg m^-3 (mean = 0.12 mg m^-3) while T_SCM (SCM thickness) ranged between 33 and 100 m (mean = 69 m). Major factors affecting the Z_SCM were light and nutrients, since Z_SCM was closely related to Z_eu (euphotic depth) and Z_N (nitracline depth). Positive relation between Chl_max and micro phytoplankton suggests micro phytoplankton to be a major contributor in increasing Chl_max. T_SCM was associated to Z_T (thermocline depth) and nutrient concentration. Increase in stratification and oligotrophy, resulted in deeper, thicker peaks of lower magnitude, and such nature of peaks could increase in continued warming scenarios. Dinoflagellates increased in warmer oligotrophic environments. Nanophytoplankton were higher at deeper Z_SCM, possibly due to their adaptability to low light. Upwelling caused shoaling of SCM with higher Chl_max in SEAS. SCM deepened in SAS and SBOB, dominated by picophytoplankton due to the influence of ASMWP (in SAS) and anticyclonic eddy (in SBOB). Ammonia and nutrient inputs from submarine vents in ANS, caused shoaling of Z_N and Z_SCM. Globally, multiple physical processes operate at short spatiotemporal scale, causing SCM variability, and the same should not be overlooked while estimating primary production or carbon export to deep ocean, through generalisations established at larger scale in the world ocean.