Picophytoplankton community characteristics at the subsurface chlorophyll maxima in the Northern Indian Ocean

Autotrophic picoplankton are integral to marine ecosystems, contributing nearly half of oceanic primary productivity, particularly in warm, oligotrophic regions. Given that subsurface chlorophyll maxima (SCM) account for over 80 % of depth-integrated primary production, this study aimed to explore the variations in picophytoplankton community composition at the SCM under dynamic environmental conditions in the Northern Indian Ocean, using flow cytometry and chemotaxonomy. We found that picophytoplankton dominated the SCM (∼60 %), with picochlorophytes being the most prevalent group (∼50 %). Picochlorophytes showed an inverse relationship with nitrate concentrations and a positive relationship with photosynthetically active radiation (PAR), suggesting their dominance in SCMs of oligotrophic and stratified regions, with higher light availability (PAR_SCM > 3 Em^-2d^-1). As PAR reduced, Synechococcus and picocryptophytes increased (collectively accounting for 50 % of picophytoplankton community), indicating their significant presence in SCMs with lower light conditions. These groups are well-adapted to low-light environments due to the presence of the green light-harvesting pigment phycoerythrin and their capacity for photoacclimation. In the SCM of coastal regions, we observed a shift toward microphytoplankton dominance (80 %), especially diatoms, which flourish in conditions with greater light availability and nutrient concentrations. Prochlorophytes, on the other hand, were more abundant in SCMs of warmer (> 28 °C) and highly oligotrophic waters (nitrate < 1 μM). With picophytoplankton contribution expected to increase in ocean warming scenario, our study concludes that they undergo complex changes in response to dynamic environmental conditions, which could have substantial impacts on marine ecosystems and biogeochemical cycles.