Spatiotemporal Variability in Phytoplankton Size Class Modulated by Summer Monsoon Wind Forcing in the Central Arabian Sea

The Arabian Sea is an area of intense ocean-atmospheric coupling that impacts its physicochemical, and biological processes. During the summer monsoon (June-September), in the central Arabian Sea, a low-level atmospheric jet blows parallel to the Arabian Peninsula causing open ocean upwelling in the north and downwelling in the south of the jet axis. High wind speeds are witnessed in the jet axis which advect nutrients from the Somali coast. Consequently, hydrography and physicochemical parameters show high spatial variability. In this dynamic region, phytoplankton size class distribution, directly controlled by nutrient supply, can be modulated by monsoon wind variability via upwelling, advection, and entrainment, however, has not been investigated recently. We studied phytoplankton size class related to atmospheric forcing, hydrography, and nutrient stoichiometry in August 2017 and 2018 along 64 degrees E (11 degrees N-21 degrees N). Chemical taxonomy-based analysis (CHEMTAX and manual) revealed that in the north, upwelling-driven nutrient enrichment supported microphytoplankton, mostly diatoms contributing similar to 50% to phytoplankton biomass. A stronger upwelling due to higher wind forcing in 2018 compared to 2017 resulted in an enhanced nutrient supply as well as an increased contribution of diatoms. Prymnesiophytic nanophytoplankton distribution was linked to mixed layer depths with insignificant spatial variability (20%-30%). Conversely, in the nutrient-poor southern region, zeaxanthin and DV-Chla containing picocyanobacteria and prochlorophytes contributed >50% to the total phytoplankton biomass and were associated with high temperatures. Likely, increasing warming and related changes in monsoon intensity might directly influence phytoplankton size classes impacting trophic transfer and carbon cycling.