The Importance of Intermonsoon Environmental Regulators for Dimethylated Sulfur Compounds Cycling: Evidence from the Central Arabian Sea

Complex environmental characteristics in the central Arabian Sea are important for the regional climate, ecological systems, and regulation of vertical exchanges in biogenic dimethylated sulfur compounds of the upper layer with the atmosphere. One noteworthy aspect we focus on is oceanic dimethylsulfide (DMS) responses to environmental forcing associated with spring intermonsoons. Hotspots of subsurface dimethylsulfoniopropionate (DMSP) and concurrent surface DMS were accompanied by subsurface chlorophyll maxima above nutriclines due to the volatility of DMS, degradation of DMSP by zooplankton grazing on phytoplankton, and bacterial consumption. In surface waters, significant northwest-southeast gradients emerged, with elevated biogenic sulfur concentrations in the northwest, caused by cyclonic eddy-induced upwelling, shallow nutriclines, winter nutrient availability, and abundant phytoplankton dominated by haptophytes and Synechococcus. Low concentrations in the southeast were attributed to strong thermal stratification, downwelling, deep nutriclines, and poor DMSP-producers. Owing to thermal sensitivity of DMSP lyase and temperature-driven modulation of microbial sulfur cycling, the dissolved DMSP (DMSPd) pool was consumed at an average rate of 7.0±5.5 nM/d. The entire DMSPd reservoir was effectively recycled by bacteria roughly three times per day. The maximum DMS photooxidation rate occurred at the cyclonic eddy periphery and was associated with sufficient substrate and photosensitizers. Instantaneous DMS sea-to-air fluxes in the northwest were 1.82 times higher than those in the southeast, despite an obvious increase in the surface temperature from northwest to southeast. Our findings highlight the sensitivity of biogenic sulfur cycling to complicated physical-biogeochemical-ecological dynamics in the oligotrophic Arabian Sea.