Carbonate chemistry and air-sea CO₂ exchange in a highly urbanized tropical coastal system

Abstract Tuticorin Bay (TB), a shallow semi-enclosed coastal system on the southeast coast of India, exemplifies the escalating challenges faced by urbanized tropical bays, where anthropogenic stressors interact with complex biogeochemical processes. This study investigates the spatial and seasonal variability of carbonate system dynamics and air–sea carbon dioxide (CO₂) fluxes across three hydrologically distinct periods: the southwest monsoon (SWM, August 2018), northeast monsoon (NEM, October 2018), and summer (SUM, May 2019). Surface waters exhibited a wide range of pCO₂ (380–1036 μatm), revealing spatial heterogeneity across distinct biogeochemical regimes. Elevated pCO₂ levels in most regions confirm the bay’s dominant role as a CO₂ source; however, localized undersaturation (~ 380 μatm) during the SWM indicates short-term CO₂ uptake, associated with intense biological production and high chlorophyll-a concentrations (> 50 μg L⁻ 1 ) in the nutrient-enriched north region of the bay. In contrast, the southern bay showed persistently high pCO₂, driven by elevated temperatures (> 36 °C) and enhanced respiration promoted by thermal effluent discharges. Dissolved Inorganic Carbon (DIC) exhibited non-conservative behaviour across seasons, with ΔDIC ranging from –487 to + 639 μmol kg⁻ 1 , highlighting the influence of terrestrial inputs and local metabolic processes. Peak air–sea CO₂ fluxes reached 17.23 mmol C m⁻ 2 d⁻ 1 during SUM, particularly in low-oxygen zones (< 60% saturation) of the southern bay. These findings demonstrate how seasonally shifting physical conditions and localized anthropogenic pressures govern CO₂ dynamics in a spatially confined tropical bay, underscoring the need for high-resolution assessments of such vulnerable systems to improve regional carbon budget estimates.