Highlights
- Summer monsoon upwelling drives strong acidification and deoxygenation over the EAS shelf.
- Non-upwelling DIC and TAlk variability is largely governed by conservative water-mass mixing.
- Elevated nDIC35 during upwelling confirms DIC enrichment beyond salinity stratification alone.
- AOU-nDIC35 coupling indicates respiratory amplification of upwelled CO2-rich source waters.
- Reduced buffering and lower ΩCa–ΩAr increase seasonal chemical stress on shelf ecosystems.
Abstract
Repeated measurements of inorganic carbon system parameters over one year along two coastal transects (Kochi in the southern EAS and Mangalore in the central EAS) in the eastern Arabian Sea (EAS) reveal strong seasonal coupling between upwelling, deoxygenation, acidification, and inorganic carbon accumulation on the shelf. During the non-upwelling (oxic) period, the variability of dissolved inorganic carbon (DIC) concentrations and total alkalinity (TAlk) was governed predominantly by conservative water-mass mixing, particularly between low-salinity Bay of Bengal-derived waters and more saline Arabian Sea shelf waters, as demonstrated by the marked reduction in salinity normalised DIC (nDIC35) and TAlk (nTAlk35). In contrast, during the summer monsoon (June–September), coastal upwelling transported oxygen-poor, DIC-rich subsurface waters onto the shelf, leading to pronounced subsurface inorganic carbon enrichment, hypoxia, and acidification. Vertical profiles of nDIC35 showed that elevated inorganic carbon concentrations persisted even after removing salinity effects, increasing from ∼1950–2000 μmol kg−1 at the surface to >2100–2200 μmol kg−1 below ∼40 m. Nearshore surface waters during peak upwelling exhibited a strong offset between measured DIC and nDIC35, indicating localized freshwater dilution, but salinity-normalised values confirmed that the underlying carbon inventory remained high. Apparent oxygen utilisation (AOU) and nDIC35 were positively correlated, indicating that a substantial fraction of the residual DIC enrichment was associated with oxygen consumption, although this relationship reflects the combined imprint of DIC-rich upwelled source waters and subsequent microbial remineralisation within the stratified shelf system. Thus, carbon accumulation during the summer monsoon is best explained by a two-stage mechanism: (i) physical advection of CO2-rich, oxygen-deficient upwelled waters, followed by (ii) secondary amplification through local respiration. In contrast, TAlk exhibited much weaker non-conservative modification, and the elevated alkalinity generated under low-oxygen conditions was insufficient to counteract the strong DIC-driven reduction in carbonate-system buffering capacity, thereby increasing the system’s vulnerability to pCO2 build-up and acidification. Consequently, calcite and aragonite saturation states declined sharply during upwelling, with ΩCa and ΩAr falling to ∼2.5 and ∼ 1.5, respectively, when pCO2 exceeded 1000 μatm under severe oxygen depletion. The co-occurrence of hypoxia, acidification, and weakened carbonate buffering characterises the eastern Arabian Sea shelf as a highly dynamic natural laboratory for understanding multi-stressor impacts on coastal biogeochemistry and ecosystem vulnerability.
Sudheesh V. & Gupta G. V. M., 2026. Effects of upwelling-driven acidification and deoxygenation on the dissolved inorganic carbon system over the southeastern Arabian Sea shelf. Marine Chemistry 276: 104638. doi: 10.1016/j.marchem.2026.104638. Article (restricted access).
