1563 / 2024-09-27 23:58:40

Upwelling-Induced Acidification and Deoxygenation: Implications for the Coastal Ecosystem of the Southeastern Arabian Sea

upwelling,deoxygenation,coastal ecosystem,carbon chemistry,multiple stressor

Repeat measurements of inorganic carbon parameters along two coastal transects (Kochi and Mangalore) over the southeastern Arabian Sea (SEAS) reveal significant acidification induced by the summer monsoon (June-September) upwelling that co-occurred with intense deoxygenation. The average pH (oxygen) levels were 8.096±0.073 (163±45.8 µM) during the pre-upwelling period, which dropped sharply to 7.857±0.038 (21.6±19.2 µM) when upwelled waters intruded onto the SEAS shelf. The inner shelf of Mangalore coastal waters recorded anoxic/sulphidic conditions, while the inner shelf off Kochi remained hypoxic (17.47±6.5 µM). Despite this, the pH variation between these regions was minimal (7.834±0.044 and 7.861±0.034 respectively). Upwelling of acidic waters onto the continental shelves drastically affected the carbonate system; the calcite (Ω_Ca) and aragonite (Ω_Ar) saturations of bottom waters during upwelling (2.45±0.23 and 1.61±0.15) dropped to nearly half their pre-upwelling levels (4.54±0.71 and 3.02±0.48). Ω_Ca and Ω_Ar reached as low as 2.5 and 1.5 in the nearshore waters when pCO₂ exceeded 1000 µatm under intense redox conditions, lasting about 15 days. The duration of such conditions increased with increasing depth and persisted for around three months at the outer shelf, albeit the SEAS shelves never fell below their threshold value (Ω=1). The co-existence of oxygen-deficient conditions with changes in carbonate saturation levels and buffering capacity during the summer monsoon has severe implications for ecosystem functioning. Being an essential metabolite, oxygen deficiency harms functional macrobenthic diversity. However, the effect of acidification on non-calcifying organisms in the SEAS remains unknown, as they both co-exist, making it challenging to separate their effects, necessitating further attention.