Riverine-coastal carbon dynamics, acidification, and CO2 outgassing in an intensive mariculture bay

Semi-enclosed bays offer hydrodynamic conditions favorable for mariculture, yet this activity can greatly alter coastal carbon dynamics and may transform coastal waters into bioreactors that modulate the carbonate system by stimulating organic matter (OM) inputs, respiration, primary production, and coupled oxygen consumption-acidification. We investigate seasonal variability in carbonate system dynamics and dissolved inorganic carbon stable isotopic composition (δ¹³C_DIC) in Sansha Bay, the largest large yellow croaker culture site in China, which is flushed by rivers and varying coastal water masses. Adopting a semi-analytical framework that uses a two end-member mixing model, we found that along the main channel, DIC concentrations were elevated by ∼5.3–87.5 μmol kg⁻¹, along with pH reduction of ∼0.05–0.07 units. Instead, western off-main channel with longer residence times exhibited opposing trends: winter DIC accumulation (up to 167 μmol kg⁻¹) and summer net removal (up to −75 μmol kg⁻¹), accompanying a pH decrease/increase of ∼0.12/∼0.19 units, respectively. Excess DIC was mainly attributable to OM remineralization and partially removed by phytoplankton production. The bay supplied a net CO₂ source, supported by high pCO₂ (mean: 811/562 μatm in winter/summer, respectively). Box model analysis showed that marine-derived OM remineralization combined with mariculture feed inputs caused DIC enrichment and declining oxygen consumption and pH evidenced by a −16.6 ‰ δ¹³C_ox value and 0.43–0.70 carbon/oxygen stoichiometry. Results underscore the role of interacting water masses and mariculture in modulating the carbonate system and its coupling with oxygen and pH dynamics. They provide critical insights into biogeochemical processes driving hypoxia and acidification in intensively farmed coastal ecosystems.