Massive carbon inputs from fish farming reduce carbon sequestration capacity in a macroalgae mariculture area

Macroalgae mariculture is promoted as a marine carbon dioxide removal (mCDR) strategy, particularly in East Asia. In practice, however, macroalgae is frequently co-cultured with fish and shellfish, complicating carbon budgets and potentially altering net carbon metabolism. While most work has emphasized organic carbon cycles, carbonate system responses under integrated aquaculture remain underexplored. In this study, we conducted seasonal surveys in Sansha Bay, one of the world’ largest mariculture zones. Contrary to expectation, the bay persistently outgassed CO₂ during winter (the seaweed growth peak season), spring, and fall. Sea surface CO₂ partial pressure (pCO₂) reached 500–1100 μatm with air-sea CO₂ fluxes of 2.1–7.0 mmol m⁻² d⁻¹. Against an estimated natural background of 200 μatm (a strong sink), long-term effects of cultivation diverged by trophic group: seaweed cultivation lowered pCO₂ by 42 ± 5 μatm, shellfish farming increased it by 36 ± 4 μatm, and fish farming raised it by 375 ± 18 μatm, elevating mean pCO₂ to ∼567 ± 20 μatm and transforming the system from a CO₂ sink to a source. In this semi-enclosed bay, dissolved inorganic carbon (DIC) generated from fish farming overwhelms algal uptake, driving increases in DIC and pCO₂ and reducing the region's carbon sequestration capacity. Seasonal submarine groundwater discharge added ∼30–60 μatm to pCO₂, and short-term mariculture activities could episodically elevate pCO₂ up to 1100 μatm. Analysis of the dissolved inorganic carbon stable carbon isotope (δ¹³C_DIC) indicates that seasonal increases in DIC and pCO₂ in Sansha Bay are due to the decomposition of residual seaweed biomass in late spring and organic matter respiration from fish feed in fall. To achieve mCDR and protect coastal environments, it is essential to reduce formulated feed use or develop alternative environmentally friendly fish farming methods.