Foregone carbon sequestration loss dominates greenhouse gas budget in mariculture associated with coastal wetland conversion

Abstract Coastal wetlands represent a disproportionately large global carbon (C) sink. However, they are threatened by the ever-expanding aquaculture and being lost at critical rates. Conversion of coastal wetlands to aquaculture systems has been predicted to result in significant C losses, yet accurate assessments of greenhouse gas (GHG) budgets associated with this major perturbation are rarely available. Here we show that the conversion of Spartina alterniflora saltmarsh to mariculture ponds in China induced a dramatic shift from net atmospheric GHG sink (‒13.8 Mg CO₂eq ha–¹ yr–¹) to net GHG sources (2.16 Mg CO₂eq ha–¹ yr–¹), creating a full GHG debt of 15.9 Mg CO₂eq ha–¹ yr–¹. The loss of foregone GHG mitigation capacity of saltmarsh makes the largest contribution (86.4%), while only 15.6% of the total debt arises from direct CO₂, CH₄, and N₂O fluxes in the mariculture ponds. But considering the main drivers of GHG emissions from animal protein production, mariculture has much lower GHG-cost than inland freshwater aquaculture and terrestrial beef, small ruminants, and pork production on a kg CO₂eq kg‒1 protein basis. The low-C mariculture could be further realized by avoiding devastation of vegetated coastal wetlands and minimizing CH₄ emission by carrying out in high-salinity waters.