Suspended-raft cultivation of bivalves and seaweed significantly influences carbon transport and transformation pathways in bay ecosystems, ultimately affecting air–sea CO2 flux (FCO2) and sedimentary carbon burial. Previous studies focused on whether bivalve and seaweed individuals are a carbon source or sink, but have not discussed the effects of their large-scale cultivation on carbon dynamics from the perspective of a bay ecosystem. In this study, we compared seasonal and spatial variations in air–sea FCO2 and sedimentary carbon burial flux in seaweed culture, bivalve culture, bivalve–seaweed polyculture, and non-culture areas in Sanggou Bay, China, a bay with a long history of mariculture practices. We found that in spring and summer, the air–sea FCO2 was lowest in the seaweed area; it was lowest in the bivalve area in autumn and winter. Despite this, the seaweed area annually absorbed twice the amount of atmospheric CO2 compared to the bivalve area. The bivalve area had the highest sedimentary carbon burial flux and the non-culture area had the lowest. More inorganic carbon than organic carbon was buried in sediment. Atmospheric CO2 absorption was positively correlated with sedimentary carbon burial. Seaweed culture was the main contributor, absorbing around half of the atmospheric CO2 of the bay, whereas bivalve culture exhibited the highest carbon burial, contributing about 40 % to the bay's sediment. We constructed an annual carbon budget model, and the estimated values for atmospheric absorption, seawater dissolution, and sedimentary burial were 2.09 × 104, 1.02 × 104, and 0.78 × 104 t C a−1, respectively. Taken together, our results indicate that the large-scale mariculture of bivalves and seaweed affect both atmospheric CO2 absorption and sedimentary carbon burial, as well as their relationship. Seawater dissolution is also a substantial carbon sequestration reservoir that cannot be overlooked when assessing the biogeochemical carbon cycle of bay ecosystems.
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