We examined the separate effect of Portuguese oyster Crassostrea angulata and the interactive effects of oyster and red seaweed Gracilaria lemaneiformis on seawater dissolved inorganic carbon (DIC) systems and the air−sea CO 2 flux (F CO2 ) in Daya Bay, southern China. Respiration and calcification rates of oysters were measured and the effects of oyster aquaculture on marine DIC systems were evaluated. The interactive effects on seawater DIC and air−sea F CO2 were examined using mesocosms containing oyster and seaweed assemblages. Results showed populations of C. angulata cultured in Daya Bay sequestered ca. 258 g C m −2 yr −1 for shell formation, whereas the CO 2 released due to respiration and calcification was 349 and 153 g C m −2 yr −1 , respectively. This indicates that oyster cultivation in Daya Bay is a CO 2 generator, favoring the escape of CO 2 into the atmosphere. DIC, HCO 3 − and CO 2 concentrations and the partial pressure of CO 2 in oyster−seaweed co-cultured mesocosms were significantly lower than the oyster monoculture mesocosm. These results indicated that G. lemaneiformis effectively absorbs the CO 2 released by oysters. The negative values of air−sea F CO2 in the co-culture groups represent a CO 2 sink from the atmosphere to the sea. These results demonstrated that there could be an interspecies mutual benefit for both C. angulata and G. lemaneiformis in the integrated culture system. Considering that photosynthesis of seaweed is carbon limited, we suggest that the 2 species are co-cultured at a ratio of ca. 4:1 (based on fresh weight) for efficient utilization of DIC in seawater by G. lemaneiformis, and further to increase the ocean CO 2 sink.