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In this study, the variations of the seawater carbonate system parameters and air-sea CO2 flux (FCO2) of Shen’ao Bay, a typical subtropical aquaculture bay located in China, were investigated in spring 2016 (March to May). Parameters related to the seawater carbonate system and FCO2 were measured monthly in 3 different aquaculture areas (fish, oyster and seaweed) and in a non-culture area near the bay mouth. The results showed that the seawater carbonate system was markedly influenced by the biological processes of the culture species. Total alkalinity was significantly lower in the oyster area compared with the fish and seaweed areas, mainly because of the calcification process of oysters. Dissolved inorganic carbon (DIC) and CO2 partial pressure (pCO2) were highest in the fish area, followed by the oyster and non-culture areas, and lowest in the seaweed area. Oysters and fish can have indirect influences on DIC and pCO2by releasing nutrients, which facilitate the growth of seaweed and phytoplankton and therefore promote photosynthetic CO2 fixation. For these reasons, Shen’ao Bay acts as a potential CO2 sink in spring, with an average FCO2 ranging from -1.2 to -4.8 mmol m-2 d-1. CO2 fixation in the seaweed area was the largest contributor to CO2 flux, accounting for ca. 58% of the total CO2 sink capacity of the entire bay. These results suggest that the carbonate system and FCO2 of Shen’ao Bay were significantly affected by large-scale mariculture activities. A higher CO2 sink capacity could be acquired by extending the culture area of seaweed.

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 CO2 flux (FCO2) 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 FCO2 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 CO2 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 CO2 generator, favoring the escape of CO2 into the atmosphere. DIC, HCO3- and CO2 concentrations and the partial pressure of CO2 in oyster-seaweed co-cultured mesocosms were significantly lower than the oyster monoculture mesocosm. These results indicated that G. lemaneiformis effectively absorbs the CO2 released by oysters. The negative values of air-sea FCO2 in the co-culture groups represent a CO2 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 CO2 sink.

Ascidians are globally important members of marine fouling communities. We meas- ured in situ biodeposition rates of Ciona intestinalis and Styela clava, common biofoulers of aqua- culture infrastructure, in Sanggou Bay, northern China, during September. Ascidian numbers were recorded within a scallop Chlamys farreri farming zone to assess biodeposit loading. Both ascidians were most abundant on lantern nets and scallop shells in August and September. The average densities of C. intestinalis and S. clava in the farming zone in September were approxi- mately 329 and 22 ind. m −2 , respectively, and their biodeposition rates were 32.1 and 121.2 mg dry material ind. �1 d �1 , respectively. Total daily biodeposit production by ascidians in September within the scallop farming zone may amount to 13.24 g m �2 , with daily organic matter, C, N, and P biodeposition rates of 1.88, 0.94, 0.11, and 0.98 × 10 �2 g m �2 , respectively. The predicted daily biodeposit production by C. intestinalis and S. clava within the scallop farming zone in the bay during September was 105.9 t dry material, 7.52 t C, 0.86 t N, and 0.078 t P. By comparison, drop- off to the sea floor was approximately 143.0 t of dry matter for an entire growing season, which would be a relatively small input if averaged on a daily basis. However, some of the drop-off is expected to occur as a short-duration pulse of material (e.g. during cleaning), which may be rela- tively important in terms of benthic effects. The results suggest that the biodeposition processes and drop-off of C. intestinalis and S. clava may play an important role in coupling material fluxes from the water column to the seabed.