Carbon dioxide dynamics from sediment, sediment-water interface and overlying water in the aquaculture shrimp ponds in subtropical estuaries, southeast China.

Aquaculture ponds can emit a large amount carbon dioxide (CO), with the consequence of exacerbating global climate change. Many studies about CO dynamics across the water-air interface, but CO in sediment and overlying water received relative less attention. In this study, CO concentration in sediment porewater, the diffusive CO fluxes across the sediment-water interface (SWI), and the CO production rates in the overlying water (CO) were determined in the shrimp ponds in the Min River Estuary (MRE) and Jiulong River Estuary (JRE), southeast China, to analyze the dynamics of CO among different growth stages of shrimps. Our results showed large variations in porewater CO concentrations, CO diffusive fluxes and CO rates among different growth stages, with markedly larger values in the middle stage of shrimp growth. The temporal variation of CO in both estuarine ponds followed closely the seasonal change of temperature. The internal CO production (CO) in these ponds was dominated by sediments. A significantly larger mean porewater CO concentrations, diffusive fluxes and production rate were observed in the MRE ponds than those in the JRE ponds, which could be attributed to the lower water salinity and a larger source of carbon substrates in the former estuary. Considering a total surface area of 6.63 × 10 km across the mariculture ponds in subtropical estuaries, it is estimated conservatively that approximately 100 Gigagram (Gg) of dissolved organic carbon and 190 Gg of dissolved inorganic carbon were transported annually from the mariculture ponds into China's coastal areas. Because of the substantial supply of dissolved carbon, the adjacent coastal waters receiving effluent discharge from the mariculture ponds could become "hotspots" of CO emissions. Our results highlight the role of aquaculture pond as a major CO source in China's coastal areas, and effective actions are needed to alleviate the greenhouse gas (GHG) emissions from these ponds.