Seasonal variations in source-sink balance of CO2 in subtropical earthen aquaculture ponds: Implications for carbon emission management

Aquaculture ponds serve as focal points for carbon cycling and act as anthropogenic contributors to the emission of carbon dioxide (CO2). To understand the seasonal CO2 dynamics within the ponds, we measured the CO2 concentrations in sediment porewater and the water column in aquaculture ponds in the Shanyutan Wetland in China. Subsequently, the sediment-to-water and water-to-air CO2 fluxes were calculated based on the gas transfer coefficient model. Our results showed that that CO2 flux ranged 0.01-4.58 mmol m- 2h-1 across the sediment-towater interface and -0.08 to 0.45 mmol m- 2h-1 across the water-to-air interface throughout the farming period. Photosynthetic activity was the key driver of the temporal variations in water column CO2 concentration and water-to-air CO2 flux, while the change in porewater CO2 concentration and sediment-to-water CO2 flux were governed by sediment temperature which drive the microbial decomposition of organic matter. Based on a simple mass balance approach, the apparent CO2 consumption (ACC) in the water column across all seasons ranged from 0.24 to 2.32 mmol m- 2h-1, indicating that the pond water body had a high capacity to "consume" the excess CO2. Our results highlight that the contrasting roles between the sediment compartment and water column compartment in CO2 dynamics, and the possibility to manipulate ACC to reduce the aquaculture carbon footprint.