Effects of conversion of coastal marshes to aquaculture ponds on sediment anaerobic CO2 production and emission in a subtropical estuary of China

Abstract

The extensive conversion of carbon-rich coastal wetland to aquaculture ponds in the Asian Pacific region has caused significant changes to the sediment properties and carbon cycling. Using field sampling and incubation experiments, the sediment anaerobic CO₂ production and CO₂ emission flux were compared between a brackish marsh and the nearby constructed aquaculture ponds in the Min River Estuary in southeastern China over a three-year period. Marsh sediment had a higher total carbon and lower C:N ratio than aquaculture pond sediment, suggesting the importance of marsh vegetation in supplying labile organic carbon to the sediment. Conversion to aquaculture ponds significantly decreased sediment anaerobic CO₂ production rates by 69.2% compared to the brackish marsh, but increased CO₂ emission, turning the CO₂ sink (-490.8 ± 42.0 mg m^-2 h^-1 in brackish marsh) into a source (6.2 ± 3.9 mg m^-2 h^-1 in aquaculture pond). Clipping the marsh vegetation resulted in the highest CO₂ emission flux (382.6 ± 46.7 mg m^-2 h^-1), highlighting the critical role of marsh vegetation in capturing and sequestering carbon. Sediment anaerobic CO₂ production and CO₂ uptake (in brackish marsh) and emission (in aquaculture ponds) were highest in the summer, followed by autumn, spring and winter. Redundancy analysis and structural equation modeling showed that the changes of sediment temperature, salinity and total carbon content accounted for more than 50% of the variance in CO₂ production and emission. Overall, the results indicate that vegetation clearing was the main cause of change in CO₂ production and emission in the land conversion, and marsh replantation should be a primary strategy to mitigate the climate impact of the aquaculture sector.

Keywords: Aquaculture ponds; Carbon dioxide (CO(2)) fluxes; Coastal wetland; Habitat alteration; Organic carbon decomposition.