Effect of drainage on CO2, CH4, and N2O fluxes from aquaculture ponds during winter in a subtropical estuary of China

Aquaculture ponds are dominant features of the landscape in the coastal zone of China. Generally, aquaculture ponds are drained during the non-culture period in winter. However, the effects of such drainage on the production and flux of greenhouse gases (GHGs) from aquaculture ponds are largely unknown. In the present study, field-based research was performed to compare the GHG fluxes between one drained pond (DP, with a water depth of 0.05m) and one undrained pond (UDP, with a water depth of 1.16m) during one winter in the Min River estuary of southeast China. Over the entire study period, the mean CO₂ flux in the DP was (0.75±0.12) mmol/(m²·hr), which was significantly higher than that in the UDP of (‑0.49±0.09) mmol/(m²·hr) (p<0.01). This indicates that drainage drastically transforms aquaculture ponds from a net sink to a net source of CO₂ in winter. Mean CH₄ and N₂O emissions were significantly higher in the DP compared to those in the UDP (CH₄=(0.66±0.31) vs. (0.07±0.06) mmol/(m²·hr) and N₂O=(19.54±2.08) vs. (0.01±0.04) µmol/(m²·hr)) (p<0.01), suggesting that drainage would also significantly enhance CH₄ and N₂O emissions. Changes in environmental variables (including sediment temperature, pH, salinity, redox status, and water depth) contributed significantly to the enhanced GHG emissions following pond drainage. Furthermore, analysis of the sustained-flux global warming and cooling potentials indicated that the combined global warming potentials of the GHG fluxes were significantly higher in the DP than in the UDP (p<0.01), with values of 739.18 and 26.46mgCO₂-eq/(m²·hr), respectively. Our findings suggested that drainage of aquaculture ponds can increase the emissions of potent GHGs from the coastal zone of China to the atmosphere during winter, further aggravating the problem of global warming.