Fluxes of carbon dioxide and methane across the water-atmosphere interface of aquaculture shrimp ponds in two subtropical estuaries: The effect of temperature, substrate, salinity and nitrate.

While aquaculture pond is a dominant land use/cover type and a distinct aquatic ecosystem in the coastal zones of China and southeast Asia, their contributions to the fluxes of greenhouse gases (GHGs) have only been poorly quantified. Fluxes of CO2 and CH4 in the shrimp ponds with different salinities were simultaneously measured in situ using the floating chamber technique in two different subtropical estuaries, namely, the Min River Estuary (MRE) and Jiulong River Estuary (JRE). The average CO2 and CH4 fluxes in the shrimp ponds over the observation periods varied from -2.09 to 3.37mmol CO2 m-2h-1 and from 0.28 to 16.28mmol CH4 m-2h-1, respectively, with higher fluxes being detected during the middle stage of aquaculture. The temporal variation of CO2 and CH4 fluxes in both estuaries ponds closely followed the seasonal cycle of temperature. Higher CH4 emissions were observed in MRE ponds than in JRE ponds because of the lower water salinity and N-NO3- concentrations as well as a greater supply of carbon substrates. Our findings suggested that shrimp ponds were CH4 emission "hotspots" in the subtropical estuaries of China. Based on a new global warming potential model, we conservatively estimated an anuual GHG emission rate of approximately 63.68Tg CO2-eq during the culture period from aquaculture ponds across the subtropical estuaries of China. Our results demonstrate the importance of aquaculture ponds as a major GHG source and a contributor to climate warming in the subtropical estuarine regions of China, and call for effective regulation of GHG emissions from these ponds for climate mitigation in future.