Variations in CO₂ fluxes from grass carp Ctenopharyngodon idella aquaculture polyculture ponds

Monthly and diurnal CO₂ fluxes at the water-air interface were measured in 3 grass carp Ctenopharyngodon idella polyculture ponds using the closed-chamber method during the farming season from April to September 2013. The results showed that the mean CO₂ emission rate from the 3 ponds was 97.8 mg m^-2 h^-1 (range 50.6 to 151.0 mg m^-2 h^-1). Apparent seasonal and diurnal variations in CO₂ fluxes were observed; the lowest (1.73 mg m^-2 h^-1) occurred in April and the highest occurred in July and August (212.9 and 181.9 mg m^-2 h^-1, respectively). The night-time to daytime CO₂ emission ratio was <1 (range 0.44 to 0.83) in May during the early part of the farming season. Night-time CO₂ emissions in early August were higher than those during the daytime (range 1.06 to 1.37). The night-time to daytime CO₂ emission ratios were 0.84 to 1.36 during late September. The CO₂ fluxes from the freshwater aquaculture ponds were higher than those of lakes and reservoirs, whereas they were lower than those of marshes in previous studies. The increases in nutrient load and pelagic respiration from feeding the fish were significantly associated with CO₂ emissions from the freshwater aquaculture ponds. Freshwater aquaculture ponds in temperate monsoon climate regions contribute an estimated 2.98 million t yr^-1 CO₂ based on the mean investigated CO₂ emission rate, contributing 0.0088% of the current annual global CO₂ emissions, and represent a small but previously unquantified CO₂ emission source.