Temporal Variation And Influencing Factors Of CO₂ Flux In Typical Aquaculture Ponds In The Yangtze River Delt

As an important source of CO₂ emission,aquaculture ponds have been included in the IPCC’s estimated list of greenhouse gases.China is the largest freshwater aquaculture country in the world.Aquaculture ponds account for 52%of the total freshwater aquaculture area in China.Therefore,to clarify the temporal variations and impact factors of CO₂ flux in the aquaculture pond can provide an important basis for the accurate estimation of China’s fishery,agricultural CO₂ emissions and formulate CO₂ emission reduction measures.In this study,a typical aquaculture pond in Guandu Village,Quanjiao County,Anhui province was chosed,which located in the Yangtze River Delta,a key aquaculture region in China.Based on eddy covariance observations and quantitative evaluation of spatial representation of flux data by using footprint model（FFP）,the temporal variations and impact factors of CO₂flux in the aquaculture pond were clarified.The main results are as follows:（1）Spatial representation of aquaculture pond CO₂ fluxIn different seasons and atmospheric stability conditions from 2016 to 2019,the aquaculture pond flux signal was not affected by ecosystems outside the aquaculture pond.The flux contribution weight of the target aquaculture pond was more than 80%and 64%respectively in upwind and non-upwind direction.Except for the non-upwind wind direction in the spring of 2017 and 2018,the maximum flux contribution location was in pond water surface area.Under different atmospheric stability conditions,the flux contribution weight of the target aquaculture pond water surface was more than 72%.The maximum flux contribution location was in the aquaculture pond water surface area under stable,unstable and neutral atmospheric conditions.Therefore,it is considered that the CO₂ flux measured by eddy covariance method mainly come from the water surface of the target aquaculture pond.（2）Temporal variations of CO₂ flux in aquaculture pondOn the daily scale,the diurnal variation of CO₂ flux in aquaculture pond in spring,summer and autumn is significant.The CO₂ flux reached the peak absorption,-0.043,-0.062 and-0.015mg·m^-2·s^-1 in a day from 12:30 to 18:00 in spring,summer and autumn,respectively.The mean daytime and night CO₂ flux of the whole year were-0.019 and 0.017 mg·m^-2·s^-1,respectively;On the seasonal scale,spring and winter aquaculture pond are weak sinks of CO₂.The CO₂flux was-0.0032 and-0.0056 mg·m^-2·s^-1,respectively,in spring and winter.The aquaculture pond were the source of CO₂ in summer and autumn.The CO₂ emission in summer was weaker than that in autumn,and the CO₂ flux was 0.0089 and 0.065 mg·m^-2·s^-1,respectively,in summer and autumn.On the inter-annual scale,the total annual CO₂ flux of the aquaculture pond from 2016to 2019 were-925.98,-393.14,2913.42 and 3946.96 kg·ha^-1,respectively.Based on the four years observation,the aquaculture pond were the source of CO₂.The averaged total annual CO₂flux for the four years is 1385.32 kg·ha^-1·a^-1.（3）Impact factors of CO₂ flux in aquaculture pondOn half-hour scale and daily scale,air temperature,water temperature and solar radiation are the main influencing factors.On the half-hour scale,the CO₂ absorption increased with the increase of air temperature,water temperature and solar radiation in spring,summer and autumn.Thus,the CO₂ flux decreased significantly.In winter,CO₂emissions increased with temperature increase.On the daily scale,with the increase of air temperature and water temperature,both CO₂ absorption and emission flux increased significantly.The CO₂ emission flux decreased significantly with solar radiation flux density increase.On the monthly scale,NDVI（Normalized Differential Vegetation Index）is the main influencing factor.The CO₂absorption flux increased with NDVI.On the inter-annual scale,human management in the aquaculture pond were the main influencing factors.In the crayfish-fish polyculture model,a large number of aquatic plants were artificially planted,which enhance CO₂ absorption.However,the increase of feed supply enhanced CO₂ emission.