| As an important part of freshwater aquaculture and inland small water bodies,aquaculture ponds are hot spots of CH4 emissions.Derterming temporal variations of CH4 flux and its environmental and anthropogenic control mechanism in aquaculture pond can reduce uncertainty of carbon balance estimation in inland waters.This study was carried out in a large aquaculture zone in Quanjiao county,Anhui Province,which located in the Yangtze River Delta,the most developed aquaculture area in China.The Eddy covariance（EC）,Flux-gradient（FG）,and closed dynamic chamber methods were used to in situ observe greenhouse gas fluxes in aquaculture ponds.FG and EC methods were evaluated for measuring the fluxes of CO2,CH4 and H2O in small ponds.Based on the continuous and high-frequency observation of CH4 flux in aquaculture ponds,the temporal variation and total emission of CH4 flux were clarified.Meanwhile,the environmental and anthropogenic effects on CH4 flux were also analyzed at multiple time scales.Furthermore,the carbon emission from CH4 flux in the carbon cycle was calculated in the aquaculture pond.Finally,the comprehensive greenhouse effects produced by carbon exchange was assessed in the aquaculture ponds.The main results showed as follows:（1）Evaluating EC and FG methods for measuring greenhouse gas flux in the small pondsThe two reasons resulted in EC fluxes measurement error:one is high frequency flux loss and the other is footprint contamination.The EC flux measurement error caused by frequency loss for CH4,water vapor,and CO2 flux were on average 18%,8%and 14%,respectively.CH4 flux suffered the largest high frequency loss（18%）due to low EC measurement height and long optical path of the CH4 analyzer.Despite the low measurement height was set in order that the flux signals were from the target aquaculture pond,the EC fluxes were still influenced by sources outside the target aquaculture pond.The effects of footprint contamination on CO2 and CH4 flux were the most severe and the least severe,respectively.For the FG method,an important uncertainty was caused by the eddy diffusivity calculation.Three eddy diffusivity models,the aerodynamic（AE）model deploying the full Obukhov stability correction,the modified Bowen-ratio model（MBR）using H2O as a tracer,and the wind profile model（WP）for neutral stability,were evaluated.Based on the eddy diffusivity calculated by AE model,the best agreement and the minimum mean error were found by comparing FG observed results with EC observed results.Our results support Horst’s（1999,Boundary-Layer Meteorology 90,171）theoretical prediction in which the footprint of the AE flux based on a two-level concentration profile measurement should be much smaller than that of the gradient flux footprint and the EC flux footprint based on the geometric mean of the two heights.These results indicated the most appropriate micrometeorological method for measuring fluxes in small water bodies is a hybrid scheme,whereby sonic anemometer/thermometer is deployed to measure the eddy diffusivity and a precision gas analyzer is used to obtain the concentration gradient of the target gas.（2）Temporal variation and emission pathway of CH4 flux in aquaculture pondsBased on the observation of CH4 flux from 2016 to 2019 in the site,the poly culture ponds acted as stable CH4 sources.The annual mean value of CH4 flux was 3.68±2.95 μg m-2 s-1 during the observation periods.The value was significantly higher than the other aquatic systems,such as small nature ponds,lake and reservoir and so on.Obvious diurnal variation did not show in the pond.But strong seasonal and inter-annual variability were showed.Ebullition emission was the main CH4 emission pathway from the aquaculture pond,contributing approximately 74%to the total CH4 emission.（3）Control factors of CH4 flux at multiple time scales in aquaculture pondsSediment temperature was the main driver factor of CH4 flux and CH4 ebullition flux at different temporal scales.Sediment temperature showed a significant positive correlation relationship with CH4 flux.At daily scale,CH4 fluxes increased exponentially with air temperature.The threshold air temperature value was approximately 15℃.More than 60%variation of CH4 flux can be explained by daily air temperature variation.Significant negative linear relationship between air pressure and CH4 fluxes was showed at different temporal scales.The result indicated that CH4 emission decreased with increase in the atmospheric pressure.The effects of artificial management（drying pond,aeration,and feed supply）on CH4 flux at different temporal scales were also investigated in the study.The aeration by turbulent aeration equipment didn’t significantly increase or depress CH4 flux during the aeration period.Drying pond can significantly increase daily CH4 emission during the early dyring pond period.The CH4 flux in the zone with a relatively high fertilizer input was higher than that in other zones in the same aquaculture pond.The air temperature and organic carbon input provided by feed supply can explain the inter-annual variation of CH4 flux.（4）The carbon cycle and comprehensive greenhouse effects caused by carbon emission in aquaculture pondsAnalyzing carbon cycle in the pond,more than 95%of organic carbon were supplied by feed or fertilizer,while the output carbon was mainly composed by two parts.One was the carbon output by aquaculture creature harvest and the other was CH4 emission.The feed use efficiency from 2016 to 2019 was 12%,14%,18%,and 22%,respectively.Because of the lower feed use efficiency,the carbon was emitted in the form of CH4.The amount of carbon emission by CH4 flux accounted for 80%（±20%）of total carbon emission and 13%（±4.86%）of input organic carbon.CH4 was the main pathway of carbon emission from the aquaculture ponds.Excluding the discharged organic carbon,the remaining organic carbon stayed in water or deposited in the sediment.The amount of the remaing organic carbon accounted for more than 65%of the total organic carbon input.The result indicated that the carbon content in the sediments cannot be ignored in the carbon cycle of aquaculture ponds.The synthetical global warming potential of CO2 and CH4 emission was on average 32.5 Mg CO2 eq ha-1 in aquaculture pond from 2016 to 2019.The CH4 flux contributed more than 90%. |
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