The Variations Of CO₂ And CH₄ Fluxes And Impact Factors In East Lake Taihu For Pre-and Post-returning Aquaculture To Lakes

As the hotspot of CO₂ and CH₄ emissions,inland lakes play an important role in the global carbon cycle.Aquaculture is an anthropogenic disturbance that seriously affects the structure and function of lakes.In recent years,the policy of returning aquaculture to lakes was carried out in China.Accurately evaluating the variations of lake CO₂ and CH₄ fluxes for pre-and post-returning aquaculture to lakes has become the key topic in the researches of greenhouse gas emissions from lakes in China.This study was conducted in East Lake Taihu,situated in the southeast corner of Lake Taihu,which owned a 40-year aquaculture history.Based on the eddy covariance method,CO₂ and CH₄ fluxes in East Lake Taihu during the period of pre-and post-returning aquaculture to lakes(from 2018 to 2021)were measured continuously.Combined with the auxiliary meteorological factor data,water quality data and aquatic vegetation growth dynamic data,the variations of lake CO₂ and CH₄ fluxes and their corresponding impact factors over multi-temporal scales during the period of pre-and post-returning aquaculture to lakes were determined.Accordingly,the effect of the policy of returning aquaculture to lakes on the carbon flux at the water-air interface and resulting comprehensive greenhouse effect in East Lake Taihu were evaluated.The main results of this study are as follows:(1)The variations of CO₂ flux and its impact factors over multi-temporal scales in East Lake Taihu for pre-and post-returning aquaculture to lakesThe CO₂ flux showed similar diurnal variation during the growing season in East Lake Taihu for pre-and post-returning aquaculture to lakes,with strong absorption in the day and weak emission at night.The obvious diurnal variation appeared from May to August during the aquaculture period,while it occurred from May to October during the ecological restoration period,and the amount of CO₂ absorption in the daytime augmented year by year.The monthly variation of CO₂ flux was different in the four years.The peak value of CO₂ absorption during the aquaculture period appeared in May and June,and the shift of CO₂source/sink appeared in September,while the peak value of CO₂ absorption during the ecological restoration period occurred in summer,and the shift of CO₂source/sink occurred in November.The peak value of monthly CO₂ absorption increased year by year.On the annual time scale,East Lake Taihu acted as the sink of atmospheric CO₂ for pre-and post-returning aquaculture to lakes,with annual cumulative CO₂-C absorption of 41.92,120.21,167.10,and 167.07 g C m^-2 in 2018,2019,2020,and 2021,respectively.The average annual cumulative CO₂-C absorption during the ecological restoration period was 3.6 times higher than that during the aquaculture period.On the hourly time scale,the solar radiation was the dominant factor regulating the diurnal variation of CO₂ flux.On the daily time scale,the solar radiation was the dominant factor affecting CO₂ flux during the aquaculture period,while both the solar radiation and the water temperature had significant impacts on CO₂ flux during the ecological restoration period.CO₂flux decreased linearly and exponentially with the increasing solar radiation and water temperature,respectively.On the monthly time scale,the CO₂ flux was mainly affected by total nitrogen during the aquaculture period,while it was mainly affected by the vegetation growth dynamics during the ecological restoration period.(2)The variations of CH₄ flux and its impact factors over multi-temporal scales in East Lake Taihu for pre-and post-returning aquaculture to lakesThe CH₄ flux exhibited obvious diurnal variation in summer and autumn in East Lake Taihu,but the CH₄ flux showed an unimodal diurnal pattern as the nighttime flux was higher than the daytime flux in 2018 and 2019.In 2020 and 2021,the diurnal variation changed to a bimodal W-shaped pattern with peak values appeared at midnight and around noon.The CH₄flux showed similar monthly variations,with the warm-season flux higher than the cold-season flux.On the annual time scale,East Lake Taihu acted as the source of atmospheric CH₄ for pre-and post-returning aquaculture to lakes,with annual cumulative CH₄-C emission of 36.01,23.69,22.80,and 26.45 g C m^-2 in 2018,2019,2020,and 2021,respectively.The average annual cumulative CH₄-C emission during the ecological restoration period represented a decrease of 32%from the value during the aquaculture period.Ebullition was always the dominant pathway of the CH₄ flux.However,under the joint influence of the purse seine removal and the increase of floating-leaved plants coverage,the growing season ebullition ratio showed an interannual variation of first increasing and then decreasing.A contribution of CH₄diffusion emission to total flux was about 13%?30%,indirectly estimating that about 25%of CH₄ was transported to the atmosphere through the plant-mediated pathway.On the hourly time scale,the diurnal variation of CH₄ flux in 2018 and 2019 was mainly affected by the thermal stability of the water body.Thereafter,the dynamic of CH₄flux during the day was also affected by solar radiation due to the emergence of the CH₄ aerobic production mechanism and the plant-mediated pathway.On the daily time scale,the CH₄ flux showed an exponential dependence on sediment temperature for pre-and post-returning aquaculture to lakes,with the exponential fit explaining more than 83%of the observed variations in the daily CH₄ flux.On the monthly time scale,the CH₄ flux was only affected by sediment temperature during the aquaculture period,while it was affected by both sediment temperature and NDVI during the ecological restoration period,increasing exponentially and linearly with the increasing sediment temperature and NDVI,respectively.(3)The variations of carbon flux at the water-air interface and resulting comprehensive greenhouse effect in East Lake Taihu for pre-and post-returning aquaculture to lakesDuring the ecological restoration period,there was a significant positive correlation between CH₄ flux and net ecosystem productivity on the half-hourly and the daily time scale,which indicated the regulation of floating-leaved vegetation stomata on CH₄ transport and the promotion of aquatic vegetation root exudates on CH₄ production,respectively.On the annual time scale,East Lake Taihu acted as the sink of atmospheric carbon for pre-and post-returning aquaculture to lakes,with annual cumulative carbon flux of-5.91,-96.52,-144.30,and-140.62 g C m^-2 in 2018,2019,2020,and 2021,respectively.The result indicated that the annual cumulative carbon absorption of East Lake Taihu increased 21 times after the ecological restoration.When considering the comprehensive greenhouse effect caused by carbon flux,East Lake Taihu showed positive effect on global warming for pre-and post-returning aquaculture to lakes,with annual cumulative comprehensive greenhouse effect of 1190.8,443.5,238.5,and374.9 g CO₂eq m^-2,respectively.The policy of returning aquaculture to lakes reduced the net warming effect from East Lake Taihu by 70%.