Analysis Of CO₂ Flux Variation And Its Driving Factors In A Submerged Macrophytes Habitat Of Lake Taihu

Inland lake constitutes a small fraction of global land surface,but plays a disproportionate and crucial role in earth climate system.In the recent decade,the importance of lake for carbon cycling has been recognized.Carbon dioxide can be processed in lakes and emitted into the atmosphere through water-atmosphere gas exchange,acting as either "sink" or "source".Carbon emission from lakes was estimated by large scale empirical model.But,the estimation cannot be verified,because of lacking of long term observations.And,measured CO₂ flux can be different because of differences among measurement methods.Thereby,carbon flux and carbon cycle processing mechanism in lake are still under debate,especially for the lakes covered by hydrophyte.The hydrophyte can change carbon sink and source of lakes.In this study,we used a 6-year CO₂ flux data measured by eddy covariance（EC）method over a submerged macrophytes habitat over a large shallow lake,Lake Taihu,in China from 2012 to 2017.The CO₂ flux dataset,meteorological data and k-s model simulation results are used to explore the temporal variation pattern of CO₂ flux,find out its impact factors and estimate CO₂ flux magnitude over this submerged macrophytes habitat of Lake Taihu.Major results are concluded as follows:（1）CO₂ flux has a clear diurnal pattern over this submerged macrophytes habitat in Lake Taihu.Obvious negative CO₂ flux appeared in the night and this habitat acted as CO₂ sink of the atmosphere,and CO₂ flux in daytime was higher than the night and this habitat acted as a weak CO₂ sink of the atmosphere.Diurnal variations of CO₂ flux indicate larger CO₂ absorption at night than during the day,and CO₂ flux was usually at a minimum in the early morning around 5:00 am and gradually increased during forenoon.This pattern is almost the same during the 6-year measurement period that CO₂ flux fluctuated between-1.67 and 0.09μmol m^-2 s^-1.The nocturnal negative CO₂ flux was observed during the whole observation period,proving the absolute CO₂ absorption from the atmosphere.CO₂ fluxes in summer and autumn were much lower than in winter and spring.Monthly mean CO₂ flux values ranged from^-2.23 to 0.19 μmol m^-2-1,the total mean value was 0.19 μmol m^-2 S-1.Annual mean CO₂ flux was-0.73 μmol m^-2 s^-1 according to the 6-year measurements,acting as CO₂ sink of the atmosphere.Mean CO₂ flux was lower in the previous 3 years(-0.89 μmol m^-2 s^-1)than the other 3 years(-0.57 μmol m-1 s^-1).（2）Submerged macrophytes habitat in Lake Taihu has a clear nocturnal CO₂ absorption character,and the magnitude of nighttime can be as low as^-19.28 μmol m^-2 s^-1 on individual nights and about-1.14 μmol m^-2 s^-1 on average.Crassllacean acid metabolism（CAM）of submerged macrophytes can absorb CO₂ in the night in this habitat which is the main biological reason of nocturnal CO₂ absorption.Incoming solar radiation,water temperature,friction velocity,kinetic energy and kinetic coefficient have negative correlations with CO₂ flux.When incoming solar radiation and water temperature were high and friction velocity and turbulence were strong,submerged macrophytes were much easier to absorb CO₂ from the atmosphere in the night.Water depth has a positive correlation with CO₂ flux which is not benefit for submerged macrophytes absorb CO₂ from the atmosphere.Solar radiation,water temperature,friction velocity,kinetic energy,kinetic coefficient and water depth are the abiotic factors of CO₂ flux variation.In daytime,incoming solar radiation and water temperature have less effects on CO₂ flux than the night,but friction velocity,kinetic energy and kinetic coefficient have negative correlations with CO₂ flux variation.Water depth still has a positive correlation with CO₂ flux in daytime.（3）Comprehensive analysis has done to find impact factors of CO₂ flux over submerged macrophytes habitat in Lake Taihu.For day timescale,solar radiation,water temperature,friction velocity,simulated turbulence kinetic energy and kinetic coefficient have negative correlations with CO₂ flux,while only water depth has a positive correlation with CO₂ flux.For month timescale,CO₂ flux relates to solar radiation and water depth,and low monthly CO₂ flux value happened in high solar radiation and low water depth.When the water depth was relatively shallow,CO₂ flux was still high.As there is not enough data for year timescale,annual impact factors are not clear.（4）Estimated annual CO₂ flux values are very close based on different gap filled methods.The standard deviation is between 0.02 and 0.1μmol m^-2 s^-1 of annual mean CO₂ flux among these 3 methods.On annual timescale,this submerged macrophytes habitat acted as a net CO₂ sink.The mean annual C absorption was 303 g C m^-2 yr^-1,and the most negative value could reach-377 g C m^-2 yr^-1 n 2012.The mean C absorption during the nighttime(203 g C M-2 yr^-1)over the 6-year period was approximately 2 times of the daytime(101.11 g C m^-2 yr^-1).Our observation indicatesthis habitat in Lake Taihu generally acts as CO₂ sink of the atmosphere.The yearly mean of C absorption varied from 184 to 370 g C m^-2 yr^-1.