Diurnal Variation Pattern Of Greenhouse Gases At Water-air Interface In An Eutrophic Shallow Lake Of Semi-arid Area

Lakes are an important part of the global carbon cycle,among which eutrophic shallow lakes are important objects for studying the carbon cycle of lakes because of their frequent material exchange,high primary productivity,and abundant organic matter.In this paper,the eutrophic meadow shallow lake Ulansuhai in Urad Qianqi,Inner Mongolia Autonomous Region,was selected as the study area.S1 site with Huangtai outbreak and the S2 site without Huangtai outbreak in the submerged vegetation area in the lake were selected,and in situ diurnal monitoring was conducted once a month from April to October 2019(the ice-free period of the lake).The partial pressures of greenhouse gases(pCO₂,pCH₄)at the surface and bottom layers of the water column were measured by the headspace equilibrium method,and the greenhouse gas fluxes(FCO₂,FCH₄)at the water-gas interface were measured by the floating chamber method.Then,the diurnal variation patterns of greenhouse gas partial pressures,greenhouse gas fluxes,and the relationships with environmental factors,were analyzed.The main results are as follows.1.Diurnal variation of pCO₂ and FCO₂ at S1 and S2 sites in the Lake Ulansuhai:The overall variation of surface and bottom pCO₂ at S1 site showed decrease gradually after sunrise and reached the minimum value in the afternoon,and then the pCO₂gradually increases at night and reached the maximum value before sunrise.By comparing the bottom and surface pCO₂,we found that the bottom pCO₂ was usually larger than the surface pCO₂,and the variation of surface pCO₂ within a day was usually larger than that of bottom pCO₂.The FCO₂ at S1 site was positive during nighttime and was the source of CO₂,and the emission rate gradually increased after sunrise and reached the maximum before sunrise,and the emission at S1 site during nighttime accounted for most of the emission throughout the day.During the daytime,the change of FCO₂ is that the emission rate gradually decreases after sunrise and the absorption rate gradually increases,and generally reaching the minimum emission rate or the maximum absorption rate in the afternoon.The pCO₂ in the surface and bottom layer at S2 site showed that a decreasing trend during the day and night,but the change during the day is greater than during the night,and there is a small difference between pCO₂ in the surface layer and pCO₂ in the bottom layer during the day.The pCO₂ in the surface layer and bottom layer at S2 site have the same change pattern during the daytime and nighttime.The emission rates daytime and nighttime are basically same when S2 site is the source of CO₂,however,the absorption of CO₂mainly comes from the daytime,and the absorption during nighttime only occupied a smaller part of the absorption throughout the day when S2 site is the sink of CO₂.2.Monthly variation of pCO₂ and FCO₂ at S1 and S2 sites in the Lake Ulansuhai:The pCO₂ of surface and bottom layer at S1 site showed a trend that decreased firstly,then increased,and finally decreased during the ice-free period.However,S2 site was a source of CO₂ in April and July(cloudy days),and a sink of CO₂ in May,June,August,September,and October.In addition,both S1 site and S2 site show that the period after the ice melt is a critical period for the release of CO₂ from the lake,accounting for a larger proportion of CO₂ release during the ice-free period.Then,S1 site acted as an overall source of CO₂ during the ice-free period with an average FCO₂ emission rate reaching 5.54 mmol m^-2 d^-1;however,S2 site was a sink of CO₂ with an average FCO₂ absorption rate reaching-7.30 mmol m^-2 d^-1.3.Diurnal variation of pCH₄ and FCH₄ at S1 and S2 sites in the Lake Ulansuhai:The pCH₄ of surface and bottom layer at S1 and S2 sites showed a general trend gradually increasing during the day and decreasing during the night,and the pCH₄ value during nighttime were larger than during the daytime at sometimes.In addition,the pCH₄ at bottom layer is larger than the surface layer in most of the daytime and nighttime,and the FCH₄ at both S1 and S2 sites showed a larger emission in the daytime and a smaller emission in the nighttime.4.The monthly variation of pCH₄ and FCH₄ at S1 and S2 sites in the Lake Ulansuhai:The pCH₄ of surface and bottom layer had very low value in April,May,and October,while very high in June to September.In addition,the pCH₄ at both S1 and S2sites showed bottom layer was significantly higher than surface in June to September,while the difference between surface and bottom was not significant in April and October when the temperature was low.The FCH₄ had significant monthly variation pattern,with very low CH₄ emissions in April,May,and October but very high emissions in June to September.In addition,the lake was a source of CH₄ at both S1 and S2 sites during ice-free period,and the average emission rate of FCH₄ was 10.44mmol m^-2 d^-1 and 9.05 mmol m^-2 d^-1,respectively.5.The impact of environmental factors on carbon cycle in the Lake Ulansuhai:The pCO₂ and FCO₂ in the lake are strongly influenced by the outbreak of Huangtai and submerged aquatic vegetation(SAV),and eutrophication has a positive effect on the reduction of CO₂ emission.Then,weather conditions,dissolved oxygen,and temperature also could affect the dynamic of pCO₂ and FCO₂.The pCH₄ and FCH₄ are greatly influenced by temperature,and it also affected by Huangtai,SAV,dissolved oxygen,and other environmental factors.During the diurnal dynamics,FCO₂ in the lake is mainly influenced by photosynthesis and respiration of Huangtai and SAV,making a trend of absorption firstly enhanced and then weakened or turned to release from morning to evening,while FCH₄ had more release during the daytime and less release during nighttime due to the strong influence of temperature.