On The Process And Mechamism Of Methane Emission From Eutrophic Ponds

The concentrations of greenhouse gases such as carbon dioxide?CO2?and methane?CH4?in the atmosphere have increased greatly since the industrial revolution.The concentration of CH₄ has increased from 0.72 ppm to the current 1.78 ppm.However,the cause of atmospheric CH₄ concentration variation is not clear.For example,the concentration of atmospheric CH₄ was relatively stable for more than 10years around the 1990s,but it began to increase again after 2007,which also led to the uncertainty of predicting future climate change scenarios.Freshwater ecosystems are an important source of CO₂ and CH₄.The majority of reported studies on carbon cycling about freshwater ecosystems target medium-large lakes and reservoirs.However,the area of ponds and lakes with more than 90%of the world's total area is less than<0.01km²,which does not include very small ponds with an area of 0.0001–0.001 km².The latter has a population of up to 3.2 billion and a total area of about 0.8×106 km².According to preliminary estimates,ponds with very small areas account for only about8.6%of the total area of lakes and ponds in the world,but they release 40.6%of total inland waters released by molecular diffusion.However,unlike deep-water lakes and reservoirs,the amount of CH₄ released by bubbling in shallow-water freshwater ecosystems is much higher than that released by molecular diffusion.In short,the importance of ponds in the global carbon cycle is seriously underestimated.In addition,the use of chemical fertilizers in agriculture has led to a two-and four-fold increase in nitrogen and phosphorus available for primary production at the global scale in the last century.In this century,the eutrophication of water bodies has expanded into a global problem,especially in tropical developing countries.This has led to the creation of more endogenous organic matter in the corresponding environment.In this paper,the eutrophic landscape ponds and agricultural ponds in Yichang City were taken as examples to carry out monitoring of gas fluxes and environmental factors at different time scales in order to clarify the characteristics and regularity of CH₄ flux.I also explore gas transfer velocity of CH₄ at low wind speeds.The main results of the paper are as follows:?1?Methane gas transfer velocity rate at the water-air interface of ponds at low wind speedsThe gas transfer velocity is a key environmental factor for calculating the gas flux at the water-air interface using thin boundary layer theory.In this paper,a static buoyancy chamber method is used to monitor the methane gas flux across the water-air interface.Taking a drinking water reservoir and the five eutrophication ponds as subjects,the seasonal regularity of the changes of gas flux and gas transfer velocity was studied.The results showed that the gas transfer velocity of the five ponds during the one-year observation period ranged from 0.20 to 1.99 cm/h,with an average value of approximately 0.50 cm/h,which was lower than those observed by Crusius and Wanninkhof at low wind speeds.Small water bodies that are usually shielded,or water bodies that are essentially free of wind or at very low wind speeds,can be represented by surface water temperatures and wind speeds.Convective cooling at low wind speeds contributes significantly to the exchange of gases on the lake surface.The diel surveys conducted in wild boar forest ponds in summer and autumn showed that the average k₆₀₀-CH₄?0.55 cm/h?in wild boar forest ponds was much lower than that in summer?1.64 cm/h?on a diurnal scale.In summer,the average diffusion flux of CH₄ is 4-5 times that of autumn.In summer,the gas transfer velocity of CH₄ is much higher than that of autumn.The k₆₀₀-CH₄ is mainly affected by weather conditions and primary productivity.The nighttime cooling of the water surface in the summer makes the gas diffusion flux increase at night.When the water temperature is lower than the temperature in autumn,the dependence of k₆₀₀-CH₄ on low wind speed is also observed.Low wind speed is not a key factor affecting the summer k₆₀₀-CH₄;in colder weather,low wind speed still dominates the exchange rate of CH4 at the water-air interface.?2?Static flotation method was used to observe the air-water interface gas fluxes of landscape ponds for one day a month and diel observations.Combined with simultaneous monitoring of water environmental factors and atmospheric environmental factors,the gas flow was analyzed.The diurnal and seasonal patterns of quantity?molecular diffusion and bubbling patterns?;the proportion of methane flux released by bubbling and diffusion in different seasons were revealed;and the environmental factors affecting methane flux in different seasons were discussed.The average value of methane diffusion flux during the three-month observation period in winter was the highest in February?0.1851 mg/m2/h?,which was 2.87 times and 2.43 times in December and January respectively.The correlation analysis showed that the methane diffusion flux and water temperature all showed a significant positive correlation during the three-month observation period,and there was no significant correlation with the temperature in December,but it was significant in January and February respectively.Negative correlations and significant positive correlations were two diametrically opposed phenomena;there was no significant correlation with wind speed in January,but there was a significant positive correlation between December and February;with Chl-a in December and 2 The month has a negative correlation,while it shows a positive correlation in January,which is consistent with the correlation between humidity and humidity.The mean value of methane diffusion flux during the spring observation period was the highest in May?0.2008 mg/m2/h?,which was nearly 2 times that of April,and the diffusion flux was the lowest in March.The diurnal diffusive flux of methane in April has a significant correlation with many water environmental factors,including a positive correlation with water temperature,air temperature,pH,wind speed and DO,and a negative correlation with Chl-a and humidity;in March,the proliferation Except that methane flux showed no significant correlation with water temperature and DO,the relationship between the methane flux and other environmental factors was consistent with that observed in April.During the May observation period,there was no significant correlation between all monitored environmental factors and diurnal diffusive flux of methane.The mean diurnal diffusion flux of methane increased gradually during the summer observation period,and was highest in August?0.1782 mg/m2/h?.Methane diffusion flux showed significant positive correlation with water temperature,pH,temperature,and wind speed during the observation period of June,and had a significant negative correlation with Chl-a and humidity,and no significant correlation with DO and atmospheric pressure.In July,there was no significant correlation with all monitored environmental factors;in the August observation period,there was only a correlation with water temperature and temperature.The mean value of methane diffusion flux during the autumn observation period was much higher in September?0.1605 mg/m2/h?than in October and November.Correlation analysis showed that the correlation between diurnal diffusive flux of methane and environmental factors was generally poor,showing only a significant negative correlation with atmospheric pressure during the observation period of October,and a negative correlation with pH and DO values in November.Correlation.The period of high diffusion flux of methane water-gas interface in Suoxi pond was concentrated from April to August,followed by February.The maximum occurs in May,about 21 times the minimum?November?.Comprehensive standard deviation and coefficient of variation,February and May are the periods when the methane diffusion flux has changed the most.The frequency of methane diffusion flux in a single observation showed a normal distribution,and the flux size in the range of 0.01-0.20 mg/m2/h accounted for 86.86%of all observations.The diffusion flux of methane was positively correlated with water temperature,air temperature,and DO,but negatively correlated with humidity.Methane bubbling flux has a strong time variability between months of the year,with the average being the largest in June 2016,followed by December 2015 and May2016,and the minimum occurring in November 2016.At the annual scale,the average flux of methane bubbles is about 4.959 mg/m2/h,the diffusion flux of methane is about0.098 mg/m2/h,and the effervescence flux accounts for the total methane flux at the water-air interface?5.057 mg/About 98.06%of m2/h?.The methane bubbling flux showed a significant positive correlation with water temperature,DO,Chl a,temperature,and methane diffusion flux.?3?Using a self-made device,the methane consumption rate of the pond water was measured at different temperatures.Using the indoor sediment culture method,the methane production rate of sediments at different temperatures was measured.The rate of methane depletion?0-20°C?in Suoxi pond water was determined using a novel rapid water-vapor exchanger and a DLT-100 greenhouse gas analyzer.With this device,the equilibrium of CH4 between the body of water and the top air body can be basically achieved in 15 minutes;at the same time,the device is an excellent reproducible measure of the methane consumption rate in the water body.The experimental results show that the methane consumption rate at all temperatures is closely related to the dissolved concentration in the water,and it shows a linear positive correlation.The methane consumption rate rises with the increase of the temperature under the condition that the dissolved methane concentration of the water body is similar.The sediment culture experiments showed that the methane yield was generally stable during the experiment at the same temperature,and the low temperature was more stable than the high temperature conditions.The average methane yields were about 0.02,0.04,and 0.36?mol/L/h at 5°C,15°C,and 25°C,respectively;the methane yield was nearly 10 times that at 15°C at 25°C.