A Preliminary Study On The Distribution Characteristics And Production And Consumption Process Of Greenhouse Gases In Subtropical Reservoirs

The aggravation of greenhouse effect has become one of the global environmental issues of common concern.Identifying and accurately quantifying the sources and sinks of greenhouse gases has become a key challenge in the study of climate change and global warming,and recent studies have shown that reservoirs are a non-negligible source of global greenhouse gases.Reservoir is a kind of artificial ecosystem,its construction and operation have completely changed various water quality parameters.There are two distinct states of thermal stratification in summer and vertical mixing in winter in subtropical reservoirs.These seasonal variation characteristics also have an impact on the production and consumption of greenhouse gases in reservoirs.In this study,the Shidou Reservoir in Jimei District,Xiamen City was taken as the research object.Based on the concentration of three major greenhouse gases（CO₂,CH₄ and N₂O）in the water of the reservoir,the distribution characteristics of vertical greenhouse gas concentration in the reservoir were defined and the greenhouse gas emission of the reservoir was estimated.The production and consumption process of CH₄ and N₂O were mainly discussed.The results are as follows:The variation characteristics of vertical greenhouse gas concentration in Shidou Reservoir are as follows:（1）During the strong stratification period in summer,the N₂O concentration in the oxygen thermocline has a maximum value,and then rapidly decreases to 0,and this pattern continues until before vertical mixing in winter.In winter,the concentration of N₂O increased rapidly in the oxygen thermocline and slowly increased in the deeper anoxic layer after spring.（2）CH₄ concentration increased gradually from top to bottom from summer to autumn,and the concentration at the bottom increased month by month.In winter,the concentration difference between the upper and lower levels is small,and in spring,it returns to the distribution pattern of summer and autumn,but the concentration of CH₄ at the bottom is much lower than that in summer and autumn.（3）The distribution of CO₂ concentration is similar to that of CH₄,but its measurement error is higher than that of CH₄ and N₂O,and its parallelism is poor.A one-dimensional convective diffusion model was used to estimate the production and consumption rates of greenhouse gases in Shidou Reservoir:the maximum N₂O consumption rate at 1.5m is 0.32 nmol·-L^-1·d^-1,and the N₂O consumption rate below 6.5m is much lower than that at the upper layer,with the range of 0.22-2.30 nmol·L^-1·yr^-1.The CH₄ consumption rate ranges from 0.59-4.08 umol·L^-1·yr^-1 above 6.5m,and rapidly increases with deep CH₄ consumption rate at 6.5m,reaching a maximum value of 1.48 umol·L^-1·d^-1 at 11.5m,namely 539.24 umol·L^-1·yr^-1.CO₂ is generated from 1.5m to 6.5m,and the production rate ranges from 0.25 to 1.76 umol·L^-1·d^-1,decreasing gradually with depth,and consumption below 6.5m and increasing gradually with depth,with a maximum value of 0.67 umol·L^-1·d^-1 at 11.5m.The CH₄ release flux of Shidou reservoir is the largest in spring,and the smallest in winter,and the variation is small in summer and autumn,and the reservoir is the source of CH₄ throughout the year.For N₂O,the emission flux is the largest in autumn,followed by spring,and smaller in summer and winter.Reservoir is the source of atmospheric N₂O throughout the year.The CO₂ emission flux is larger in spring and autumn,but smaller in winter.The total emission of N₂O,CH₄ and CO₂ in Shidou Reservoir from June 2020 to April 2021 is estimated to be 113.8kg,2040.1kg and 1029.6t,respectively.The warming potential GWP of Shidou reservoir is 18.79 gCO₂·m-2·d^-1.