Characteristics And Influencing Factors Of Carbon Dioxide And Methane Emission From Lakes In Eastern China

The rising concentration of greenhouse gases in atmosphere is the main causes of global warming.Aquatic ecosystem is an important source of greenhouse gas emission,and qualifying greenhouse gas emission from lakes is of great importance for having a knowledge of global climate change and developing strategic sustainable goals.In this study,the research object is divided into two parts:the typical lakes are Lake Hongze and Lake Luoma,and the eastern lakes are 30 lakes in the middle and lower reaches of Yangtze River and Huaihe River basin.The diffusive coefficient method was used to calculate the concentration and effluxes of CO₂ and CH₄,and the temporal and spatial variation of CO₂ and CH₄ emissions was discussed,and the influencing factors were preliminarily clarified based on environmental factors,nutrient levels,dissolved organic matter composition and source characteristics of the lakes.The findings of this study are as follows:（1）The emissions of CO₂ and CH₄ from Lake Hongze and Lake Luoma were determined,and the results showed that the CO₂ concentration and efflux in the surface water of Lake Hongze were 31.6±21.4μmol/L and 43.5±63.5 mmol/（m²·d）,and the CH₄ concentration and efflux from Lake Luoma were 0.12±0.09μmol/L and 21.0±18.5 mmol/（m²·d）.For all the 30 lakes and reservoirs in Eastern China,the mean concentration of dissolved CO₂ was 44.7±24.4μmol/L and the mean efflux was 22.6±26.1 mmol/（m²·d）,and the mean concentration of CH₄ dissolved concentration was0.21±0.28μmol/L and the mean efflux was 153.4±187.9μmol/（m²·d）.（2）Temperature had positive relationship with greenhouse gas emission,and it was also an important driver of CO₂ and CH₄emissions,suggesting that the foreseeing global warming will result in an elevated emission of greenhouse gases from aquatic ecosystems.Because the changes in microbial activity of dissolved organic carbon,gas solubility,dissolved oxygen level that associated closely with temperature.Hydrological changes also played an important role in the concentration and effluxes of CO₂ and CH₄ from the water column in the lakes and reservoirs of Eastern China,with notably higher gas emissions in the wet season than in the dry season（t-test,P<0.05）,and further higher than in the wet-to-dry transition season（t-test,P<0.05）.（3）Terrestrial input of nutrients including nitrogen and phosphorus to the lakes and reservoirs not only increased the nutrient level of the water bodies,but also promoted the production and emission of CO₂ and CH₄.Nitrogen and phosphorus concentrations showed positive correlations with both concentrations of dissolved CO₂and CH₄ and their effluxes,while chlorophyll-a displayed a weak linkage with greenhouse gas emissions from water bodies.Trophic level index（TLI）was calculated for all these lakes based on total nitrogen（TN）,total phosphorus（TP）and chlorophyll-a concentrations,and we found that more than 90%lakes in the study area were eutrophic,and both the concentration of dissolved CO₂ and CH₄and their effluxes increased with increasing TLI（P<0.01）,indicating that the eutrophication accelerated the production and emission of CO₂ and CH₄,and promotes water bodies to be a carbon source.（4）Dissolved organic matter（DOM）in the water columns provided an important carbon substrate for both CO₂ and CH₄ production and emission from lakes.The optical composition of DOM in the water column were analyzed,and their sources were mainly classified into terrestrial and autochthonous production.The input of terrestrial humic-like substances could significantly increase the levels of effluxes of both CO₂ and CH₄from the water column,and the terrestrial humic-like substances are closely related to human activities such as the discharge of urban sewage,agricultural irrigation and industrial wastewater,which reflected the profound impact of human activities on lake and reservoir carbon cycle and global climate change.