Carbon Dioxide Emissions From Typical Lakes And The Potential Linkages With The Composition Of Dissolved Organic Matter

In aquaticecosystems,the cycling of organiccarbon and dynamics of source and sink of greenhouse gases are important issues concerning global warming.Lakes receive and process a large quantity of organicand inorganiccarbon from the catchment,and carbon cycling in lake ecosystems has become increasingly complex due to the enhanced eutrophication caused by accelerated urbanization and increasing frequency of extreme weather events.Dissolved organicmatter（DOM）is the largest organiccarbon pool on earth,and its transportation and transformation affect carbon mineralization and carbon dioxide（CO₂）emission,thus influence global climate change.Therefore,the eutrophicshallow lake Chaohu and the oligotrophicdeep-water lake Qiandao were chosen as the research objects,and excitation-emission matrices（EEMs）combined with parallel factor analysis（PARAFAC）,together with ultrahigh-resolution mass spectrometry（FT-ICR MS）to resolve the chemical composition of dissolved organicmatter（DOM）in lakes under different hydrological conditions,and determines dissolved CO₂（cCO₂）in the water column by diffusive coefficient approach and calculates the diffusion flux（FCO₂）to investigate the characteristics of CO₂ emissions from typical lake and reservoir,and the potential linkages with the chemical composition of DOM.The main findings are shown as follows.There were significant seasonal variability in the composition and sources of DOM in Chaohu Lake,with significantly higher DOM concentrations in the wet and dry periods than in the wet-to-dry transition period（t-test,p<0.001）and no significant differences between the wet and dry periods（t-test,p>0.05）,with terrestrial humic-like inputs during the wet period contributed primarily to the DOM pool in the western inflowing lake area,and tyrosine-like from algal degradation during the wet-to-dry transition period was the main source of DOM in the open water lake area.Terrestrial humic-like component decreased from the western inflowing area to the eastern outflowing lake area,and terrestrial humic-like substances increased with increasing total nitrogen（TN）total phosphorus（TP）and chlorophyll-a（Chl-a）（p<0.01）,showing that DOM of Lake Chaohu is closely associated with the migration of N and P elements.This can be explained by the large amount of plant residues and household effluents accumulated in the western inflowing river mouth during rainfall events.The emission of CO₂ from Lake Chaohu varied significantly.The mean cCO₂ of surface water was higher in rainy summer July than in April,and thereafter January,and the overall cCO₂ was higher in the western inflowing lake area than in the central and estern outflowing lake areas.Correspondingly,the FCO₂ is higher in July than in April and January,with high efflux found in the western inflowing lake areas than in the open water and eastern outflowing areas,similar pattern with that of DOM optical properties.cCO₂ and FCO₂ both increased significantly with increasing Chl-a（p<0.001）and PC1 of principal component analysis that associated positively with terrestrial norganicmatter input,indicating that terrestrial nutrient input had a significant contribution to CO₂ production and that eutrophication might lead to elevated CO₂ emissions.In addition,both cCO₂ and FCO₂ increased with significantly with increasing terrestrial humic-like fractions C1 and C4（p<0.001）,and increased with decreasing DO in the western inflowing lake area（p<0.001）,indicating that the input of terrestrial humic-like substances and the subsequent mineralization of organicmatter can significantly contribute to CO₂ emissions from the lake,especially from the inflowing river mouth.Extreme weather events,such as rainstorms can significantly increase carbon emissions from the inflowing river mouth of Lake Qiandao,with cCO₂ increased from 8.2±4.0μmol L^-1 before the rainstorm to 31.7±14.0μmol L^-1 after the rainstorm（p<0.001）and diffuse efflux FCO₂ increased from-3.8±3.0 mmol m^-2 d^-1to 13.2±9.3 mmol m^-2 d^-1（p<0.001）.Both FCO₂ and cCO₂ increased with increasing DOM absorption a₃₅₀,terrestrial humic-like C1,specificultraviolet absorbance SUVA₂₅₄,PO₄^3--P,NH₄⁺-N,NO₃^--N,and increased with decreasing Chl-a,DO,and p H.This indicated that CO₂ emission from Lake Qiandao during rainstorm events is influenced by many environmental factors,among which the inputs of N,P,and DOM are the main influencing factors,while water temperature and Chl-a had relatively minor impacts on carbon emission during the rainstorm.In addition,the enhanced soil carbonate erosion from the catchment during rainstorm also favored the emission of CO₂.Terrestrial input of soil organiccarbon was the main source of DOM in Lake Qiandao during rainstorm events,and photodegradation of highly aromaticDOM can provide sufficient carbon source for microorganisms,and the stable carbon isotopicsignatures further proved that CO₂ mainly comes from the degradation of DOM.Laboratory anoxicbio-incubation experiment results further proved that terrestrial humic-like DOM was closely associated with CO₂ production and emission from Lake Qiandao.