Molecular Characteristics Of Dissolved Orgaic Matter In Meiliang Bay Of Lake Taihu Over The Algal Blooming-disappearance Cycle

Eutrophication not only severely influences lake’s biogeochemical processes,but also intensively modifies lake’s carbon cycle and carbon source-sink signature.As an important component of the carbon cycle,dissolved organic matter(DOM)is an important medium for the dynamic of organic carbon transformation.Investigation on the source and dynamics of DOM,as well as their controlling factors are important for understanding the carbon cycle of eutrophic lakes.Among lakes with server eutrophication conditions,Lake Taihu draws a lot of attention due to its strong eutrophication condition and its massive influence on the society.However,the source and dynamics of DOM under the algal blooming-recession cycle are still under throughout investigations.In this paper,we chose Meiliang Bay,one of the most serious areas of eutrophication in Lake Taihu,as the research object,and collect water samples though at every two weeks during 2017-2019.We applied total carbon content,stable carbon isotope,UV-visible absorption spectroscopy,three-dimensional fluorescence Spectroscopy,Fourier transform ion cyclotron resonance mass spectrometry,in order to explore the source and the dynamics of the DOM and its driving factors,as well as the potential impact on the dynamics of the DOM from the absolute eutrophication strength.Here are our major findings and conclusions:The results of total carbon content,stable carbon isotope show that the DOM of waters in the Meiliang Bay are of mixed sources from both autochthonous and allochthonous sources.The spectroscopy parameters suggest that the FDOM are mainly from autochthonous algae,while the CDOM are mixed with both autochthonous and allochthonous sources.Data from Fourier transform ion cyclotron resonance mass spectrometry also indicate that SPE-DOM in the Meiliang Bay simultaneously receives the influence of terrestrial sources,algae and other biomass and human pollution.Compared with lakes over the globe,DOM in the Meiliang Bay are more saturated,containing higher proportions of CHO and sulfur-containing molecules,and aliphatic compounds,and lower proportion of aromatics and polyphenol compounds.The complex sources for DOM in the Meiliang Bay might be mainly affected by the interaction among factors such as residual DOM pool,input from the algal blooming,terrestrial input and anthropogenic contaminations.The dynamic characteristics of DOM in the Meiliang Bay during algal bloomingrecession cycle indicate an important role of algal blooming-recession on DOM modifications.Specifically,during the algal blooming periods,the DOC concentrations,δ13Cvalues,averaged molecular saturation,the proportion of CHO,CHON and aliphatic compounds increased,while the average molecular aromaticity decreased.The characteristic molecules during the algal blooming periods include low mass(m/z =150 ～ 250)lipid-like molecules and high mass(m/z = 500 ～ 650)lignin-like molecules.During the algal recession periods,the DOC concentrations,δ13Cvalues,averaged molecular saturation,the proportion of CHO,CHON and aliphatic compounds decreased,while the average molecular aromaticity increased.The characteristic molecules during the algal blooming periods include low mass(m/z = 250 ～ 500)ligninlike molecules.Between the 2017-2018 cycle and the 2018-2019 cycle,the algal blooming-recession process in 2017-2018 has a greater impact on DOM transformation in the Meiliang Bay,which is consistent with the fact that the eutrophication strength in 2017 is stronger than that in 2018.The results of statistical analysis further confirmed the strong influence of algal blooming-recession dynamic on the variation of the DOM characteristics.Basically,DOM in the Meiliang Bay at algal blooming stage are mainly impacted by terrigenous input,total nitrogen and nitrate nitrogen ion.DOM in the Meiliang Bay at the algal recession stage are mainly impacted by DOC concentration,terrigenous input,ammonia nitrogen ion and phosphate ions.In summary,the results from water samples in the Meiliang Bay collected in two whole algal blooming-recession cycles indicate that the residual DOM pool(mainly mineralized or of terrestrial source)is the main component for the DOM in the waters from the Meiliang Bay.Under this background,the algal blooming would induce the input of in-situ fresh DOM,which is influenced by the eutrophication strength.These fresh DOM would mostly degrade at the Subsequent algal recession period.Therefore,although the dynamics of DOM in the waters from the Meiliang Bay is characterized by the algal blooming-recession process,the whole lacustrine eutrophication cycle does not change the carbon souce-sink characteristic of DOM in lake.Therefore,in order to eventually evaluate the importance of eutrophication in lakes on lake’s carbon cycle and carbon source-sink signature,the future studies should focus on the source,dynamics and mechanism of particle organic matters in lakes.