Characterizing The Sources,Composition,and Bioavailability Of Chromophoric Dissolved Organic Matter In The Two Largest Freshwater Lakes Connecting To The Yangtze River

Climate change and human activities are the two major challenges encountered to lake ecosystems.Urbanization and land use intensification have resulted in an excessive loading of nutrients being discharged into downstream receiving rivers and lakes,and thereby a deterorating ecological environments.The middle and lower reaches of the Yangtze River is the largest lake groups in China.It is densely populated,urbanized and highly vulnerable to pollution.Chromophoric dissolved organic matter（CDOM）is one of the most important occurrence forms of natural organic matter,and is riched in carbon-,nitrogen-,phosphorus-containing compounds.CDOM is constantly mineralized in the process of water migration,accumulating the nutrients and favoring the processes of lake eutrophication.Lakes connected to large river continuum share characteristics of enhanced seasonality,and the composition,bio-lability,and flux of CDOM in these lakes are distinct under different hydrological scenarios.Absorbance,three-dimensional fluorescence coupled with parallel factor analysis,and Fourier transform ion cyclotron resonance mass spectrometry were used to trace the characteristic of CDOM in Lake Poyang and Lake Dongting.The sources,composition,fluxes,and bioavailability of CDOM of these two lakes and river connecting to the lakes under different hydrological scenarios were explored,and the results showed that:The sources of CDOM in Lake Poyang and Lake Dongting can be divided into terrestrial humic-like and protein-like compounds.The abundance and optical composition of CDOM in the two lakes were significantly different under different hydrological conditions,and so were the contribution of different fluorescence components.In the flood season,the input of terrestrial humic-like substances contributed importantly to the CDOM pool,while protein-like compounds in the dry season contributed notably to the CDOM pool,and the contribution percentages of protein-like and humic-like components were roughly equal during the dry-to-flood-transition season.The mean of HIX in Lake Poyang was significantly higher than Lake Dongting in the flood season（t-test,P<0.01）,while the mean of BIX was higher in Lake Poyang than in Lake Dongting in the dry season.（t-test,P<0.001）,indicating that the soil leaching organic matter input in Lake Poyang in the flood season resulted in an enhanced humification of CDOM,while the discharge of protein-like fluorescent components from the surrounding residential areas may exist in the dry season.DOC concentration in Lake Poyang correlated positively to water level and also humic-like components under different hydrological scenarios（r²=0.99,P<0.01）,suggesting that rainstorm followed by elevated water levels in Lake Poyang in the flood season will bring a large amount of soil organic substances from the upstream watersheds.Tryptophan-like components in East Lake Dongting was negatively correlated with water level and also protein-like components,implying that there might be discharge of domestic waste water from the surrounding cities of Lujiao and Yueyang surrounding the lake in the dry season.The DOC and CDOM inflowing fluxes to Lake Poyang and Lake Dongting were significantly higher in the flood than in the dry season（t-test,P<0.01）,indicating that inflowing runoff significantly affects the seasonal flux characteristics of Lake Poyang and Lake Dongting.Compared with Lake Poyang,Lake Dongting is a notable sink of DOC and CDOM fluxes in the flood season,CDOM inputs from upstream tributaries were degraded significantly in the lake before being discharged into the downstream-linked Yangtze River,thus affecting the eutrophication process of lakes.Lake Poyang and Lake Dongting are both DOC and CDOM source fluxes in the dry season.Our results further showed that the four inflowing rivers from the south were significantly affected by human activities,and protein-like substances in the Xiangjiang River were higher than other inflowing rivers of the Lake Dongting watershed in the flood season,suggesting that anthropogenic disturbances discharged more domestic sewages in the upper reaches of Xiangjiang River compared with other inflowing rivers.In July,the contribution of three inlets to the DOC and CDOM fluxes in Lake Dongting was greater,and this can be ascribed to the larger inflow discharge of the three inlets.Under different hydrological scenarios,DOC,S_275-295,humic-like and protein-like fluorescence intensity decreased notably after 28 days of bio-incubation in both Lake Poyang and Lake Dongting,indicating that bio-lability of CDOM in the lakes connecting to the Yangtze River was stronger,and can be mineralized as inorganic nutrients to maintain the metabolisms in the lake ecosystem.DOC of the four rivers in Lake Dongting is less consumed by microorganisms than that of the three inlets,and can be ascribed by the four rivers are more biologically stable.This was consistent with the the high contribution percentage of highly unsaturated phenols analyzed by FT-ICR MS in Lake Dongting in the dry season.During the migration of CDOM from Songzi to Chenglingji,H/C ratio and the contribution percentages of aliphatic and polypeptide was higher and with corresponding lower O/C ratio indicating that aliphatic and polypeptide components could be utilized during the biogeochemical cycling of CDOM in these lakes.