Characterizing The Sources,Composition And Bioavailability Of Chromophoric Dissolved Organic Matter In Key Lakes Connecting The South-to-North Water Transfer Project

The South-to-North Water Transfer Project（SNWTP）supplies drinking water to approximately 4.38 billion residents in northern cities of China.The five lakes of Lake Gaoyou,Lake Hongze,Lake Luoma,Lake Nansi,and Lake Dongping are the key lakes connecting the eastern route of the SNWTP.Water quality in these lakes is directly related to water supply security to more than 1 billion population.Dissolved organic matter（DOM）is the main organic carbon pool on of the global carbon cycle in the hydrosphere.Large-scale inter-basin water transfer projects directly change the regional carbon cycling.Chromophoric dissolved organic matter（CDOM）is the fraction of the DOM that can strongly absorb ultraviolet radiation and blue light.High concentration of CDOM results in a sour taste of water and CDOM produces carcinogens during water treatment processes.High CDOM level not only adversely affects technological processes of water treatment,but also damages water processing instruments.This paper focuses on the key lakes along the eastern route of the SNWTP,and the response of sources,composition,and bioavailability of CDOM in the five lakes to different hydrological scenarios and the potential driving factors.There main results are as follows:（1）DOC concentration and CDOM absorption a₂₅₄ in the five lakes in the wet season is higher than that in the dry season.This indicates that the input of organic matter was higher in the wet season,and source of CDOM is also mainly terrestrial.The specific UV absorbance SUVA₂₅₄ of Lake Gaoyou and Lake Hongze was significantly higher in the wet season than that in the dry season,and SUVA₂₅₄ in other lakes is also higher in wet season,indicating higher aromaticity of CDOM and higher input of humus-like acid in the wet season than in the dry season.（2）Three fluorescent components are obtained in five lakes using excitation-emission matrices（EEMs）and parallel factor analysis（PARAFAC）,including a humic-like acid C1,a tryptophan-like C2 and a tyrosine-like C3.The above three fluorescent components are also obtained for the CDOM samples collected from Lake Hongze and Lake Luoma after 28 days of bio-incubation.（3）The responses of CDOM optical composition in the five lakes to different hydrological scenarios are different.Fluorescence intensity(F_max)of three fluorescent components of Lake Gaoyou,Lake Hongze and Lake Luoma in the wet season is significantly higher than in the dry season.In contrast,Lake Nansi and Lake Dongping have higher fluorescence intensity in the dry season than in the wet season.Fluorescence intensities of the three components in Lake Gaoyou,Lake Hongze,and Lake Luoma,especially in the wet season,are significantly higher in the inflowing river mouths than in the remaining lake regions.This indicates that the river input has a greater contribution to the CDOM in the wet season.The three fluorescent components in the lower of Lake Nansi is higher than that in the upper in different hydrological scenario.The spatial distribution of three fluorescent components in Lake Dongping in different hydrological scenarios is less affected,and humic-like acid and tryptophan-like are higher in inflowing river mouths（Lake Dawen）in dry season.（4）Humic-like acid and protein-like components in Lake Gaoyou,Lake Hongze,and Lake Luoma contributed importantly in different hydrological scenario.The humic-like acid and protein-like components are mainly imported from the terrestrial source in the wet season,while the tryptophan-like is mainly from the autogenous source.The terrestrial source has a less contribution to humic-like acid,and the protein-like is more from the autogenous source in the dry season,that is,the contribution of microorganisms or algae.There is a small impact on the composition of CDOM in Lake Nansi and Lake Dongping in different hydrological scenarios.The protein-like of the two lakes mainly origins from endogenous sources and also is affected by human activities in the dry season,while humic-like acid is lower than protein-like,indicating that the input of terrestrial source is lower.（5）The concentration of DOC and a₂₅₄ in the water transfer period are lower than that in the non-transfer period,and the concentration of DOC,fluorescence intensity of a₂₅₄ and two protein-like components in Lake Nansi and Lake Dongping are significantly higher than those in the other three lakes in the water transfer period,indicating that the water transfer has a cumulative effect on the CDOM of the lake in the lower reaches of the eastern line.Humic-like acid and the degree of humification in the five lakes in the water transfer period is lower than that in the non-transfer water period.（6）The bioavailability of DOC in Hongze Lake and Luoma Lake in the dry season（%BDOC,17±4%and 15±4%,respectively）is higher than that in the wet season（%BDOC,5±5%and 10±7%,respectively）,and the%BDOC in the dry season is higher in the inflowing river mouths than the remaining lake regions.SUVA₂₅₄ and humic-like acid of the two lakes in the dry season post-incubation are significantly higher than those pre-incubation,while the tyrosine-like fluorescence intensity is significantly lower than pre-bio-incubation,indicating that the bioavailability of tyrosine-like is higher in the dry season than in the wet season.This causes the accumulation of humic-like acids and increases the degree of humification of CDOM.In summary,the river input was the main source of CDOM in Lake Gaoyou,Lake Hongze,and Lake Luoma in the wet season.The humification of CDOM is high and CDOM is dominated by endogenous of the three lakes in the dry season,i.e.algal or microbial activity.The source and composition of CDOM in Lake Nansi and Lake Dongping are more affected by the dry season,and both endogenous and anthropogenic pollution play major role in the contribution of protein-like in the two lakes in the dry season.Furthermore,the water diversion will cause the cumulative effects on water environment in the Lakes Nansi and Lake Dongping and reduce the concentration of CDOM,which promote the improvement of water quality.The bioavailability of CDOM in Lake Hongze and Lake Luoma is relatively high in the dry season,and it is mainly concentrated in the area where in the inflowing river mouths.In addition,the role of microorganisms has increased the degree of humification of CDOM.