Characteristics And Influencing Factors Of Colored Dissolved Organic Matter In Typical Lakes In The Plains And Hills Of The Taihu Lake Basin

Dissolved organic matter?DOM?is one of the largest reduced carbon pools on the planet.It is ubiquitous in various environments and its transportation and transformation are of great significance to the global carbon cycle and climate change.Chromophoric DOM?CDOM?is the fraction of DOM that strongly absorb light in the ultraviolet and visible region and is composed of a series of humic acids,fluvic acids,aliphatic and aromatic hydrocarbons.Lakes are hotspots of the surface carbon cycling that receive terrestrial CDOM input,storage,migration and transformation.Under the current threats of accelerated climate change and urbanization,the cycling of CDOM in lakes and reservoirs has become one of the most important research fields in environmental sciences.Based on data collected from Lake Gehu,Lake Yangcheng,Lake Dianshan and Lake Tianmu in the Lake Taihu watershed,coupled with CDOM emission-excitation matrices?EEMs?and parallel factor analysis?PARAFAC?,the optical characteristics and fluorescence characteristics of different catchments are investigated,and the sources of CDOM and associated driving factors are further unraveled.In Lake Gehu,Lake Yangcheng and Lake Dianshan in the plain area,PARAFAC results exhibited that all the three lakes had fulvic acid-like C1,tryptophan-like C2,tyrosine-like C3,and humic acid-like C4.Among the fluorescence components,fulvic acid-like C1 represents non-point source agricultural effluents and was significantly and positively correlated with total phosphorus concentration?TP?,total nitrogen concentration?TN?,and chlorophyll concentration?Chla??p<0.001?.This indicated that external nutrient loading affects importantly the water quality in these lakes.The fluorescence intensity of protein-like C2 and C3,associated closely with point-source pollution and algae degradation,were significantly and positively correlated?p<0.01?with the fluorescence intensity of terrestrial humic acid-like C4,suggesting that external source rivers might be engulfed in sewage generated by residents activities into lakes.The mean of fluorescence intensities of C2-C4 in the wet season were significantly lower than in the dry season?t-test,p<0.05?,and the mean of fluorescence intensity of fulvic-like C1 in the wet season was higher than that of the dry season.The high levels of CDOM absorption coefficient a₂₅₄,DOC,and fluorescence intensity of CDOM components were found in lake areas in the nearshore of agricultural or cultivated areas,further indicated that human activities were important for the optical composition of CDOM in the shallow lakes in the Yangtze River Delta.For the Lake Tianmu in the hilly area,western of the Lake Taihu watershed,seven fluorescence components were obtained via PARAFAC modeling,including humic-like C1,C3,and C6,microbial humic-like C2,tryptophan C4-C5,tyrosine-like C7.High levels of a₂₅₄ and terrestrial humic-like C1,C3 and C6 were found in the southern part of the lake with large inflows.There were maximum protein-like fluorescence intensities in the northern and northern areas with intense urbanized areas.These indicated that riverine inflows to the lake and point-source effluents were important factors affecting the CDOM optical composition in Lake Tianmu.a₂₅₄ and the fluorescence intensities of all the seven components except for C5 in the wet season were significantly higher than in the dry season?ANOVA,p<0.05?.S_275-295 and a₂₅₀:a₃₆₅ were significantly higher in the dry season than in the wet season?ANOVA,p<0.05?,indicating that rainfall events increased the concentration of CDOM in the lake and terrestrial soil organic-rich signals.Combining inflow discharge and land use data,we found that DOM in the southern lake areas with large inflows showed stronger terrestrial signals than that of remaining lake regions.Forest land use?%?was significantly and negatively correlated with a₃₅₀ and fluorescence component intensities of C1-C6?p<0.001?,and was significantly and positive correlation with a₂₅₀:a₃₆₅?p<0.05?.The fluorescence intensities of C1-C6 and a₃₅₀ increased with increasing proportion of anthropogenic land use?cropland+urban??p<0.001?.This indicated that forest land use can effectively reduce the input of terrestrial soil organic matter.Human activities mobilized the inputs of terrestrial soil organic matter and point source effluents.A further comparison of two different types of lakes,i.e.Lake Tianmu,and Lake Gehu,Lake Yangcheng,Lake Dianshan,found that a₂₅₄,fluorescence intensity of the humic-like and protein-like materials,total nitrogen concentration?TN?,total phosphorous concentration?TP?,and Chla concentration in hilly Lake Tianmu were significantly higher than those in plain lakes?p<0.05?,indicating that the CDOM abundance and trophic levels of hilly regional lakes were significantly lower than those of plain shallow lakes.Lake Tianmu in hilly area was mainly humic-like?%humic-like is about 55%?,with high HIX and low BIX levels,while plain lakes were mainly protein-like?%protein-like is about 89%?,with low HIX and high BIX levels.This meant that the lake in the hilly area showed strong terrestrial humic-rich signals,while the lakes in the plain areas showed strong protein-like signals.For the lakes in plain areas,the management of lake aquaculture should be taken as an important engineering strategy for ecological restoration.For the hilly Lake Tianmu,better management of tourism and agricultural activities in the river basin,maintain the coverage of forest land use,and intercept the point source pollution at the lake entrance are needed.Our results had shed light on the governance of similar lakes.