Effects of land use on the characteristics and composition of fluvial chromophoric dissolved organic matter (CDOM) in the Yiluo River watershed, China

Chromophoric dissolved organic matter (CDOM) is a key indicator of water quality, biogeochemical state and nutrient content in freshwaters. Land use affects the concentration, quality, source, and age of CDOM in rivers. To investigate the characteristics and composition of fluvial CDOM and the effects of land use on carbon components, 37 sites were sampled during May 2017 in the Yiluo River watershed, China. In addition, water parameters, absorption and fluorescence spectral parameters of CDOM were measured in situ and in the lab. Some samples showed atypical CDOM absorption curves with an absorption shoulder at 390 nm. CDOM absorption at 440 nm [a(CDOM)(440)] was positively correlated with dissolved organic carbon (DOC) and fluorescence intensity (Fn). Fluorescence excitation-emission spectra indicated the presence of strong humic-like and tryptophan-like fluorophores revealing terrestrial, anthropogenic influence. Low recent autochthonous contribution index (BIX) and humification index (HIX) values indicated that autochthonous production was an important contributor to CDOM levels. Cropland-dominated subwatersheds (SWs) exhibited high a(CDOM)(440), Fn and DOC and forest-dominated SWs showed low CDOM concentration. At the riparian buffer scale, CDOM in the built-up-dominated areas showed the highest CDOM concentration. Moreover, built-up areas greatly influenced the concentrations of DOC and CDOM quantificationally at both the SW and riparian buffer scales; more than 64% of the overall variation in carbon components at the SW scale and 46% of DOC variation at the riparian buffer scale was explained by built-up areas. Overall, the results of our study showed that the properties of fluvial CDOM are dependent on the surrounding land use, further highlighting the importance of land use for CDOM photoreactivity in SWs and riparian buffer zones.