Abstract
Fluorescence (excitation-emission matrices, EEMs) spectroscopy coupled with PARAFAC (parallel factor) modelling and UV-Vis (ultraviolet visible) spectra were used to ascertain the sources, distribution and biogeochemical transformation of dissolved organic matter (DOM) in the Duliujian River catchment. Dissolved organic carbon (DOC), chromophoric dissolved organic matter (a335) (CDOM), and hydrophobic components (a260) were higher in summer than in other seasons with 53.3 m−1, while aromaticity (SUVA254) was higher in spring. Four fluorescent components, namely terrestrial humic acid (HA)-like (A/C), terrestrial fulvic acid (FA)-like (A/M), autochthonous fulvic acid (FA)-like (A/M), and protein-like substances (Tuv/T), were identified using EEM-PARAFAC modelling in this river catchment. The results demonstrated that terrestrial HA-like substances enhance its contents in summer ARE compared with BRE, whilst terrestrial FA-like substances were newly input in summer ARE, which was entirely absent upstream and downstream, suggesting that rain events could significantly input the terrestrial soil-derived DOM in the ambient downward catchments. Autochthonous FA-like substances in summer BRE could derive from phytoplankton in the downstream waters. The results also showed that DOM from wetland exhibited lower fluorescent intensity of humic-like peak A/C and fulvic-like peak A/M, molecular weight (SR) and humification index (HIX) during the low-flow season. Built-up land, cropland, and unused land displayed higher a335 (CDOM). A higher proportion of forest and industrial land in the SCs showed higher SUVA254 values. Humic-like moiety, molecular weight and aromaticity were more responsive to land use during stormflow in summer. Rainfall could increase the export of soil DOM from cropland and unused land, which influences the spatial variation of HIX. The results in this study highlighted that terrestrial DOM has a significant influence on the biogeochemical alterations of DOM compositions and thus water quality in the downward watershed catchments, which might significantly vary according to the land-use types and their alterations by human activities.
Highlights
The influence of different land-use types associated with seasonal change on the content of dissolved organic matter (DOM) and characteristics in the SCs was investigated in a typical river of Northern China
Four fluorescent components were identified by excitation emission matrix (EEMs)-PARAFAC analysis and they were denoted as terrestrial humic acid-like, terrestrial fulvic acid-like, autochthonous fulvic acid-like, and protein-like substances
The contents of Dissolved organic carbon (DOC), Chromophoric dissolved organic matter (CDOM) and hydrophobic DOM were higher in summer, but aromaticity was lower in spring
Summary
The fluorescence spectroscopy technique by a 3D excitation emission matrix (EEMs) has been widely used to characterize the fluorescent dissolved organic matter (FDOM) of CDOM in an aquatic system [3,4,5,6]. EEM-parallel factor analysis (EEM-PARAFAC) modelling is widely applied to ascertain fluorescent DOM (FDOM) sources and their biogeochemical alterations in surface waters [6]. They are a major fraction of bulk DOM and useful indicators that can identify and characterize biogeochemical processes. Terrestrial DOM became dominant sources in rivers after rainfall events, which has influence on the phytoplankton community dynamics and subsequent influences on DOM dynamics and its compositions in lakes [11]
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