Components of dissolved organic carbon in relation to environmental factors in lakes along an altitudinal gradient in central China

Dissolved organic carbon (DOC) is an important organic substance that affects biogeochemical processes and underwater light conditions in aquatic ecosystems. To explore altitudinal variations in DOC components and water quality, 66 surface water samples were collected along an altitudinal gradient (10–1857 m) in three regions of central China, including a montane region (Enshi Prefecture) and two floodplain regions (Dongting Lake Basin and Wuhan City). DOC components were measured using a three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy, in conjunction with fluorescence regional integration (FRI) and fluorescence indices. Generally, lakes at high elevations (Enshi Prefecture) had higher DOC concentrations (a mean value of 6.43 mg L⁻¹) than floodplain lakes in Dongting Basin (4.51 mg L⁻¹) and Wuhan City (3.89 mg L⁻¹). Fluorescence index (FI, a range of 1.37–1.92) and biological index (BIX, a range of 0.47–0.74) indicated that DOC mainly originated from allochthonous sources in the three regions. Enshi Prefecture had relatively lower FI and BIX values, suggestive of a larger contribution of allochthonous carbon in this montane region in comparison with the floodplain regions. Humic-like substances were more abundant than other four fluorescent components, including tyrosine-like, tryptophan-like, fulvic acid-like, microbial-like substances. Spearman rank correlation analyses showed that tryptophane-like substances were positively correlated with K⁺, Mg²⁺, and Na⁺, while humic-like substances were negatively correlated with these cations. Taken together, the results can improve our understanding on altitudinal variations in DOC components and potential driving forces.