Spectroscopic indices trace spatiotemporal variability of dissolved organic matter in a river system with Karst characteristic

River acts as a continuum of dissolved organic matter (DOM) transport and transformation. However, underlying controls of fluvial DOM spatiotemporal variability has yet to be revealed. Here, we explored the DOM component, molecular weight and source in the Longchuan River, a headwater tributary of the Yangtze with Karst characteristic. Concentrated rainfall, severe soil erosion and in-situ metabolism primarily controlled DOM component and molecular weight in the Karst river, resulting in accumulation of recalcitrant (high molecular weight) DOMs from tributaries to the main stem in the post-wet period. We found a robust relationship between DOM absorption coefficients (a280 and a350) and dissolved organic carbon (DOC) concentrations from a multiple linear regression model: [DOC] = 0.098a285 + 0.591a350 + 1.337 (R2 = 0.83, p < 0.001). High fluorescence index (FI) values (>1.9) indicated a dominant origin of autochthonous DOM. Biological index (BIX) values exhibited an increasing trend along the flow direction, suggesting a large amount of recently produced DOM from phytoplankton downstream. Our findings demonstrated that both allochthonous and autochthonous inputs regulated DOM dynamics in a river continuum with Karst characteristic, highlighting spatiotemporal variability of DOM response to River Continuum Concept (RCC).