Dissolved organic matter spiraling in a forested stream across hydrological regimes and chemical compositional characteristics

Dissolved organic matter (DOM) is being continuously modified and removed along the fluvial continuum. Current assessments and predictions of DOM transformation and uptake within drainage networks are based mostly on data from laboratory incubations, which do not simulate the collective influences of hydrodynamic transport, photochemical degradation, and microbial processing. Field data of DOM uptake rates remain limited, and their variation as a function of hydrological regimes and chemical compositions is poorly understood. In this study, we evaluated reach-scale uptake rates of DOM from leaf leachates in a forested stream across hydrological regimes and chemical compositional characteristics. We combined hydrological transport modeling with seven slug addition experiments evaluated by a novel method assessing carbon spiraling (i.e., Tracer Additions for Spiraling Curve Characterization, TASCC). DOM was characterized by high-resolution, in situ sensor logging, and laboratory measurements of optical properties and biodegradability. We found that DOM uptake rate (U) and length (Sw) were positively correlated with water discharge, whereas the uptake velocity (Vf) was negatively impacted by discharge. In the literature, laboratory water-column DOM biodegradation is regularly performed to evaluate biodegradable DOM, but our data show that this process accounted for only less than 5% of total DOM uptake in the field. The Break Through Curves (BTCs) of various fluorescence components show that terrestrially derived, humic-like compounds were preferably removed than microbially derived, humic-like and protein-like compounds, resulting in a longer uptake length of microbially sourced DOM. These results suggest that sediment sorption may be the primary mechanism mediating reach-scale DOM removal. Our study highlights the importance of incorporating and assessing hydrological transport and physical removal of DOM to better quantify and predict the fate of DOM within drainage networks.