Delineating source contributions to stream dissolved organic matter composition under baseflow conditions in forested headwater catchments

Dissolved organic matter (DOM) composition in streams reflects the dynamic interplay between DOM sources, mobilization mechanisms, and biogeochemical transformations within soils and receiving water bodies. The information regarding DOM sources being mobilized during baseflow can improve our ability to predict hydrological and biogeochemical responses to environmental changes, with implications for catchment management strategies. The objective of this study was to characterize the spatial changes in DOM composition along a headwater stream during baseflow and to link the findings in-stream with possible DOM sources in the catchment. DOM was monitored over 1.5 years at three sites in the Große Ohe catchment (19.2 km2), within the Bavarian Forest National Park, using UV-Vis absorption indicators of aromaticity (SUVA) and molecular weight (E2:E3) from high-frequency probe measurements. Additionally, discrete samples were collected and analyzed by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). At baseflow conditions, DOC concentrations – a proxy of DOM amount – ranged from 1.5 to 4.7 mg L-1 and were similar along the slope gradient. However, DOM quality exhibited clear spatial patterns, with overall high aromatic and low molecular weight DOM in the lower part of the catchment. Moreover, molecular data revealed that oxygen-rich, aromatic compounds increased their abundance at high DOC concentrations in both upper and lower part of the catchment, with also additional input of oxygen-depleted, aromatic compounds identified in the lower part. In contrast, nitrogen-rich, aliphatic compounds were negatively correlated with DOC concentration, indicating a higher contribution of deep groundwater flows.