Sources, Transformation, and Fate of Dissolved Organic Matter in the Gravel Bar of a Prealpine Stream

Gravel bars (GBs) are hotspots of biogeochemical activity, likely impacting carbon dynamics instreams and rivers. However, it remains unclear how GBs process dissolved organic matter (DOM) receivedfrom stream water and groundwater. Here we investigate the spatial and temporal variability of DOMconcentration and composition within a GB using DOM absorbance andfluorescence measurements. Wefound clear seasonal and diurnal patterns in DOM composition within the GB, indicating changingcontributions of DOM sources and transformation processes. While DOM composition was characterized bymore protein‐like DOM in summer, with increasing contributions of humic‐like DOM toward winterwithin the stream water and GB, groundwater DOM exhibited a strong aromatic and humic‐like characteryear‐round. DOM composition and concentration varied diurnally during autumn and winter within theGB and stream water, while seasonally higher groundwater inputs in summer likely muted the diurnalpattern, pointing to the importance of seasonal hydrological controls on DOM dynamics. Mixing modelanalysis showed GB DOM characteristics to differ from that predicted by stream water‐groundwater mixingwithin the GB, particularly in summer. Dissolved organic carbon concentration decreased alongflow pathsthrough the GB with increasing residence time, likely pointing to microbial uptake and/or adsorption tosediment surfaces within the GB, with concurrent clear shifts in DOM composition. While freshly producedand more humified DOM increased along GBflow paths, protein‐likefluorescence (C3) was removed,indicating the simultaneous production and removal of DOM. Together, ourfindings highlight the role of GBsin DOM removal and transformation, influenced by seasonal shifts in temperature and hydrodynamics.