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 in streams and rivers. However, it remains unclear how GBs process dissolved organic matter (DOM) received from stream water and groundwater. Here we investigate the spatial and temporal variability of DOM concentration and composition within a GB using DOM absorbance and fluorescence measurements. We found clear seasonal and diurnal patterns in DOM composition within the GB, indicating changing contributions of DOM sources and transformation processes. While DOM composition was characterized by more protein-like DOM in summer, with increasing contributions of humic-like DOM toward winter within the stream water and GB, groundwater DOM exhibited a strong aromatic and humic-like character year-round. DOM composition and concentration varied diurnally during autumn and winter within the GB and stream water, while seasonally higher groundwater inputs in summer likely muted the diurnal pattern, pointing to the importance of seasonal hydrological controls on DOM dynamics. Mixing model analysis showed GB DOM characteristics to differ from that predicted by stream water-groundwater mixing within the GB, particularly in summer. Dissolved organic carbon concentration decreased along flow paths through the GB with increasing residence time, likely pointing to microbial uptake and/or adsorption to sediment surfaces within the GB, with concurrent clear shifts in DOM composition. While freshly produced and more humified DOM increased along GB flow paths, protein-like fluorescence (C3) was removed, indicating the simultaneous production and removal of DOM. Together, our findings highlight the role of GBs in DOM removal and transformation, influenced by seasonal shifts in temperature and hydrodynamics.