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.

中文翻译：

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高山前溪流砾石中溶解有机物的来源，转化和命运

碎石（GB）是生物地球化学活动的热点，可能会影响溪流和河流中的碳动态。但是，尚不清楚GBs如何处理从河水和地下水中获得的溶解有机物（DOM）。在这里，我们使用DOM吸光度和荧光测量研究GB内DOM浓度和组成的时空变化。我们发现GB内DOM组成具有明显的季节性和昼夜模式，表明DOM来源和转化过程的变化。尽管夏季DOM的组成特征是蛋白质类DOM增多，但在河水和GB中冬季对腐殖质类DOM的贡献越来越大，而地下水DOM全年却表现出强烈的芳香和腐殖质特征。在GB和溪流水的秋季和冬季，DOM的组成和浓度每天都在变化，而夏季季节性增加的地下水输入量可能会使昼夜模式减弱，这表明季节性水文控制对DOM动力学的重要性。混合模型分析表明，GB DOM特性不同于GB内的溪流-地下水混合所预测的特性，特别是在夏季。随着停留时间的增加，沿GB流动的溶解有机碳浓度降低，这可能表明微生物吸收和/或吸附到GB内的沉积物表面，同时DOM组成同时发生明显变化。当新鲜产生的和更腐烂的DOM沿GB流径增加时，蛋白样荧光（C3）被去除，表明DOM的同时产生和去除。