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 in a small mountainous catchment 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 (Bavarian Forest National Park, Southeast Germany) which strongly varied in slope between upper and lower part of the catchment, 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, dissolved organic carbon (DOC) concentrations, a proxy of DOM amount, ranged from 1.5 to 4.7 mg L⁻¹, but were similar along the stream. However, DOM quality exhibited clear spatial patterns, with overall high aromatic and low molecular weight DOM in the lower part of the catchment, which has a higher proportion of hydromorphic soils. Moreover, molecular data revealed that oxygen-rich, aromatic compounds increased in their abundance at high DOC concentrations in both the steep upper and the flatter 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 at low DOC concentrations.

中文翻译：

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描绘森林源头集水区基流条件下溪流溶解有机物成分的来源贡献

河流中的溶解有机质 (DOM) 组成反映了 DOM 来源、动员机制以及土壤和接收水体内的生物地球化学转化之间的动态相互作用。有关在基流期间动员的 DOM 源的信息可以提高我们预测对环境变化的水文和生物地球化学响应的能力，并对流域管理策略产生影响。本研究的目的是表征基流期间小山区流域上游源流中 DOM 组成的空间变化，并将流中的发现与流域中可能的 DOM 源联系起来。在 Große Ohe 集水区（巴伐利亚森林国家公园，E ₂ : E ₃ ) 来自高频探头测量。此外，通过超高分辨率傅里叶变换离子回旋共振质谱 (FT-ICR-MS) 收集和分析离散样品。在基流条件下，溶解有机碳 (DOC) 浓度（DOM 量的代表）范围为 1.5 至 4.7 mg L ^-1，但沿溪流相似。然而，DOM质量表现出明显的空间格局，流域下部整体高芳烃和低分子量DOM，水形土比例较高。此外，分子数据显示，在流域陡峭的上部和较平坦的下部，富氧芳香化合物的丰度在高 DOC 浓度下增加，并且在下部确定了额外的缺氧芳香化合物输入。相比之下，富氮脂肪族化合物与 DOC 浓度呈负相关，表明在低 DOC 浓度下深层地下水的贡献更大。