Climate change is decreasing watershed glacial coverage throughout Alaska, impacting the biogeochemistry of downstream ecosystems. We collected streamwater fortnightly over the glacial runoff period from three streams of varying watershed glacier coverage (0–49%) and a subglacial outflow to assess how glacier recession impacts the relative contributions of glacier and terrestrial plant derived dissolved organic matter (DOM) inputs to streams. We show an increase in the fraction of old dissolved organic carbon (up to ∼ 3200 yr old radiocarbon age) with increasing glacial meltwater contribution to streamflow. We use a dual isotopic mixing model (δ¹³C and Δ¹⁴C) to quantify the relative contribution of terrestrial and glacial sources to streamwater DOM. The endmember contributions were further compared to DOM molecular compositional data from Fourier‐transform ion cyclotron resonance mass spectrometry to assess whether DOM composition can be linked to streamwater DOM source in watersheds with varying contributions of glacial runoff. This approach revealed the glacial fraction was positively correlated with percent relative abundance of heteroatom‐containing DOM molecular formulae, aliphatics, and peptide‐like formulae, while the terrestrial fraction was positively correlated with condensed aromatics and polyphenolics. These results provide information about how the retreat of mountain glaciers will impact the composition and thus biogeochemical role of DOM delivered to downstream ecosystems. Our findings highlight that combining an isotopic mixing model and ultrahigh resolution mass spectrometry data can provide novel insights into how changes in watershed landcover impact the source and chemical properties of streamwater DOM.

中文翻译：

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通过组成和碳同位素模型描绘的冰川化流域中的溶解有机物源

气候变化正在减少整个阿拉斯加的流域冰川覆盖，影响下游生态系统的生物地球化学。我们每两周从冰川径流期间每两周收集一次不同的分水岭冰川覆盖率（0-49％）和冰川下流出的溪流，以评估冰川衰退如何影响冰川和陆地植物衍生的溶解有机物（DOM）输入对流。我们发现，随着冰川融水对水流的贡献增加，旧的溶解有机碳的比例（直到3200年以前的放射性碳年龄）增加。我们使用的双同位素混合模型（δ ¹³ C和Δ ¹⁴C）量化陆地和冰川源对河水DOM的相对贡献。将最终成员的贡献与来自傅里叶变换离子回旋共振质谱的DOM分子组成数据进行了进一步比较，以评估DOM组成是否可以与流域中的水DOM来源相关，而冰川径流的贡献也不同。该方法表明冰川分数与含杂原子的DOM分子式，脂族和肽样式的相对丰度百分比呈正相关，而陆地分数与缩合的芳族化合物和多酚呈正相关。这些结果提供了有关高山冰川的退缩将如何影响其组成的信息，从而提供了将DOM传递到下游生态系统的生物地球化学作用的信息。