The Greenland Ice Sheet (GrIS) is losing mass at a remarkable rate as a result of climatic warming. This mass loss coincides with the export of dissolved organic matter (DOM) in glacial meltwaters. However, little is known about how the source and composition of exported DOM changes over the melt season, which is key for understanding its fate in downstream ecosystems. Over the 2015 ablation season, we sampled the outflow of Leverett Glacier, a large land‐terminating glacier of the GrIS. Dissolved organic carbon (DOC) concentrations and DOM fluorescence were analyzed to assess the evolution of DOM sources over the course of the melt season. DOC concentrations and red‐shifted fluorescence were highly associated (R² > 0.95) and suggest terrestrial inputs from overridden soils dominated DOM early season inputs before progressive dilution with increasing discharge. During the outburst period, supraglacial drainage events disrupted the subglacial drainage system and introduced dominant protein‐like fluorescence signatures not observed in basal flow. These results suggest that subglacial hydrology and changing water sources influence exported DOC concentration and DOM composition, and these sources were differentiated using fluorescence characteristics. Red‐shifted fluorescence components were robust proxies for DOC concentration. Finally, the majority of DOM flux, which occurs during the outburst and post‐outburst periods, was characterized by protein‐like fluorescence from supraglacial and potentially subglacial microbial sources. As protein‐like fluorescence is linked to the bioavailability of DOM, the observed changes likely reflect seasonal variations in the impact of glacial inputs on secondary production in downstream ecosystems due to shifting hydrologic regimes.

中文翻译：

---

冰川流出溶解的有机物，成为通向季节性变化碳源的窗口：格陵兰的勒沃里特冰川

由于气候变暖，格陵兰冰原（GrIS）的质量正以惊人的速度减少。这种质量损失与冰川融化水中的溶解性有机物（DOM）出口相吻合。但是，人们对熔解季节出口DOM的来源和组成如何变化知之甚少，这是了解其在下游生态系统中命运的关键。在2015年的消融季节，我们采样了Leverett冰川（GrIS的大型陆地终止冰川）的流出量。分析了溶解的有机碳（DOC）浓度和DOM荧光，以评估在融化季节过程中DOM源的演变。DOC浓度和红移荧光高度相关（R ²> 0.95），并建议在被逐渐稀释并排放增加之前，来自被覆土的地面输入占DOM早期季节输入的主导。在爆发期，冰川上排水事件破坏了冰川下排水系统，并引入了基流中未观察到的显性蛋白样荧光信号。这些结果表明，冰下水文和不断变化的水源影响出口的DOC浓度和DOM组成，并且利用荧光特性对这些水源进行了区分。红移荧光成分是DOC浓度的可靠代表。最后，大部分的DOM通量发生在爆发期和爆发后时期，其特征是来自冰川上和潜在的冰川下微生物来源的类蛋白质荧光。