Montana Creek Alaska hydrology and biogeochemistry

High northern latitudes are experiencing dramatic changes in precipitation and air temperatures. These changes can impact river discharge and concentrations of bioreactive elements like carbon and nitrogen, but the pathways linking soil source pools to streams remain poorly constrained. Here we use high frequency water isotope and biogeochemical concentrations from a temperate forested watershed in Southeast Alaska to better understand how rainfall can influence water flow pathways, stream discharge, and concentrations of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN). We found that shallow groundwater traveling through organic matter rich surface horizons dominates contributions to stream water discharge during both wet and dry periods. Additionally, DOC yields generally increase with heightened shallow groundwater contributions and greater discharge, while TDN is more disconnected from these hydrologic processes. However, increased future groundwater contributions from shallow flowpaths to streams, particularly during repeated storm events, in a warmer and wetter region could lead to temporary periods of DOC source limitations, but will have little impact on TDN. Further, our findings suggest that concentration-discharge relationships for TDN appear to be controlled primarily by reach-scale factors, which could also shift in the future due to changes like warming stream temperatures due to shrinking snow and glacial meltwater inputs to surface water. These stream DOC/TDN dynamics could also impact nutrient availability in coastal regions that have limited mixing with the open ocean.

高纬度北部地区正经历着降水与气温的显著变化。此类变化可影响河流径流与碳、氮等生物反应性元素的浓度，但从土壤源库到溪流的关联路径仍未得到充分阐明。本研究利用阿拉斯加东南部温带森林流域的高频水同位素与生物地球化学浓度监测数据，以深入探究降雨如何影响水流路径、河川径流，以及溶解性有机碳（dissolved organic carbon, DOC）与总溶解性氮（total dissolved nitrogen, TDN）的浓度。研究发现，在干湿期内，流经富有机质表层的浅层地下水均为河川径流的主要补给来源。此外，DOC产率通常随浅层地下水补给占比升高与径流增大而上升，而TDN则与这些水文过程的关联性较弱。然而，在未来气候暖湿化的背景下，若浅层径流路径对溪流的地下水补给占比进一步提升（尤其是在频发的风暴事件期间），可能会引发暂时性的DOC源限制现象，但对TDN几乎无影响。进一步而言，本研究结果表明，TDN的浓度-径流关系主要受河段尺度因素调控；而随着积雪减少、冰川融水补给地表水导致河流水温升高等变化，这类河段尺度因素未来也可能发生改变。这类溪流DOC/TDN的动态变化，还可能对与外海混合作用有限的近岸海域的养分可利用性产生影响。