Nutrient availability is an important control on ecosystem processes in streams. In this study, we explored how an extreme summer drought affected spatial patterns of nutrient availability along a fourth-order stream network in western Oregon. Droughts are expected to become increasingly common and more severe across western North America and around the world. Understanding how nutrient availability changes locally and throughout a stream network during low-flow periods provides important insight into drought impacts on stream ecosystems. We quantified nitrate (NO₃⁻) and phosphate (PO₄³⁻) concentrations every 50 m along 11.5 km of a headwater stream network during three summer periods of different drought intensity that encompassed some of the lowest discharges observed in this system over its 70-year hydrologic record. Semi-variogram analysis indicated that concentrations of the dominant limiting nutrient, NO₃⁻, became increasingly spatially heterogenous during the most extreme drought conditions, whereas spatial variability of PO₄³⁻ concentrations remained similar across all three flows. Synoptic sampling during the most severe low-flow period revealed hotspots of biogeochemical processing that would be missed if sampling were conducted during higher flows when surface water dilution and more rapid transport of limiting nutrients would dampen local signals. Along a 3 km section of the upper mainstem, an increase in N availability during the drought led to a reduction in the degree of N-limitation and a potential shift toward P-limitation. Our results suggest that projected climate-induced changes in hydrology in this region will modify local nutrient availability.

养分可用性是对河流生态系统过程的重要控制。在这项研究中，我们探讨了夏季极端干旱如何影响俄勒冈州西部四级河流网络沿线养分可用性的空间格局。预计干旱将在北美西部和世界各地变得越来越普遍和严重。了解在低流量期间养分可用性如何在当地和整个河流网络中变化，可以为干旱对河流生态系统的影响提供重要的见解。我们量化了硝酸盐（NO ₃ ^-）和磷酸盐（PO ₄ ^3-) 在三个不同干旱强度的夏季期间，沿源头河流网络 11.5 km 的每 50 m 浓度，其中包括该系统在其 70 年水文记录中观察到的一些最低排放量。半变异函数分析表明，在最极端的干旱条件下，主要限制养分 NO ₃ ^{-的浓度在空间上变得越来越异质，而 PO} ₄ ^{3-的空间变异性}三种流中的浓度保持相似。在最严重的低流量期间的天气采样揭示了生物地球化学处理的热点，如果在高流量期间进行采样，当地表水稀释和限制性营养物质的更快速运输会抑制当地信号时，这些热点将被遗漏。沿着主干上部 3 公里的部分，干旱期间 N 可用性的增加导致 N 限制程度的降低和向 P 限制的潜在转变。我们的研究结果表明，该地区预计的气候引起的水文变化将改变当地的养分供应。