Extreme drought conditions increase variability of nitrate through a stream network, with limited influence on the spatial patterns of stream phosphate.

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. [ABSTRACT FROM AUTHOR]