Stable Spatial Patterns of Nitrate in Headwater Stream Networks Allow Identification and Mitigation of Critical Source Areas.

Nutrient pollution of freshwater and estuarine water bodies is degrading ecological functioning and ecosystem services at a global scale. Economic damage from nitrate contamination alone is estimated to cost 0.2 to 2.3 trillion USD annually—up to 3% of the global gross domestic product. In the past 50 years, global fertilizer use increased by over 500% , and nitrogen and phosphorus pollution are expected to increase until the middle of the century. Most excess nutrients enter stream networks in headwater catchments, but modeling and managing these systems is hindered by high spatial and temporal variability and a breakdown of the relationship between water quality and land use. While continuous monitoring of thousands of small streams is prohibitively expensive, particularly in developing countries where water-quality is degrading fastest, what if we could learn what we needed about where and when to implement conservation efforts with periodic sampling of headwater catchments? To address this question, we quantified spatial and temporal variability of nitrate and other solute concentrations over 12 years in 56 agricultural catchments in France ranging from 4 to 370 km². Spatial variability of dissolved carbon, nitrogen, phosphorus, and major ions collapsed moving downstream, with variance thresholds occurring between 18 and 68 km² for most solutes, corresponding to the size of sink and source patches in the landscape. The smallest subcatchments expressed the greatest diversity of temporal variability regimes, but overall temporal variability did not decrease moving downstream for most solutes, partly due to synchronous variability among subcatchments. While land use was not strongly associated with spatiotemporal patterns of stream chemistry, the spatial structure of water quality among subcatchments was stable on annual and decadal timescales for nitrate and most solutes. These observations demonstrate that while subcatchment variability cannot be extrapolated among subcatchments, periodic sampling of headwaters provides valuable information about solute sources and inherent resilience of subcatchments, informing the optimal location for high-frequency monitoring and restoration interventions. If human activity were realigned based on this assessment of catchment vulnerability to disturbance, water quality could be improved while maintaining or increasing crop yields.