Cross-scale interactions and non-linearity of stream nutrient concentrations with urban and agricultural development across the United States.

Concentrations of nitrogen and phosphorus are key to aquatic ecosystem function and services, where high concentrations in the streams and rivers of watersheds with urban and agricultural development frequently degrade local and downstream receiving waters. Almost ubiquitously, development increases stream nutrient concentrations; however, relationships between land cover and concentrations differ substantially among studies. We hypothesize that regional physiographic characteristics (climate and topography) mediate the relationship between development and nutrient concentrations. We tested this by characterizing the relationships between urban and agricultural land cover intensity and mean annual concentrations of phosphorus (P), total nitrogen (TN), ammonia (NH₃) and nitrate (NO₃) for 1047 streams across 76 ecoregions in the conterminous U.S. We used Hierarchical Linear Models (HLM) to determine if relationships between nutrients and land cover differs across ecoregions and whether there are cross-scale interactions with regional biophysical factors.

Across the entire U.S, we found no clear relationship between urban and agricultural land cover and nutrient concentrations. At the ecoregion scale, relationships between urban and agricultural land cover and nutrient concentrations differed and were often non-linear. Climate and topography drove cross-scale interactions altering relationships between P concentrations and both urban and agricultural development, where P concentrations increased more rapidly with development in wetter climates and flatter landscapes. Relationships between nitrogen species and land cover were only affected by topography, where concentrations increased faster with development in flatter landscapes. These cross-scale interactions indicate that resilience of watersheds to mediate development disturbances depends on topography and climate, where watersheds in wetter, flatter lands are more sensitive and require different land development strategies to lower watershed nutrient concentrations.