Urban and agricultural development increases surface water nutrient concentrations. In general, as developed area increases so do nutrient concentrations; however, the nature of these relationships differ substantially among studies. We hypothesize regional landscape and physiographic characteristics may alter how increasing development intensity changes nutrient concentrations. To test this, we 1) modelled 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 watersheds across 76 ecoregions in the conterminous U.S, then 2) used Hierarchical Linear Models (HLM) to determine if relationships differed across ecoregions and regional landscape and biophysical factors explained those differences.
Results/Conclusions
At the U.S. scale, the relationship between nutrient concentrations and percent urban and agricultural land cover was positive, but highly variable. At the ecoregion scale, relationships between land cover and nutrient concentrations differed and were often non-linear, ranging from sublinear (β ≈ 0) to linear (β ≈ 1). We found that regional agriculture development and precipitation moderated how concentrations of some nutrient increased with local agricultural development. Regional topography altered relationships between urban land cover and nutrient concentrations, where concentrations increased faster with development in ecoregions with flatter landscapes. These cross-scale interactions indicate certain watersheds are more sensitive to development, where nutrient concentrations in regions with greater agriculture development, wetter climates, or flatter lands continually increase and require different mitigation strategies.