Limited progress in nutrient pollution in the U.S. caused by spatially persistent nutrient sources
Human agriculture, wastewater, and use of fossil fuels have saturated ecosystems with nitrogen and phosphorus, threatening biodiversity and human water security at global scales. Here, we applied a new ecohydrological framework to ~10,000 water samples from streams and lakes across the contiguous United States collected by the U.S. Environmental Protection Agency. We quantified temporal trends in nutrient concentrations, the persistence of spatial patterns through time, and the size of nutrient sources and sinks. Despite efforts to reduce nutrient pollution, nutrient and carbon concentrations stayed the same or increased between 2000 and 2012, except for NO3- in lakes, which decreased. The spatial patterns of nutrient and carbon concentrations were persistent across and within ecoregions. Watersheds showed strong critical source area dynamics, with 2–8% of the land area accounting for 75% of the observed flux. Variability in nutrient contribution was greatest in catchments smaller than 250 km2 for most parameters. An ensemble of 4 machine learning analyses revealed that nutrient concentrations were strongly associated with land use and land cover, demonstrating how human activity and inherent nutrient resilience interactively determine nutrient state. Together, these results suggest that targeted nutrient interventions in a small portion of the landscape could substantially improve water quality at continental scales.