Abstract
Reference conditions pertain to conditions without anthropogenic influence and serve to gauge the degree of river pollution and identify the best attainable water quality. Here we show estimates of the global human footprint of nitrogen and phosphorus concentrations and potential for related nuisance or harmful algal growth in rivers. We use statistical models based on 1.2 million stream nutrient measurements (from 2005 to 2013) and find global human enrichment of river total nitrogen and total phosphorus is 35% and 14% respectively. The greatest enrichment is in Europe (86 and 30% respectively) and the least in Oceania (9 and 2% respectively). The levels of enrichment translated into an almost doubling of the catchment areas with rivers predicted to have anthropogenically elevated levels of potentially harmful or nuisance algae, affecting ~40% of the world's population. Focusing management on the difference between current and reference conditions can help protect good water quality while avoiding unrealistic goals where nitrogen and phosphorus are naturally high.