Defining nutrient ecoregions for reference nitrogen and phosphorus concentrations in rivers from the major South American biomes

Abstract Knowing reference (i.e., baseline) riverine nutrient concentrations is essential to understand fundamental processes of biogeochemical transport from continents to the ocean, describing ecological conditions, and informing managers of best attainable conditions for nutrient abatement and controlling anthropogenic eutrophication. We used data from 434 Brazilian watersheds representative of six South American biomes to estimate expected nutrient levels in the absence of anthropogenic influence. We used a novel watershed-based approach to describe spatial patterns throughout Brazil and for the entire Amazon basin. We found reference total nitrogen concentrations were most closely related to biome, whereas total phosphorus levels were to soil properties in addition to climatic factors influencing biomes. We found that many areas have intrinsically high background phosphorus and relatively low nitrogen, suggesting nitrogen-limitation of freshwaters could be widespread in South America, and nitrogen-fixing cyanobacterial blooms could have a competitive advantage. These results represent the first broad-scale analyses of spatial distribution of baseline nutrient levels for tropical and subtropical waters.