Global N removal by freshwater aquatic systems using a spatially distributed, within‐basin approach

We explored the role of aquatic systems in the global N cycle using a spatially distributed, within‐basin, aquatic nitrogen (N) removal model, implemented within the Framework for Aquatic Modeling in the Earth System (FrAMES‐N). The model predicts mean annual total N (TN) removal by small rivers (with drainage areas from 2.6–1000 km 2 ), large rivers, lakes, and reservoirs, using a 30′ latitude × longitude river network to route and process material from continental source areas to the coastal zone. Mean annual aquatic TN removal (for the mid‐1990s time period) is determined by the distributions of aquatic TN inputs, mean annual hydrological characteristics, and biological activity. Model‐predicted TN concentrations at basin mouths corresponded well with observations (median relative error = −12%, interquartile range of relative error = 85%), an improvement over assumptions of uniform aquatic removal across basins. Removal by aquatic systems globally accounted for 14% of total N inputs to continental surfaces, but represented 53% of inputs to aquatic systems. Integrated aquatic removal was similar in small rivers (16.5% of inputs), large rivers (13.6%), and lakes (15.2%), while large reservoirs were less important (5.2%). Bias related to runoff suggests improvements are needed in nonpoint N input estimates and/or aquatic biological activity. The within‐basin approach represented by FrAMES‐N will improve understanding of the freshwater nutrient flux response to anthropogenic change at global scales.