COUPLED HYDROLOGICAL, BIOGEOCHEMICAL, AND GEOMORPHIC PROCESSES AFFECTING NUTRIENT LOADINGS IN FLUVIAL NETWORKS

Inputs of reactive nitrogen (all N species except for N₂) to landscapes have been increasing worldwide, largely due to human activities associated with food production and energy consumption via the combustion of fossil fuels and biofuels. Despite the obvious essential benefits of a plentiful supply of food and energy, the adverse consequences associated with the accumulation of N in the environment are large. Most of the N created by human activities is released to the environment, often with unintended negative consequences. The greater the inputs of N, the greater the potential for negative effects -- caused by greenhouse gas production, ground level ozone, acid deposition, and N overload; which in turn can contribute to climate change, degradation of soils and vegetation, acidification of surface waters, coastal eutrophication, hypoxia, habitat loss, and loss of stratospheric ozone. Here we present a contemporary inventory of reactive N inputs to major water regions in the United States, and discuss systematic accounting methods for quantifying N sources and transport. Furthermore, we quantify loadings of N from terrestrial headwaters downstream to coastal estuaries and embayments, as affected by river hydrogeomorphology and its cumulative influence on water quality. N delivery to downstream waters is influenced by nutrient sources as well as coupled hydrological and biogeochemical processes occurring along the river corridor (e.g., travel time distributions, denitrification, and storage) that scale with stream size and are affected by impoundments such as lakes and reservoirs. This underscores the need to account for the nonlinear interactions of aquatic transport processes with watershed nutrient sources in developing efficient nutrient reduction strategies. Such work provides a scientific perspective from which to understand drivers of nutrient pollution problems and to explore potential intervention strategies and restoration tactics to mitigate nutrient pollution problems. Our results stem from the EPA Integrated Nitrogen Advisory Committee, the EPA Center for Integrated Multi-Scale Nutrient Pollution Solutions, and the John Wesley Powell Center River Corridor Working Group.