The controls of watershed nutrient export

Anthropogenic nitrogen (N) loading has increased considerably in recent times, yet only a small proportion is generally exported to the coast from most watersheds. This dissertation addresses some of the factors controlling N export from watersheds. The focus is on (1) how N export and N retention change with increased urbanization, and (2) how river networks modify N that is loaded to them. Urbanization is a major perturbation of the land surface. Chapter 1 uses a budget approach to assess how N retention changes with increased urbanization in small headwater catchments in northeastern Massachusetts, USA. Water runoff, N loading, and N exports were higher in an urban compared to a forested site. N exports increased at a faster rate than N loading, indicating that the capacity of urban catchments to retain nutrients has declined. Impervious surfaces are likely a major factor leading to this decline. Aquatic systems can store or denitrify a large amount of N, and can therefore potentially buffer increased N loading from terrestrial systems (non-point sources) and point sources. Chapter 2 uses a modeling approach to assess the role of river networks at the global scale. The strength of aquatic N removal varied considerably by watershed, depending on the spatial distribution of N loading, hydraulic characteristics, and biological activity. The representation of biological activity strongly influenced predicted N removal of aquatic systems, pointing out the need to better understand biological controls in different regions of the world. A great deal of attention has recently been given to the role of river networks in modifying nutrient exports. Chapter 3 is an exploration of the biological and hydrological controls of nutrient removal at the scale of river networks. In particular, the role of stream size and nutrient concentrations are emphasized. The analysis suggests that river network models should clearly separate the biological and hydrological parameters to (1) facilitate understanding of model behavior, (2) facilitate comparability with field measurements, (3) improve the ability to apply/test models across temporal and spatial domains, and (4) improve the ability to explore the relative influences hydrological and biological controls of river network nutrient removal