Factors controlling aquatic dissolved inorganic nitrogen removal and export in suburban watersheds

Human activity has accelerated the nitrogen (N) cycle and enriched the landscape with N which can result in eutrophication, especially in coastal zones where N is typically limiting. N exported to coastal zones is a function of both N loading to aquatic systems and N removal in transit through the river network. To determine drivers of dissolved inorganic nitrogen (DIN) removal and export from suburban river networks, we compared 2 well-studied suburban New-England watersheds. The Lamprey River watershed (474 km²) in NH has a mean population density of 53 inhabitants per km² and feeds into the Great Bay estuary which is designated as N impaired. The Ipswich River (400 km²) in MA has a much higher population density with 302 inhabitants per km² and feeds into the Plum Island estuary, which is not N impaired. Median (2000 - 2009) watershed DIN export was 171 kg km^-2 y^-1 for the Ipswich and 77 kg km^-2 y^-1 for the Lamprey. We used the Framework for Aquatic Modeling in the Earth System (FrAMES) to evaluate the relative importance of anthropogenic N loading and river network DIN processing in determining N export from these watersheds. FrAMES is a spatially distributed and time varying coupled hydrologic and biogeochemical model for river networks. We hypothesized that greater N export relative to population density in the Lamprey watershed was due in part to less aquatic N processing caused by interactions among: 1. The distribution of development/sources in the watershed (i.e. mean flow path length N has to travel), and 2. The area and distribution of intact fluvial wetlands in the watershed. We conducted a sensitivity analysis to determine the relative importance of these factors in limiting aquatic N removal in the Lamprey river watershed. Our results suggest that the distribution of loading within a river system has important influence on nutrient export to coastal zones.