Sources and fluxes of carbon and nutrients in human-impacted watersheds and estuaries

Urbanization alters watershed hydrology and has increased nutrient pollution, leading to eutrophication and hypoxia in downstream coastal ecosystems. In order to manage increased nutrient exports and loss of biogeochemical functions in urban streams and rivers it is important to better understand how management of both point and diffuse sources affects carbon and nutrients under various flow conditions. The objective of this research was to investigate the impacts of urban development on sources, transformations, and fluxes of carbon (C) and nutrients along urban headwater streams and a large coastal estuary. Chapter 1 covers the impact of headwater stream burial on nitrate uptake and stream metabolism. Chapter 2 focuses on the impact of age and extent of urban infrastructure and watershed management on hydrologic variability and pulsing of C and nutrient sources and fluxes. Chapter 3 examines the capacity of a large river-estuary system to transform and remove inputs of nitrogen (N) and C from a major metropolitan area and the world's largest advanced wastewater treatment plant. Results from Chapter 1 indicate that burial of headwater streams significantly reduces nitrate uptake, gross primary productivity, and ecosystem respiration, compared to non-buried streams, by factors of 9.6, 11, and 5, respectively. It was also estimated that at the watershed scale, stream burial decreases N uptake by 39 ? 5% and C production by 105 ? 103%. Results from Chapter 2 indicate that urbanized streams with higher percent impervious surface cover and older infrastructure have greater inputs of N, C, and phosphorus (P) from leaky sanitary sewers and flashier hydrology, resulting in higher concentrations and more variable and pulsed exports of C, N, and P. Results from Chapter 3 indicate that large river-estuary systems have the potential to transform and remove N loads from point-source wastewater treatment plant inputs over relatively short spatial scales. There were seasonal patterns in N removal in the Potomac River Estuary, with greater denitrification and uptake rates in the summer and fall, accompanied by lower net exports, but during the spring when flows were highest, exports were also highest. Overall, the research in this dissertation indicates that urbanization reduces nutrient retention through stream burial and increases the amount and variability of C and nutrient exports in headwater streams, while river-estuarine systems have a large capacity to assimilate nutrient inputs from major point sources