Global transport of nitrogen (N), carbon (C), and phosphorus (P) in river ecosystems has been dramatically altered due to urban population growth and wastewater.  We examined the capacity of a major tributary of the Chesapeake Bay, the Potomac River, to transform carbon, nitrogen, and phosphorus inputs from the world’s largest advanced wastewater treatment facility.  Historical effluent data on C, N, and P loadings were compared with river-flow data and long-term interannual records of carbon, nitrogen, and phosphorus levels within 11 downstream stations along the Potomac River.  Stoichiometric ratios of C, N, and P were determined across sites and related to changes in climatic conditions and management of effluent loadings.  In addition, surface water samples were collected seasonally along targeted longitudinal transects of the Potomac River.  Samples were analyzed for major dissolved and particulate forms of carbon, nitrogen, and phosphorus.  The source and quality of organic matter was characterized using fluorescence spectroscopy, excitation emission matrices (EEMs), and PARAFAC modeling.  Sources of nitrate were tracked using stable isotopes of N and O.  Information on sources, land use, and historical water chemistry was also compared to assess the relative importance of nonpoint sources from land-use change versus point sources of carbon and nutrients.
Results/Conclusions

Improvements in wastewater treatment technology showed significant long-term decreases in effluent nitrogen levels over time for Washington D.C., with corresponding long-term decreases in nutrients at downstream sampling stations.  Preliminary data from EEMs suggested that more humic-like organic matter is important above the wastewater treatment plant, but more protein-like organic matter is present below the treatment plant.  Levels of nitrate and ammonia decreased rapidly downstream of the wastewater treatment plant, potentially indicating nutrient uptake and/or denitrification.  Stoichiometric ratios along the river indicate longitudinal increases in C/N ratios downstream, but no trend with C/P ratios.  Despite large inputs of nutrients and organic matter, there can be significant variations in sources, transformations, and ecological stoichiometry along large rivers, and knowledge of how urban wastewater management and climate change impacts these transformations will be critical in predicting downstream changes in coastal hypoxia.