title: Decadal-scale export of nitrogen, phosphorus, and sediment from the Susquehanna River basin, USA: Analysis and synthesis of temporal and spatial patterns.
abstract: The export of nitrogen (N), phosphorus (P), and suspended sediment (SS) is a long-standing management concern for the Chesapeake Bay watershed, USA. Here we present a comprehensive evaluation of nutrient and sediment loads over the last three decades at multiple locations in the Susquehanna River basin (SRB), Chesapeake's largest tributary watershed. Sediment and nutrient riverine loadings, including both dissolved and particulate fractions, have generally declined at all sites upstream of Conowingo Dam (non-tidal SRB outlet). Period-of-record declines in riverine yield are generally smaller than those in source input, suggesting the possibility of legacy contributions. Consistent with other watershed studies, these results reinforce the importance of considering lag time between the implementation of management actions and achievement of river quality improvement. Whereas flow-normalized loadings for particulate species have increased recently below Conowingo Reservoir, those for upstream sites have declined, thus substantiating conclusions from prior studies about decreased reservoir trapping efficiency. In regard to streamflow effects, statistically significant log-linear relationships between annual streamflow and annual constituent load suggest the dominance of hydrological control on the inter-annual variability of constituent export. Concentration-discharge relationships revealed general chemostasis and mobilization effects for dissolved and particulate species, respectively, both suggesting transport-limitation conditions. In addition to affecting annual export rates, streamflow has also modulated the relative importance of dissolved and particulate fractions, as reflected by its negative correlations with dissolved P/total P, dissolved N/total N, particulate P/SS, and total N/total P ratios. For land-use effects, period-of-record median annual yields of N, P, and SS all correlate positively with the area fraction of non-forested land but negatively with that of forested land under all hydrological conditions. Overall, this work has informed understanding with respect to four major factors affecting constituent export (i.e., source input, reservoir modulation, streamflow, and land use) and demonstrated the value of long-term river monitoring.
authors.affiliation: Johns Hopkins University, Department of Geography and Environmental Engineering, 3400 North Charles Street, Baltimore, MD 21218, USA. Electronic address: qzhang19@jhu.edu.
authors.forename: Qian
authors.initials: Q
authors.lastname: Zhang
authors.affiliation: Johns Hopkins University, Department of Geography and Environmental Engineering, 3400 North Charles Street, Baltimore, MD 21218, USA; Chesapeake Research Consortium, 645 Contees Wharf Road, Edgewater, MD 21037, USA. Electronic address: bball@jhu.edu.
authors.forename: William P
authors.initials: WP
authors.lastname: Ball
authors.affiliation: U.S. Geological Survey, Virginia Water Science Center, 1730 East Parham Road, Richmond, VA 23228, USA. Electronic address: dlmoyer@usgs.gov.
authors.forename: Douglas L
authors.initials: DL
authors.lastname: Moyer
mesh_terms: D004784:Environmental Monitoring; D019015:Geologic Sediments / Q000737:chemistry*; D062070:Hydrology; D008396:Maryland; D009584:Nitrogen / Q000032:analysis*; D010414:Pennsylvania; D010758:Phosphorus / Q000032:analysis*; D045483:Rivers / Q000737:chemistry*; D012621:Seasons; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D014874:Water Pollutants, Chemical; D010758:Phosphorus; D009584:Nitrogen
keywords: Chemostasis; Chesapeake bay; Lag time; Land use; Legacy source; Reservoir modulation; Streamflow