title: Evaluating the sources and fate of anthropogenic dissolved inorganic nitrogen (DIN) in two contrasting North Sea estuaries.
abstract: Nitrogen isotope ratios (delta(15)N) were used to help elucidate the sources and fate of ammonium (NH(4)(+)) and nitrate (NO(3)(-)) in two northeastern English estuaries. The dominant feature of NH(4)(+) in the heavily urbanised Tyne estuary was a plume arising from a single point source; a large sewage works. Although NH(4)(+) concentrations (ranging from 30-150 microM) near the sewage outfall varied considerably between surveys, the sewage-derived delta(15)N-NH(4)(+) signature was remarkably constant (+10.6+/-0.5 per thousand) and could be tracked across the estuary. As indirectly supported by (15)N-depleted delta(15)N-NO(3)(-) values observed close to the mouth of the Tyne, this sewage-derived NH(4)(+) was thought to initiate lower estuarine and coastal zone nitrification. In the more rural Tweed, NH(4)(+) concentrations were low (<7 microM) compared to those in the Tyne and delta(15)N-NH(4)(+) values were consistent with mixing between riverine and marine sources. The dominant form of dissolved inorganic nitrogen (DIN) in the Tweed was agricultural soil-derived NO(3)(-). A decrease in riverine NO(3)(-) flux during the summer coinciding with an increase in delta(15)N-NO(3)(-) values was mainly attributed to enhanced watershed nutrient processing. In the Tyne, where agricultural inputs are less important compared to the Tweed, light delta(15)N-NO(3)(-) (ca. 0 per thousand) detected in the estuary during one winter survey pointed to a larger contribution from precipitation-derived NO(3)(-) during high river discharge. Regardless of the dominant sources, in both estuaries most of the variability in DIN concentrations and delta(15)N values was explained by simple end-member mixing models, implying very little estuarine processing.
authors.affiliation: School of GeoSciences, University of Edinburgh, Edinburgh, EH9 3JW, UK. j.ahad@qub.ac.uk
authors.forename: Jason M E
authors.initials: JM
authors.lastname: Ahad
authors.forename: Raja S
authors.initials: RS
authors.lastname: Ganeshram
authors.forename: Robert G M
authors.initials: RG
authors.lastname: Spencer
authors.forename: Günther
authors.initials: G
authors.lastname: Uher
authors.forename: Robert C
authors.initials: RC
authors.lastname: Upstill-Goddard
authors.forename: Greg L
authors.initials: GL
authors.lastname: Cowie
mesh_terms: D000383:Agriculture; D004784:Environmental Monitoring; D009566:Nitrates / Q000032:analysis; D009584:Nitrogen / Q000032:analysis*; D009587:Nitrogen Isotopes; D009659:North Sea; D000644:Quaternary Ammonium Compounds / Q000032:analysis; D045483:Rivers; D006113:United Kingdom; D014865:Waste Disposal, Fluid; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D009566:Nitrates; D009587:Nitrogen Isotopes; D000644:Quaternary Ammonium Compounds; D014874:Water Pollutants, Chemical; D009584:Nitrogen