Evaluating the sources and fate of anthropogenic dissolved inorganic nitrogen (DIN) in two contrasting North Sea estuaries

Nitrogen isotope ratios (delta N-15) were used to help elucidate the sources and fate of ammonium (NH4+) and nitrate (NO3-) in two northeastern English estuaries. The dominant feature of NH4+ in the heavily urbanised Tyne estuary was a plume arising from a single point source; a large sewage works. Although NH4+ concentrations (ranging from 30-150 mu M) near the sewage outfall varied considerably between surveys, the sewage-derived delta N-15-NH4+ signature was remarkably constant (+10.6 +/- 0.5 parts per thousand) and could be tracked across the estuary. As indirectly supported by N-15-depleted delta N-15-NO3- values observed close to the mouth of the Tyne, this sewage-derived NH4+ was thought to initiate lower estuarine and coastal zone nitrification. In the more rural Tweed, NH4+ concentrations were low (< 7 mu M) compared to those in the Tyne and delta N-15-NH4+ 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 NO3-. A decrease in riverine NO3- flux during the summer coinciding with an increase in delta N-15-NO3- values was mainly attributed to enhanced watershed nutrient processing. In the Tyne, where agricultural inputs are less important compared to the Tweed, light delta N-15-NO3- (ca. 0 parts per thousand) detected in the estuary during one winter survey pointed to a larger contribution from precipitation-derived NO3- during high river discharge. Regardless of the dominant sources, in both estuaries most of the variability in DIN concentrations and delta N-15 values was explained by simple end-member mixing models, implying very little estuarine processing. (c) 2006 Elsevier B.V. All rights reserved.