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

Nitrogen isotope ratios (δ15N) 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 μM) near the sewage outfall varied considerably between surveys, the sewage-derived δ15N-NH4+ signature was remarkably constant (+ 10.6 ± 0.5‰) and could be tracked across the estuary. As indirectly supported by 15N-depleted δ15N-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 μM) compared to those in the Tyne and δ15N-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 δ15N-NO3− values was mainly attributed to enhanced watershed nutrient processing. In the Tyne, where agricultural inputs are less important compared to the Tweed, light δ15N-NO3− (ca. 0‰) 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 δ15N values was explained by simple end-member mixing models, implying very little estuarine processing.