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

Abstract Nitrogen isotope ratios ( δ 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 μM) near the sewage outfall varied considerably between surveys, the sewage-derived δ 15 N-NH 4 + signature was remarkably constant (+ 10.6 ± 0.5‰) and could be tracked across the estuary. As indirectly supported by 15 N-depleted δ 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 ( δ 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 δ 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 δ 15 N-NO 3 − (ca. 0‰) 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 δ 15 N values was explained by simple end-member mixing models, implying very little estuarine processing.