Biogeochemical factors that influence the stable nitrogen isotope ratio of dissolved ammonium in the Delaware Estuary

Abstract The isotopic composition ( δ 15 N ) of dissolved ammonium (NH 4 + ) in the Delaware Estuary was related to reactions in the nitrogen cycle occurring in different regions of the estuary and at different rates throughout the year. The range of values at any one location (as great as +10 to +40%.) was dependent on either nitrification, algal uptake, and microbial remineralization, or on a combination of these reactions. Specifically, observations of isotopic discrimination during nitrification in the riverine portion of the estuary were similar to those reported in other estuaries. In addition, the first calculation of the isotopic fractionation during algal uptake in the field is reported. Algal assimilation of NH 4 + in the estuary had an estimated fractionation factor (ϵ) of −9.1%. This estimated ϵ for the field data and fractionation factors measured in culture ( − 14 to − 20%.) were compared in a numerical simulation of NH 4 + transport and uptake in the estuary. Model results for the period of the spring bloom resembled the field data more closely when the isotopic fractionation estimated with the in situ data was used rather than greater isotopic fractionations measured in culture.