A time series of nitrogen speciation and nitrogen isotope fractionation during nitrification in a eutrophic coastal embayment

In aquatic environments, processes and conditions affecting distribution and isotopic compositions of nitrogen (N) compounds are complex and highly variable in both space and time. Verification studies that can be used to test understanding of mechanisms through comparison of both N species concentrations and N isotopic signals with microbial N sources and sinks are rare. A particularly effective approach is to exploit naturally occurring “experiments”, in which temporally changing physical and hydrochemical conditions drive changes in N cycling that can be followed through observation of temporal changes in N concentrations and isotopic ratios. Here, we present an annual cycle of physicochemical parameters and N isotopic data obtained in the frame of the Bedford Basin Time Series, a coastal monitoring program in the Northwestern Atlantic near Halifax, Canada.

During spring and early summer, high export production and remineralization of phytoplankton-derived organic matter resulted in increasing levels of particulate N and accumulation of ammonium (NH₄⁺) in the basin bottom waters (60m). Elevated bottom water nitrate (NO₃^-) concentrations as well as an increase in δ¹⁵N-NH₄⁺ and a corresponding decrease in δ¹⁵N-NO₃^- clearly indicated active NH₄⁺ and nitrite (NO₂^-) oxidation. In mid-summer, inflow of more saline Scotian Shelf water into the basin was observed and nitrifying activity markedly increased, likely driven by an increase in temperature and/or bottom water oxygen concentrations. Decreasing surface productivity in autumn was followed by a decline in subsurface NH₄⁺ concentrations and a complete oxidation of the NH₄⁺ pool to NO₃^-. The N isotopic compositions of NH₄⁺, NO₂^-, and NO₃^- followed a Rayleigh-type fractionation, with a fractionation factor for NH₄⁺ oxidation of ~15 ‰.