The influence of ocean acidification on nitrogen regeneration and nitrous oxide production in the northwest European shelf sea

The assimilation and regeneration of dissolved inorganic nitrogen, and the concentration of N₂O, was investigated at stations located in the NW European shelf sea during June/July 2011. These observational measurements within the photic zone demonstrated the simultaneous regeneration and assimilation of NH₄⁺, NO₂⁻ and NO₃⁻. NH₄⁺ was assimilated at 1.82–49.12 nmol N L⁻¹ h⁻¹ and regenerated at 3.46–14.60 nmol N L⁻¹ h⁻¹; NO₂^- was assimilated at 0–2.08 nmol N L⁻¹ h⁻¹ and regenerated at 0.01–1.85 nmol N L⁻¹ h⁻¹; NO₃⁻ was assimilated at 0.67–18.75 nmol N L⁻¹ h⁻¹ and regenerated at 0.05–28.97 nmol N L⁻¹ h⁻¹. Observations implied that these processes were closely coupled at the regional scale and that nitrogen recycling played an important role in sustaining phytoplankton growth during the summer. The [N₂O], measured in water column profiles, was 10.13 ± 1.11 nmol L⁻¹ and did not strongly diverge from atmospheric equilibrium indicating that sampled marine regions were neither a strong source nor sink of N₂O to the atmosphere. Multivariate analysis of data describing water column biogeochemistry and its links to N-cycling activity failed to explain the observed variance in rates of N-regeneration and N-assimilation, possibly due to the limited number of process rate observations. In the surface waters of five further stations, ocean acidification (OA) bioassay experiments were conducted to investigate the response of NH₄⁺ oxidising and regenerating organisms to simulated OA conditions, including the implications for [N₂O]. Multivariate analysis was undertaken which considered the complete bioassay data set of measured variables describing changes in N-regeneration rate, [N₂O] and the biogeochemical composition of seawater. While anticipating biogeochemical differences between locations, we aimed to test the hypothesis that the underlying mechanism through which pelagic N-regeneration responded to simulated OA conditions was independent of location. Our objective was to develop a mechanistic understanding of how NH₄⁺ regeneration, NH₄⁺ oxidation and N₂O production responded to OA. Results indicated that N-regeneration process responses to OA treatments were location specific; no mechanistic understanding of how N-regeneration processes respond to OA in the surface ocean of the NW European shelf sea could be developed.