Pelagic Nitrogen Cycle Observations In The Arctic Ocean – How Might They Change In Response To Ocean Acidification?

Phytoplankton forms the base of marine food webs by assimilating nutrients and generating biomass that supports higher trophic levels. Conversely, marine heterotrophs degrade organic matter produced by phytoplankton and recycle nutrients, maintaining food web integrity. We investigated the assimilation and regeneration of dissolved inorganic nitrogen (DIN) at stations located in the Arctic Ocean. In addition, we measured the concentration of nitrous oxide, a by-product of N-regeneration (specifically nitrification) and a climatically active gas. Measurements demonstrated the simultaneous regeneration and assimilation of ammonium, nitrite and nitrate at open ocean, ice-edge and within-ice locations. Ammonium was regenerated and assimilated within the range 0.2-4.5 nmol·L^-1·h^-1 and 0.5-24.8 nmol·L^-1·h^-1 respectively. Nitrite was regenerated and assimilated within the range 0.1-9.2 nmol·L^-1·h^-1 and 0.0-6.9 nmol·L^-1·h^-1 respectively. Nitrate was regenerated and assimilated within the range 0.3-372.7 nmol·L^-1·h^-1 and 0.1-48.3 nmol·L^-1·h^-1 respectively. Results indicated that the ice-edge was associated with enhanced DIN assimilation. The concentration of nitrous oxide (<100m) averaged 11.8±2.2 nmol·L^-1, which was approximately 15% higher than measured in the European shelf seas and most likely related only to temperature. In separate experiments, the influence of ocean acidification (OA) upon nitrogen cycle processes was investigated. The carbonate system of photic zone seawater (depth <10m) was modified to achieve a range of PCO₂ concentrations using bioassay experiments. Preliminary results indicated that NH₄⁺ oxidation and the concentration of nitrous oxide did not respond in a clear or consistent way to OA treatments. In contrast, the regeneration of NH₄⁺ increased in response to elevated PCO₂. The bacterial degradation of organic matter may be enhanced in the Arctic Ocean in response to OA, potentially modifying DIN pool composition and concentration in the future.