Inorganic nitrogen assimilation and regeneration in the coastal upwelling region of the Iberian Peninsula

Darren R. Clark, Peter I. Miller, E. Malcolm, S. Woodward and Andrew P. Rees

Limnol. Oceanogr., 56(5), 2011, 1689-1702 | DOI: 10.4319/lo.2011.56.5.1689

ABSTRACT: N-regeneration and N-assimilation rates were investigated in surface waters of the Iberian Peninsula, northeast Atlantic, during a period of seasonal upwelling. Stations were selected on the basis of near–real-time remotely sensed data to provide a contrast between upwelling and oligotrophic conditions. Inorganic nitrogen (NO⁻₃ and NH⁺₄) was simultaneously regenerated and assimilated in the short term (i.e., hours), highlighting the importance of N-regeneration in sustaining productivity. The regeneration of both NH⁺₄ and NO⁻₃ would introduce error in N-assimilation rate estimations using ¹⁵N techniques, if not accounted for. The rate of ammonium assimilation (ρNH⁺₄) frequently exceeded the rate of NO⁻₃ assimilation (ρNO⁻₃). Although the assimilation of NO⁻₃ and NH⁺₄ continued at night, dark NO⁻₃ assimilation was lower than during the day. The separate oxidation steps of the nitrification process occurred at similar rates at most stations, and NO⁻₂ oxidation made a significant, but variable, contribution to hourly phytoplankton NO⁻₃ uptake. NO⁻₂ accumulated in near-surface waters of stations experiencing upwelling, suggesting an imbalance between production and consumption. At these stations, NO⁻₂ release by phytoplankton may have stimulated NO⁻₂ oxidation, uncoupling it from NH⁺₄ oxidation. ¹⁵N-based f-ratio as an estimate of exportable production must be corrected for isotope dilution, and this requires information on NH⁺₄ regeneration and nitrification rate; in addition, the assumed close coupling between the separate oxidation steps of nitrification must be verified. These corrections may be especially pertinent to upwelling regions, which represent an important route for new-N input to the surface ocean.