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