MEPS 337:79-92 (2007)  -  doi:10.3354/meps337079

ABSTRACT: Nitrate and ammonium assimilation, nitrification and ammonium regeneration were measured during field experiments in the NE Atlantic (39 to 44.5°N, 16 to 21°W; Programme Océan Multidisciplinaire Méso Echelle) in 2001. NH₄⁺ and NO₂^– concentrations were high during winter and spring, reaching maximum values of 0.3 and 0.6 µM, respectively, in the top 100 m. ¹⁵N tracer experiments allowed the measurement of nitrification throughout the water column, and rates in surface waters varied between 0.02 µM d^–1 in winter and 0.077 µM d^–1 in spring. At 30 m depth, rates could exceed 0.1 µM d^–1 during the spring bloom. For the winter season, nitrification could account for 40% of the assimilated nitrate. During spring, nitrification could support more than 20% of nitrate assimilation. Ammonium regeneration was significant during both seasons, with average rates exceeding 0.5 µM d^–1. Correction for NH₄⁺ isotopic dilution could increase estimations of regenerated production by 40%. Also, a new evaluation of the ƒ ratio (including nitrification and ammonium isotopic dilution) showed a 10% overestimation of classic estimations of export production. Additionally, active NH₄⁺ uptake by <0.7 µm particles (mainly bacteria) was observed during late spring, suggesting that a significant fraction of small particles was not retained by GF/F filters during the spring bloom, which could lead to underestimations of NH₄⁺ uptake.

KEY WORDS: Nitrification · Ammonium regeneration · NE Atlantic

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