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 f 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. © Inter-Research 2007.