Nitrogen regeneration in the NE Atlantic Ocean and its impact on seasonal new, regenerated and export production
Fernandez, C., I.; Raimbault, P.
Marine Ecology Progress Series 337: 79-92
ISSN/ISBN: 0171-8630 Accession: 016489622
Nitrate and ammonium assimilation, nitrification and ammonium regeneration were measured during field experiments in the NE Atlantic (39 to 44.5 degrees N, 16 to 21 degrees W; Programme Ocean Multidisciplinaire Meso Echelle) in 2001. NH4+ and NO2- concentrations were high during winter and spring, reaching maximum values of 0.3 and 0.6 mu M, respectively, in the top 100 m. N-15 tracer experiments allowed the measurement of nitrification throughout the water column, and rates in surface waters varied between 0.02 mu M d(-1) in winter and 0.077 mu M d(-1) in spring. At 30 m depth, rates could exceed 0.1 mu M V 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 mu M 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 mu m 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.