A seasonal study of diatom dynamics in the North Atlantic during the POMME experiment (2001): Evidence for Si limitation of the spring bloom

International audience; The Si cycle in the northeast Atlantic (40°–43.5°N, 15°–21°W) was investigated in the winter, spring, and late summer during the Programme Océan Multidisciplinaire Méso Echelle (POMME) (2001). The aim of this study was to determine the principal causes of the onset and subsequent decline of the diatom bloom, with an emphasis on nutritional limitation processes. The siliceous compartment dynamics was characterized through silicic acid distribution, size-fractionated biogenic silica and Si uptake rates, Si uptake kinetics (K S and V max), and export rates. The results of the POMME cruises indicated an important seasonal variability of the siliceous compartment in the North Atlantic together with a spatial variability following a south-north gradient in parallel to the increasing stock of nutrients. Here we present the first kinetic evidences for Si limitation of the diatom spring bloom in the northeast Atlantic, supporting previous reports of potentially limiting silicic acid concentrations in this region. Integrated Si uptake rates were very low throughout the survey, except at the northern anticyclonic eddy site, where a Pseudo-nitzschia bloom was observed during spring and ranged between 0.04 and 11.2 mmol m−2 d−1, which is comparable to values obtained in oligotrophic regions. The overall annual Si production budget for the entire POMME area (375,000 km2) was extrapolated to 9.8 × 109 mol Si yr−1, while Si export fluxes at 400 m only represented 3% of the surface production.