Two Lagrangian drift experiments were carried out at the NW Iberian margin. The first tracked a body of nutrientrich, upwelled water as it moved south along the shelf break over a 5 day period. The second experiment, of similar duration, followed a water mass as it moved into the deep ocean in an off-shelf filament. This paper describes the background to and aims of each experiment. The overall objective was to quantify chemical and biological processes relating to the additional potential for the ocean at the shelf margins to sequester atmospheric CO2 in upwelling regions. The first experiment began at a time of intense wind-driven upwelling; within 2 days, the wind speed had moderated and the system entered a relaxation period with greatly reduced upwelling. The patch of upwelled water was marked by a single buoy array and it moved south along the shelf break. Transport was initially rapid but slowed with reducing wind speed. The temperature–salinity characteristics were consistent with sampling only a single water mass throughout the experiment. A model of particle trajectories showed slight deviation from the actual movement of the marked water mass, but overall the data support the assumption that the experiment was Lagrangian. During a 5 day experimental period, nutrients were utilised with a N:P ratio of 18.3 and N:Si of 4. Nutrient concentrations first reduced in the nearsurface but depletion deepened in the water column during the experiment. At the beginning of the experiment, the highest chlorophyll concentrations were in the surface 15m but this was replaced by a subsurface chlorophyll maximum at 30m. There was a shift from a small flagellate and dinoflagellate dominated photosynthetic phytoplankton assemblage to a diatom dominated assemblage. A high biomass of heterotrophic dinoflagellates and ciliates was also present. Canonical correlation analysis between environmental variables and microplankton assemblages, as defined by principal component analysis, suggested that a considerable part of DON production resulted from trophic relationships rather than direct release from phytoplankton. The second experiment followed a water mass marked with 5 Argos drifting buoys for 5 days as the water drifted off shelf in an off-shore filament. This water mass was extremely oligotrophic; nitrate concentrations were typically <10nmol l-1 in the upper 20m and ammonium concentrations were 20–40nmol l-1. Chlorophyll concentrations were very low and the phytoplankton assemblage was dominated by picoplankton. Diatoms were largely absent from the nano- and microplankton fractions, which were dominated by dinoflagellates. The data presented in this paper are a general description of the experiments and form the background to the more detailed descriptions given in the individual papers that make up this Special Issue of Progress in Oceanography

This work was supported by the European Union in the framework of the Mast 3 Programme, contract number MAS3-CT97-0076. (OMEX II project), by the DYME Project of the CCMS Plymouth Marine Laboratory, a component of the Natural Environment Research Council and by the Spanish Comisión Interministerial de Ciencia y Tecnología grant No MAR97-1729-CE and MAR1997-2068-CE

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