Off-shelf fluxes of labile materials by an upwelling filament in the NW Iberian Upwelling System

Daily changes in the concentrations of carbon and nitrogen species were monitored during the course of a Lagrangian drifter experiment in a recurrent upwelling filament south of Cape Finisterre (NW Iberian Upwelling System). A drifting buoy released at the southern edge of the upwelling centre generated by the Cape moved 60km southwestwards from 3 to 7 August 1998. Organic matter in the 50m deep study water mass (average 77±2 μM C) consisted of: 57μM C of dissolved organic matter (DOM) with a C/N molar ratio of 19±2; 6μM C of DOM with a C/N ratio of 9±2, and 14μM C of 50% DOM and 50% suspended organic matter (POMsusp) with a C/N ratio of 6.0±0.4. Net conversion of consumed inorganic salts into accumulated TOM=POMsusp+DOM was 40% for nitrogen and 30% for carbon. Since the parcel of water crossed the shelf-edge, these conversion efficiencies are equivalent to net horizontal export-ratio of 0.4 and 0.3 respectively. A second drifter was deployed in the offshore-end of the filament, and was displaced 20km west between 14 and 17 August 1998. Nitrate was exhausted in the surface water and no significant changes were observed in the variables measured during the course of the second experiment. Low C/N ratios (6.5±0.4) and rapid POMsusp/DOM inter-conversion in the 20 μM C excess observed in the study volume points to the persistence of the labile materials formed on the shelf during transport to the ocean. Our data demonstrate a) the key role of upwelling filaments in off-shelf export of organic materials and b) the major contribution of DOM to this horizontal export, a previously unaccounted amount. The high nitrogen content of the materials exported make them attractive organic substrates for use by microbial populations in the adjacent oligotrophic ocean.  2001 Elsevier Science Ltd. All rights reserved.