Dissolved organic matter in shelf waters off the Ria de Vigo (NW Iberian upwelling system)

Abstract

Net in situ production and export of dissolved organic carbon DOC.and nitrogen DON.have been studied in shelf waters off the R´ıa de Vigo NW Spain., as part of a comprehensive hydrographic survey carried out from September 1994 to September 1995 with a fortnight periodicity. DOC and DON correlated well rsq0.78., the slope of the regression line being 12.0"0.7 mol-C mol-Ny1, about twice the Redfieldian slope of particulate organic matter, 6.5"0.2 mol-C mol-Ny1 rsq0.95.. Labile DOC and DON accumulated in the upper 50 m during the upwelling season March–September., mainly after prolonged periods of wind relaxation, when horizontal flows were reduced. This labile material represented 50% and 35% of the total dissolvedqparticulate.organic carbon and nitrogen susceptible of microbial utilisation, which assert the key contribution of dissolved organic matter DOM.to the export of new primary production in the NW Iberian upwelling system. This surface excess in shelf waters appeared to be formed into the highly productive R´ıa de Vigo a large coastal indentation. at net rates of ;4.4 mM-C dy1 and ;1.3 mM-C dy1 in the inner and outer segments of the embayment respectively, and subsequently exported to the shelf. Once in the shelf, simple dilution with the inert DOM pool of recently upwelled Eastern North Atlantic Central Water ENACW.occurred. Eventually, the DOM excess produced during the upwelling season is exported to the adjacent open ocean waters by the coastal circulation. Conversely, during the unproductive downwelling season October–February., the lowest DOC and DON levels were recorded and export was prevented by the characteristic downwelling front associated to the seasonal poleward slope current.