NUTRIENT MINERALIZATION PATTERNS IN SHELF WATERS OF THE WESTERN IBERIAN UPWELLING

A set of hydrographic data collected during several cruises to the NW Iberian upwelling system has been used to assess the patterns of nutri ent mineralization over the shelf. Mineralized nutrient ratios (N/P an d N/Si) in the lower water column are clearly below the ratios charact eristic of freshly upwelled oceanic waters. Nitrogen mineralization: i n comparison to phosphorus mineralization, is incomplete, in agreement with phosphate's faster regeneration rate and the low residence time of water, which precludes complete mineralization. Silicate shows the highest enrichment as a consequence of the selective concentration of particulate biogenic silica in the near bottom waters. Diffusion of nu trients mineralized in the sediments seems to represent an important c ontribution. The lowest mineralization of the surveyed area is found t o the north of Cape Finisterre, especially with regard to silicate. To the South, the Rias Baixas (four large embayments) return as reducing particulate organic matter (POM) a small fraction of the upwelled nut rients imported from the shelf. However, the out-flowing freshwater co ntributes to stratification of coastal waters, which favours POM produ ction, accumulation and sedimentation. In addition, high levels of new dissolved organic matter (DOM) in the outflow can also enhance primar y production in shelf surface waters. Finally, highest mineralization was found to the south of the River Mino, where there are no embayment s and the shelf is wider. Mineralization tends to: (1) increase the po tential primary production of this ecosystem by up to 50%, (nitrogen l imitation) or up to 80% (silicon limitation); (2) favour the developme nt of phytoplankton assemblages dominated by diatoms (Si enrichment do uble that of N and P); and (3) buffer the large spatial and temporal d ifferences introduced by the advected nutrients. (C) 1997 Elsevier Sci ence Ltd.