The coastal transition zone and adjacent continental shelf of the Iberian upwelling system was studied in September 1986, during the seasonal transition from upwelling-favored to downwelling-favored winds. The most striking features in the coastal transition zone were: (1) a poleward flow of high salinity Eastern North Atlantic Central Water (subtropical) off the Rías Baixas (Galician western coast); and (2) an anticyclonic eddy off Cape Ortegal (Galician northern coast). Chemical and biological similarities between both structures, clearly different from the surrounding oceanic waters, suggest that the eddy was an isolated and aged parcel of water originating from the poleward flow. The continental shelf was characterized by: (1) outwelling of chlorophyll-rich waters from the four large coastal embayments (the “Rías Baixas”) in the western coast and; (2) an upwelling front off the northern coast. The coexistence of opposite hydrographic structures, as the poleward flow and the upwelling front, was the consequence of transitional wind conditions in September–October, and we hypothesized transitional conditions to be crucial for the development of the eddy. Both the poleward flow and the eddy precluded the shelf-edge exchange of microplankton populations developed over the shelf, leading to massive in situ sedimentation and subsequent nutrient mineralization over the shelf.