The conservative chemical parameters "NO"; "CO" and "NCO" have been used to investigate the relationship between the upwelling intensity off the NW Iberian coast and the differential carbohydrate synthesis and utilisation in the water column. Differences between the observed vertical distribution of "NO" and "CO" and that expected from Redfield stoichiometry indicate that an excess of carbohydrate synthesis may occur in the surface layer when nutrients become depleted during upwelling relaxation periods. Excess carbohydrate synthesized in the upper layers is consumed in the subsurface levels. This spatial uncoupling between photosynthesis and nutrient uptake can be attributed to the presence of autotrophic migratory organisms like the photosynthetic ciliate Mesodinium rubrum and several dinoflagellates which produce carbohydrates in the upper, well illuminated layers, and take up nutrients in the sub-photic zone. Conversely, situations of intense upwelling are associated with production of organic matter following the Redfield ratio and with a decrease in carbohydrate synthesis. Deviations from Redfield stoichiometry found in other marine areas could be explained by processes comparable to those occurring in the NW Iberian upwelling, suggesting that vertical migration by photosynthetic organisms may play a greater role in the vertical transport of nitrogen and carbon than generally recognized.