Abstract Upwelling off Morocco was investigated in the summers of 1972 and 1973. In the northern zone, superficial waters result from the mixture of three water masses. The southern area is characterized by a ‘bell’ of cold water brought to the continental shelf, where nutrient regeneration is intense. Using models of water mass formation, nutrient anomalies (nitrogen, phosphorus, and silicon) are calculated in both uptake and regeneration phases and corresponding ratios are evaluated. Deviations from Redfield's ratios were observed in superficial North Atlantic central waters bathing the continental shelf, where inorganic phosphorus and silicon appear to regenerate faster than nitrogen. In the photic layer no limitation on phytoplankton growth by silicon was detected.