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Diatoms, the main group of siliceous plankton, contribute more than 40% of the marine primary productivity and dominate the plankton communities of the Southern Ocean, north Pacific and the upwelling regions of the equatorial and tropical Pacific. The Si(OH)4/NO3- >1.0 ratio in subsurface waters of the southwestern margin of Baja California (SWM-BC) suggests that diatoms grow in optimum nutrient conditions and reach a value of Si (OH)4/NO3 <3 in conditions near the limit because of Fe or P deficiency. These values are higher than the Si:N <1 ratio for many oceans, therefore if primary productivity is diatom dominated in the last millennium, the Si:N ratio must be <3. To answer this question, a multicore was recovered from a depth of 700 meters in the SWM-BC. The core was sampled every 1 cm and freze-dried to obtain the biogenic opal content (BO) and total nitrogen (TN), from which the Si:N ratio and organic phosphorus were calculated. The BO and TN content increased in a constant manner and the Si:N ratio had an average and standard deviation of 2.8±0.1 and Porg had an average and standard deviation of 1.1±0.9 mg kg-1. In the last millennium, primary productivity has been diatom dominated, a product of the silicic acid exported from the Gulf of California to the Magdalena margin, hypothesis that has to be validated, under a Fe-limited ocean, and not by nitrates and orthophosphates.

Palabras clave: Diatomeas; margen de Magdalena; ópalo biogénico; productividad primaria; razón Si:N

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