Abstract.Coastal Patagonian sea waters had been differentially nutrient‐enriched to determine (i) the magnitude of the algal biomass supported and (ii) which species were able to best compete for the available nutrients. Nitrogen was found to be the nutrient limiting growth of the natural phytoplankton population. The addition of NO3at 25 μg at 1‐1led to a 10 to 30 μg 1‐1increase in chlorophyllain all but those treatments containing trace metal additions. The best yield coefficients were obtained when a chelator (EDTA) was added with NO3. A pool of nine diatom species were common in the N03supplemented waters and the addition of vitamins along with NO3allowed two of the nine species to d