A longstanding paradigm in biological oceanography is that nutrient supply rate affects phytoplankton taxonomy, which in turn affects the intensity of nutrient recycling in surface waters, reducing the variability of biological production across the global ocean relative to the large geographic variation in nutrient supply from below. The paradigm has frequently been framed in terms of the ratio of phytoplankton growth supported by nitrate vs. ammonium. However, the required phytoplankton sensitivities and mechanistic connections lack observational confirmation. Genomic and culture data indicate that Synechococcus, among the most abundant phytoplankton in the low and mid-latitude ocean, can use multiple nitrogen sources for growth, ranging from the nitrate supplied from the subsurface to the ammonium cycling between organisms in sunlit surface waters. However, our field data suggest stark differences in the “nutrient niches” of Synechococcus and eukaryotic phytoplankton in the nitrogen-poor subtropical ocean, with the former uniformly relying on ammonium and other regenerated nitrogen forms and the latter dominating the assimilation of nitrate. One interpretation for this distinction has been that the eukaryotes outcompete Synechococcus for nitrate in nitrate-deplete waters. Here, we report evidence from the wintertime Sargasso Sea and Southwest Indian Ocean as well as the summertime subpolar North Atlantic that, even under nitrate-rich conditions, nitrate is used by eukaryotic phytoplankton but not by Synechococcus. Thus, Synechococcus maintains its nutrient niche of reliance on regenerated nitrogen despite nitrate availability, indicating a lack of plasticity that runs counter to prior expectations and to the nutrient assimilation rules included in ocean biogeochemical models.