Recent studies have described active nitrogen fixation in high-latitude waters, but the ecological controls on the occurrence or activity of nitrogen-fixing organisms (diazotrophs) in such systems remain unknown. Turnover rates and top-down controls are also general knowledge gaps for marine diazotrophs. We detected abundant UCYN-A (endosymbiotic nitrogen-fixing cyanobacteria) in the Gulf of Anadyr, western Bering Sea, which correlated with high dissolved iron to dissolved inorganic nitrogen ratios (Fe : DIN) due to riverine input. Growth and grazing mortality of UCYN-A sublineages were almost balanced with higher biomass-turnover rates compared to the whole phytoplankton community, indicating selective grazing of UCYN-A in nitrogen-depleted waters. Grazing rates on UCYN-A1 (small cells) were higher than for UCYN-A2/3/4 (large cells), consistent with the general size dependence of phytoplankton growth and grazing mortality. We found that Fe : DIN is a major determinant of UCYN-A abundances in high-latitude waters, where UCYN-A could make substantial contributions to plankton food-web cycling.