Primary production is vulnerable to iron (Fe) availability because Fe is an essential micronutrient regulating carbon sequestration in the ocean ^[1]. This is particularly true in the subpolar North Atlantic (spNAtl), which supports pronounced seasonal phytoplankton blooms ^[2][3]. Despite this importance, the role of Fe in the seasonal phytoplankton cycle in the spNAtl remains poorly constrained. Here we combine results from bioassay Fe addition experiments during the DY180 (spring) and JC269 (autumn) cruises with year-round measurements of maximum photosystem II photochemical efficiency, , across the spNAtl, spanning from shelf regions to the open ocean. We found that the onset and intensification of Fe stress during the spring bloom coincided with declining surface dissolved Fe concentrations, indicating that Fe limitation likely contributes to bloom termination in the spNAtl. While the Fe stress persisted into autumn, its severity was generally weaker than in late spring. This seasonal development of Fe stress was reduced or absent in proximity to shallow bathymetry, likely contributing to the enhanced productivity around the margins. Our results highlight a fundamental role of Fe in regulating primary productivity in the spNAtl.

References
[1] Twining, B. S., & Baines, S. B. (2013). Annu. Rev. Mar. Sci., 5(1), 191-215.
[2] Moore et al., (2006). Glob. Change Biol., 12(4), 626-634.
[3] Ryan-Keogh et al., (2013). L&O, 58(2), 533-545.