Phytoplankton iron limitation in the Atlantic Southern Ocean driven by seasonal mixed‐layer dynamics

Phytoplankton blooms in the Southern Ocean (SO) are seasonally limited by light and micronutrients. As such, regional variations in iron supply from mixed‐layer winter entrainment are expected to impact the extent of seasonal iron limitation. Here, we determined seasonal iron limitation in the Atlantic SO by conducting iron addition incubation experiments during winter, prior to the maximum mixed‐layer deepening, and in spring, prior to the peak of the summer bloom. Both the polar and subantarctic zones displayed evidence of iron limitation in spring, based on increased photosynthetic efficiency, with evidence of the subantarctic zone being limited in winter. In contrast, there was no evidence of limitation in either season in the sub‐tropical and Antarctic zones. The large degree of zonal variability in the timing of iron supply resulting from winter entrainment impacts the seasonal characteristics of iron limitation, phytoplankton physiology and the potential for growth.