Seasonal development of iron limitation in the sub-Antarctic zone

The seasonal and sub-seasonal dynamics of iron availability within the sub-Antarctic zone (SAZ, ~ 40–45° S) play an important role in the distribution, biomass and productivity of the phytoplankton community. The variability in iron availability is due to an interplay between winter entrainment, diapycnal diffusion, storm-driven entrainment, iron scavenging and iron recycling processes. Biological observations utilising grow-out iron addition incubation experiments were performed at different stages of the seasonal cycle within the SAZ to determine the importance of these supply mechanisms. Here we demonstrate that at the beginning of the growing season there is sufficient iron to meet the demands of the phytoplankton community, but as the growing season develops the supply mechanisms fail to meet this demand. Phytoplankton increase their photosynthetic efficiency and net growth rates following iron addition from mid to late summer, with no differences determined during early summer; suggestive of seasonal iron depletion and low iron resupply. The result of which is residual macronutrients at the end of the growing season, and the prevalence of the high-nutrient low-chlorophyll (HNLC) condition. We conclude that despite the prolonged growing season characteristic of the sub-Antarctic zone, which can extend into late summer/early autumn, the results suggest that the iron supply mechanisms are insufficient to maintain potential maximal growth and productivity throughout the season.