| Summary: | Phytoplankton production controls the transfer of carbon dioxide from the atmosphere into the ocean through the biological carbon pump, facilitating the sequestration of carbon dioxide and thus, contributing to the regulation of global climate. In the Southern Ocean, a high-nutrient low-chlorophyll region, productivity is limited by micronutrients including iron. The limiting role of light is also important, especially in the context of Southern Ocean phytoplankton adaptation to the low-iron conditions. To investigate the relative dominance of iron and light limitation on phytoplankton growth, we conducted a series of incubation experiments during a 2019 summer research voyage off East Antarctica. Our results show that, while light was the primary limiting factor of phytoplankton growth, iron addition became favourable for growth under high light conditions. This supports similar findings from laboratory (Strzepek et al., 2019) and field experiments in the Ross Sea (Alderkamp et al., 2019) and provides a new insight from a different region of the Southern Ocean, at a later stage of the austral summer. Similar results have previously been documented in larger diatoms, and here we provide evidence that it is also true for smaller cells. Our findings add to the body of knowledge regarding iron and light limitation in the Southern Ocean, in a poorly sampled area, leading to a better understanding of how primary production will change in a future warming ocean.
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