Phytoplankton blooms in the Southern Ocean

The Southern Ocean spring phytoplankton bloom impacts regional food webs and the marine carbon cycle, but we do not fully understand which drivers – environmental, ecological, or biological – control the timing or productivity of the spring bloom. Nutrients, and particularly iron, are likely replete in the austral winter, but the importance of underwater light availability and grazing pressure are topics of ongoing discussion. Furthermore, in the extreme polar winter, phytoplankton physiology may impart additional constraints on phytoplankton variability when ocean mixing decreases. Of particular interest is the impact of highly variable sea ice on the Southern Ocean environment, where over the last decade the Antarctic sea ice extent (SIE) has recorded record highs and lows. The anomalous low in 2023 suggested a new reduced sea-ice state, with unknown impacts on phytoplankton bloom dynamics, including bloom phenology and magnitude. Such changes in SIE will alter the physical environment, and in turn will have profound implications for the Southern Ocean ecosystem. Phytoplankton are especially sensitive to such changes in the physical environment, but understanding and predicting future changes resulting from a reduced sea-ice state remains challenging.