Under‐Ice Mixed Layers and the Regulation of Early Spring Phytoplankton Growth in the Southern Ocean

Under‐ice phytoplankton “blooms” have been observed in the Southern Ocean, although irradiance is extremely low and vertical mixing is assumed to be deep. Most under‐ice data have been collected using Argo floats, as research expeditions during austral fall and winter are limited. Hydrographic measurements under dense ice cover indicate that vertical mixing in weakly stratified systems may be less than previously suggested, and that the accepted determinations of mixed layer depths are inappropriate in regions with extremely weak stratification, such as those under ice. Vertical gradients in density suggest that mixed layers in the Ross Sea in early October are not extremely deep; furthermore, while phytoplankton biomass is low, it has begun to accumulate under ice. Growth rates indicate that phytoplankton growth in the Ross Sea begins in early September. Extending the period of growth may have substantial impacts on carbon biogeochemistry and food web energetics in ice‐covered waters. Plain Language Summary Data from profiling floats under Southern Ocean ice indicate that some growth of plankton occurs there, even though available irradiance is extremely low and vertical mixing is deep. Few research expeditions during austral fall and winter occur due to the difficulty of sampling and penetrating deep into ice. Estimates of vertical mixing in October from the Ross Sea suggest that the accepted criterion of mixed layer depths are inappropriate in regions with extremely weak stratification, such as those under ice. Mixed layers in the Ross Sea in early October are not extremely deep; furthermore, while phytoplankton biomass is low, it has begun to accumulate under ice. Growth rates indicate that the onset of phytoplankton growth in the Ross Sea likely occurs by early September, soon after positive irradiance occurs. Extending the period of growth may have substantial impacts on carbon biogeochemistry and food web energetics in ice‐covered waters. Key Points Mixed layer depths under extremely weak vertical stratification (like under ice) are overestimated using conventional criteria Phytoplankton growth in the Ross Sea is initiated soon after the start of solar day, far earlier than has been previously suggested Early spring growth can potentially alter our views of Southern Ocean carbon biogeochemistry and food web phenology