Little is known about Southern Ocean under-ice phytoplankton, despite their suspected potential-ice and stratification conditions permitting-to produce blooms. We use a distributional approach to ask how Southern Ocean sea ice and under-ice phytoplankton characteristics are related, circumventing the dearth of co-located ice and phytoplankton data. We leverage all available Argo float profiles, together with freeboard (height of sea ice above sea level) and lead (ice fractures yielding open water) data from ICESat-2, to describe co-variations over time. We calculate moments of the probability distributions of maximum chlorophyll, particulate backscatter, the depths of these maxima, freeboard, and ice thickness. Argo moments correlate significantly with freeboard variance, lead fraction, and mixed layer depth, implying that sea ice dynamics drive plankton by modulating how much light they receive. We discuss ecological implications in the context of data limitations, and advocate for diagnostic models and field studies to test additional processes influencing under-ice phytoplankton.