Antarctic landfast sea ice (fast ice) is stationary sea ice that is attached to the coast, grounded icebergs, ice shelves, or other protrusions on the continental shelf. Fast ice forms in narrow (generally up to 200 km wide) bands, and ranges in thickness from centimeters to tens of meters. In most regions, it forms in autumn, persists through the winter and melts in spring/summer, but can remain throughout the summer in particular locations, becoming multi‐year ice. Despite its relatively limited extent (comprising between about 4% and 13% of overall sea ice), its presence, variability and seasonality are drivers of a wide range of physical, biological and biogeochemical processes, with both local and far‐ranging ramifications for the Earth system. Antarctic fast ice has, until quite recently, been overlooked in studies, likely due to insufficient knowledge of its distribution, leading to its reputation as a “missing piece of the Antarctic puzzle.” This review presents a synthesis of current knowledge of the physical, biogeochemical and biological aspects of fast ice, based on the sub‐domains of: fast ice growth, properties and seasonality; remote‐sensing and distribution; interactions with the atmosphere and the ocean; biogeochemical interactions; its role in primary production; and fast ice as a habitat for grazers. Finally, we consider the potential state of Antarctic fast ice at the end of the 21st Century, underpinned by Coupled Model Intercomparison Project model projections. This review also gives recommendations for targeted future work to increase our understanding of this critically‐important element of the global cryosphere.

Phytoplankton are critical to the Antarctic marine food web and associated biological carbon pump, yet long-term shifts in their community composition are poorly understood. Here, using a machine learning framework and combining pigment samples and environmental samples from austral summertime 1997–2023, we show declines in diatoms and increases in haptophytes and cryptophytes across much of Antarctica’s continental shelf. These trends—which are linked to sea ice increases—reversed after 2016, with a rebound in diatoms and a large increase in cryptophytes, coinciding with the loss of sea ice. Significant changes (P < 0.05) across the 25-year dataset include diatom chlorophyll a (chl-a) declines of 0.32 mg chl-a m⁻³ (~33% of the climatology) and increases for haptophytes and cryptophytes of 0.08 and 0.23 mg chl-a m⁻³, respectively. The long-term shifts in phytoplankton assemblages could reduce the dominance of the krill-centric food web and diminish the biologically mediated export of carbon to depth, with implications for the global-ocean carbon sink.