Sea-ice is a key physical driver of Antarctic marine ecosystems. Understanding ecological effects of sea-ice is particularly important given current and future climate change, but a major obstacle is the impracticality of manipulating sea-ice at a relevant scale. However, large-scale anomalous events, such as those occurring in Commonwealth Bay, East Antarctica, provide opportunities for natural experiments. Historically, katabatic winds have kept Commonwealth Bay ice-free for most of each year, but since 2010, a massive grounded iceberg has resulted in year-round sea-ice cover. We surveyed benthic communities in Commonwealth Bay approximately 3 years after continuous sea-ice cover began and found algal bed communities in severe decline. The majority (*75 %) of large macroalgae were decomposing, and the remainder were discoloured or bleached, while approximately 40 % of encrusting coralline algae were bleached. Accompanying this, the presence of invertebrates such as ophiuroids and polychaetes suggests that communities are in the early stages of transitioning to an invertebrate-dominated state. With a known start date, monitoring benthic communities in Commonwealth Bay will allow quantification of rates of benthic regime shifts in response to sea-ice cover, and improve understanding of the vulnerability of polar ecosystems to climate change.