Abstract for presentation at Biodiversity Extinction Crisis Conference - A Pacific Response

At the close of each summer surface waters around Antarctica freeze to form a layer of sea-ice over 1m thick. When temperatures rise the following summer, the sea-ice gradually breaks away from the coast in patches. Some patches break out sooner than others due to stabilizing effects of coastal geomorphology, creating a mixture of ice-covered and ice-free zones. Sea-ice greatly reduces light penetration and alters the sedimentation regime, so the annual period for which subtidal communities are covered by ice may have a strong influence on the species composition and diversity below. Furthermore, global warming is likely to bring forward the timing of sea-ice breakout and postpone refreezing, which could change the structure of some communities and endanger those that rely on extended periods of cover. We investigated the effects of sea-ice duration on benthic encrusting communities by collecting boulders from a range of sites that experience breakout at different times. Sea-ice duration dramatically affected the types of communities found. Sites where ice breaks out early were dominated by coralline and turfing algae, whereas those that were covered for longer periods accommodated a diversity of sponges, bryozoans, and other invertebrates. Many species were found exclusively in the most ice-covered areas, meaning that even small changes in the sea-ice regime could result in local extinctions.