Climate change, glacier retreat and a new ice-free island offer new insights on Antarctic benthic responses

The Antarctic Peninsula is among the places on Earth that registered major warming in the last 60 yr. Massive ice losses, represented by glacier retreat, ice-shelf collapses and sea-ice reduction are among the main impacts of this regional warming. The loss of sea-bed ice coverage, on the one hand has been affecting benthic assemblages, but on the other it is opening up new areas for benthic colonisation. Potter Cove (South Shetland Islands) offered the opportunity of assessing both processes. We recently reported a sudden shift of benthic assemblages related to increased sedimentation rates caused by glacier retreat. This glacier retreat also uncovered a new island that presents a natural experiment to study Antarctic benthic colonisation and succession. We sampled the new island by photo-transects taken up to 30 m depth. Here, we report an unexpected benthic assemblage characterised by high species richness, diversity and structural complexity with a well-developed three-dimensional structure and epibiotic relationships. Filter feeders comprised the largest trophic group at all depths, mainly ascidians, sponges and bryozoans. Densities were also surprising, recording only six ascidian species with a mean of similar to 310 ind. m(-2). These values are at least an order of magnitude higher than previous Antarctic reports on early colonisation. This finding challenges the extended idea of a slow and continuous recruitment in Antarctica. However, it also opens the question of whether these complex assemblages could have been present under the glacier in ice-free refuges that are now exposed to open sea conditions. Under the current scenario of climate change, these results acquire high relevance as they suggest a two-fold effect of the Antarctic Peninsula warming: the environmental shifts that threaten coastal ecosystems, and also the opening up of new areas for colonisation that may occur at a previously unimagined speed.