Projected climate-driven shifts in coral distribution indicate tropicalisation of Southwestern Atlantic reefs

AimPredicting and acting on the future of ecosystems requires understanding species distribution shifts due to climate change. We investigated which corals are more likely to shift their distribution in the Southwestern Atlantic under a warming scenario.LocationSouthwestern Atlantic (SWA; 1 degrees N-28 degrees S).MethodsWe used spatial distribution models with a Bayesian approach to predict the current and future (2050 and 2100) coral occurrence probabilities of 12 zooxanthellate corals and hydrocorals under an intermediate scenario of increasing greenhouse gas emissions (RCP6.0) projected by the Intergovernmental Panel on Climate Change (IPCC).ResultsWe found a decline in the occurrence probabilities of all 12 taxa within the tropics (1 degrees N-20 degrees S) and an increase towards subtropical sites (20-28 degrees S) as early as 2050. The most significant declines are projected to occur between 9 degrees S and 20 degrees S, a region that currently hosts the richest reef complex in the South Atlantic, the Abrolhos bank. The imminent loss of suitable habitat in the tropics mostly threatens the Brazilian endemics and range restricted corals Mussismilia braziliensis and Mussismilia harttii, while more widely distributed taxa such as Siderastrea spp., Millepora spp. and Porites spp. are expected to expand their ranges southwards.Main ConclusionsThe projected declines in the tropical region are likely to reduce structural complexity causing biodiversity loss. The overall increase in occurrence probabilities in subtropical areas indicates tropicalisation of SWA reefs, which may benefit species already established in these areas and potentially enrich coral assemblages through the range expansion of taxa that currently do not occur in the region. These findings emphasise the need to support ecological corridors that could aid coral migration towards more suitable habitats under climate change.