Coral bleaching and mortality across a 24° latitudinal range in the Southwestern Atlantic during the fourth global bleaching event

Anthropogenic climate change combined with the 2023–24 El Niño-Southern Oscillation triggered the fourth global bleaching event, affecting coral reefs worldwide. However, the extent of its impacts on the unique and extensive reef environments in the Southwestern Atlantic remained unquantified. Here, we report results from the first large-scale, standardized bleaching monitoring effort in the Southwestern Atlantic, encompassing 18 reef sites across a broad latitudinal range (3°-27°S). The intensity and duration of the thermal stress experienced were evaluated using the Degree Heating Week metric, obtained through remote-sensing data. Bleaching and coral cover loss were monitored through photoquadrat surveys conducted before, during, and after the event. Our results revealed widespread thermal anomalies that often exceeded 20 °C-weeks and lasted 3–5 months. Bleaching incidence reached 96% in Maragogi, 84% in Porto de Galinhas, and > 70% in nearby sites on the northern coast (~ 3°–9°S). In contrast, southern sites (~ 22°–27°S) such as Alcatrazes Archipelago and Arraial do Cabo experienced < 5% bleaching. Coral mortality was highest in Maragogi (88%) and São José da Coroa Grande (53%), both within the largest nearshore Brazilian marine protected area. The species Millepora alcicornis and Mussismilia harttii experienced the highest declines, repeating patterns observed during the 2019–20 bleaching event. As key providers of structural complexity, their continued degradation threatens overall biodiversity. These results highlight the increasing impacts of global warming in a region that is generally neglected in global bleaching assessments, and underscore the urgency of sustained, standardized monitoring to track change and support effective climate conservation efforts across the Southwestern Atlantic.