Nowhere to hide: massive corals’ mortality after cumulative heat stress in a putative climate change refugium

Coral reefs worldwide are under severe threat due to a combination of local stressors and escalating intensity and frequency of heatwaves, which lead to mass coral bleaching. Although turbid-zone reefs have been considered climate change refugia, their long-term responses to heatwaves remain poorly assessed. Additionally, the relationship between morphological variation and bleaching resistance within massive corals that often dominate turbid zones remains unclear. To address these two gaps, we conducted line-transect surveys during the 2016 and 2019 mass bleaching events (peak DHW 10.9 and 19.7 °C-weeks, respectively), estimated Symbiodiniaceae densities, and fate-tracked 79 colonies of the two massive coral species in Southwestern Atlantic’s largest turbid-zone reefs using 3D photogrammetry and pulse-amplified modulated fluorometry. Fate-track sampling was performed 5 months before, during, and 5 and 24 months after the 2019 event. We found no association between morphology (colony sphericity and polyp area) and tissue mortality, either between or within species. Coral tissue area continued to decline for up to two years after the heatwave, challenging the Brazilian climate change refugia hypothesis and emphasizing the need for long-term monitoring to fully understand the consequences of bleaching events. Notably, tissue mortality was unrelated to bleaching prevalence, suggesting that the phased recovery of photophysiological parameters may be decoupled from longer-term coral health and survival. Recommendations to policy-makers and managers include prioritizing climate change mitigation and improving monitoring programs.