Legacy Effects of an Extreme Marine Heatwave on a Stress-Tolerant Coral

During the 4th Global Coral Bleaching Event (GCBE4; January 2023-September 2025), an extreme marine heatwave occurred on the Bocas del Toro Reef Complex (BTRC) in Panama. We characterized how this heatwave impacted the health and holobiont communities of the stress-tolerant coral Siderastrea siderea at four sites across the BTRC. Tagged colonies at each site (N = 30-53 colonies per site) were visited before, during, and after the heatwave (early May 2022, mid-August 2023, and late April 2024, respectively), and images and DNA samples were collected at each time point. In situ temperature logger data showed that sites reached maxima of 32.1°C-33.9°C in October 2023, resulting in the accumulation of ~12-20 maximum degree-heating weeks (DHWs). Consequently, S. siderea colonies displayed widespread bleaching (i.e., the loss of algal endosymbionts), with an increase from 8.6% to 33% of colonies bleached per site in May 2022 to 33%-70% in August 2023, followed by a decline to 15%-63% by April 2024. Colony-level partial mortality increased significantly between 2022 and 2024 at three of the four sites, and was observed even in colonies that were not bleached in August 2023. Further, many corals hosting Cladocopium spp. algal symbionts in 2022 shifted towards less diverse communities dominated by heat-tolerant Breviolum and Durusdinium spp., and most of these corals continued to host modified symbiont communities for months. The heatwave also reshaped corals' bacterial microbiomes, including increases in α-diversity and abundances of potentially pathogenic taxa (e.g., Vibrionaceae), and these shifts were persistent following the heatwave. Together, these findings demonstrate that GCBE4 had lasting impacts on S. siderea holobiont health across the BTRC, underscoring that extreme heat events can compromise even stress-tolerant coral species and induce legacy effects that will likely affect their future resilience. Rapid action to minimize further ocean warming is thus necessary to safeguard reef ecosystems.