Shallow seasonal stratification ameliorates coral bleaching during record-breaking marine heatwaves in a marginal subtropical system

Abstract Marine heatwaves are intensifying globally, with the Fourth Global Coral Bleaching Event (GCBE) beginning in 2023, yet coral bleaching outcomes remain highly variable across space and time. This study examines the role of fine-scale thermal variation in shaping bleaching impacts during two record-breaking marine heatwaves (2022 and 2024) affecting shallow coral assemblages in the marginal and urbanised environments of Hong Kong. Using high-resolution in situ temperature loggers, we quantified thermal exposure across coral-inhabited depths (1–8 m) at three sites and related this to bleaching prevalence across taxa. In 2024, surface temperatures exceeded the local bleaching threshold (29.7 °C) for 37 days, resulting in heat accumulation that reached 25.2 °C days (9.24 °C weeks) and remained above 8 °C days (°C weeks) for 46 days (20 days). However, seasonal stratification created vertical gradients of up to 9.6 °C between 2 and 8 m, reducing the magnitude and duration of heat accumulation severalfold (e.g., up to 35.4 °C days and 76 days > 8 °C days at 2 m, compared to 7.9 °C days at 8 m). Bleaching prevalence was 20–46% lower between 1 and 5 m, with Bayesian bleaching probabilities highest for Goniopora (82%) and lowest for Porites (64%). These patterns could not be explained by satellite-derived sea surface temperatures alone. Despite record heating and stress, mortality was limited (60 to 95% pigmentation recovery). Our findings provide rare fine-scale assessments of bleaching and thermal dynamics, including during the Fourth GCBE, and highlight how local stratification can create thermal refuges. Understanding how oceanographic contexts structure these refuges is essential for identifying and conserving coral communities with greater survival potential under accelerating ocean heating.