Do pre-summer temperatures influence coral bleaching prevalence and severity?

Abstract Coral reefs are threatened by rising sea temperatures that drive widespread bleaching events, leading to coral mortality and loss of diversity. Understanding the mechanisms underlying coral reef resilience to thermal stress is critical for effective conservation. This study investigates the influence of pre-summer temperature patterns upon community-level bleaching prevalence in the austral summers of 2016 and 2017. Pre-summer metrics based on sea surface temperature anomalies across the central Great Barrier Reef were developed and relationships with in-water coral bleaching observations (n = 69 sites across 27 reefs) analysed using generalised additive models. Among analysed metrics, "Winter heat" offered valuable information to assess bleaching sensitivity. Results suggest that a hot winter (when cumulative Winter heat exceeds 7.5 °C-weeks) exacerbated coral bleaching in the subsequent summertime. It also appeared that a “Goldilocks” just right range may exist, in which low-to-moderate Winter heat accumulations above normal historical winter average temperatures (across the range 1.0–7.5 °C-weeks in winter) may have helped lessen bleaching prevalence. Further testing using data with greater spatiotemporal extent; higher frequency of bleaching-level exposure for a single reef location; and higher spatial resolution is recommended to confirm these findings. Including Winter heat increased the bleaching predictive capability in both internal and external model validation exercises. Specifically, we recommend further testing (and potential refinement) of the Winter heat > 7.5 °C-weeks threshold to confirm the use of this metric in regional coral bleaching prediction models. By accounting for winter thermal history prior to the next summer heat stress, bleaching prediction models can more effectively identify reefs at risk and provide a stronger basis for prioritising management interventions to safeguard reef resilience.