Assessing local oceanographic processes that mitigate thermal stress on coral reefs of the Queensland Plateau during the 2024 marine heatwave

Details

Supervisors

Hanert, Emmanuel ; Lambrechts, Jonathan ; Legat, Vincent

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil mécanicien, à finalité spécialisée

Abstract

enCoral reef ecosystems are increasingly threatened by climate change and the unprecedented rise in ocean temperature. The growing frequency and intensity of marine heatwaves have led to more severe and widespread coral bleaching events worldwide. During the 2023-2024 global bleaching events, coral reefs around the world were exposed to an intense marine heatwave. Among those, the reefs of the Queensland Plateau were exposed to extreme heat stress but field surveys conducted during the warm season have reported unexpectedly low levels of bleaching across several reefs in the region. This thesis investigates the oceanic conditions and bleaching patterns associated with this event using SLIM, a three-dimensional baroclinic hydrodynamic model. The objective is to better understand the physical processes that may mitigate thermal stress and contribute to the relative protection of certain reefs within the Queensland Plateau region. The model is validated against in situ observations and Argo float data, demonstrating a consistent representation of the regional thermohaline structure and circulation. Our simulations reproduce the regional hydrodynamics and thermal conditions during the event, and thermal stress accumulation is quantified using the Degree Heating Weeks (DHW) metric derived the modelled temperature fields. Two selected reefs, Lihou and Diamond, are further analyzed in detail to investigate the processes responsible for reduced thermal stress at these locations. The results reveal strong spatial variability in thermal stress between surface and reef-depth conditions. In particular, some areas exhibit reduced thermal stress at reef depth despite high surface temperature. The analysis highlight the importance of multi-scale processes in modulating thermal stress at reef depth. The southern part of Lihou Reef experiences reduced thermal stress, likely due to large-scale circulation patterns that enhance the presence of cooler and deeper waters. In contrast, Diamond Reef is periodically influenced by cooler water intrusions driven by tidal processes. These findings emphasise the critical role of local hydrodynamics and subsurface processes in shaping reef thermal environments, and highlight the importance of considering reef-depth conditions when assessing coral reef vulnerability to marine heatwaves.