As global temperatures continue to rise, shallow coral reef bleaching has become more intense and widespread. Mesophotic coral ecosystems reside in deeper (30–150 m), cooler water and were thought to offer a refuge to shallow-water reefs. Studies now show that mesophotic coral ecosystems instead have limited connectivity with shallow corals but host diverse endemic communities. Given their extensive distribution and high biodiversity, understanding their susceptibility to warming oceans is imperative. In this multidisciplinary study of an atoll in the Chagos Archipelago in the central Indian Ocean, we show evidence of coral bleaching at 90 m, despite the absence of shallow-water bleaching. We also show that the bleaching was associated with sustained thermocline deepening driven by the Indian Ocean Dipole, which might be further enhanced by internal waves whose influence varied at a sub-atoll scale. Our results demonstrate the potential vulnerability of mesophotic coral ecosystems to thermal stress and highlight the need for oceanographic knowledge to predict bleaching susceptibility and heterogeneity.

Coastal managers worldwide must prepare for changes in annual wave overtopping events due to climate change and sea-level rise. Research often assesses overtopping discharges by extreme events at a sea wall crest, typically using data from physical models or empirical rules based on scaled experiments. Here, we analyse a unique 1-year field dataset of coastal wave overtopping, from SW England, to determine the number of individual waves, regardless of their size, overtopping two locations across a coastal structure. The coastal conditions causing the most frequent overtopping differ from those driving it landward, complicating hazard communications for multiuse infrastructure. These data are the first field observations covering a year of tide, wave and wind conditions that cause overtopping of a vertical sea wall. Storms have a minimal (<2%) contribution to the number of tides associated with overtopping and the prevailing wave direction was not that associated with most overtopping events. Overtopping histograms identify the variability in the most likely time of overtopping relative to high tide for different wave categories across the structure. Sea-level rise, beach lowering and climate change will influence the annual number of waves overtopping in future. Change will be a complex balance between overtopping by different wave categories due to their likelihood of coincidence with water levels that do not cause depth-limitation over the foreshore or (partial-)reflection off the structure. It is possible the number of waves overtopping will reduce at the crest of a sea wall, while more of those overtopping waves will travel further inland.

A growing reliance on (ageing) sea walls to protect people, property and infrastructure is a global coastal management issue. Wave overtopping hazard is increasing in likelihood throughout the year due to sea-level rise and is not only associated with winter storms. Adaptive management requires accurate predictive tools to explore future options and forecast hazard. Predictive tools, based on empirical rules derived mainly from scaled experiments, can be highly uncertain in real-world application due to limited multi-process representation. We analyse a unique 4-month field dataset of coastal wave overtopping at two locations, ~345 m apart, along a sea wall fronted by an emergent beach, in SW England. Nearly one order of magnitude difference in the number of waves overtopping occurs between the two sites, most likely due to the variability in the beach influencing the coastal dynamics. Higher beach levels reduce but do not always eliminate overtopping, even when the beach is emergent above the high tide level. The number of waves overtopping increases with higher water elevations. The likelihood of overtopping will potentially increase with sea-level rise while the emergent beach is sustained. During winter 2021/2022, overtopping was incorrectly predicted for 49% of the tides using an empirical approach that disregards wind influence and the temporal variability in and emergence of the beach levels in front of the sea wall. Predictive tools and approaches for beach-fronted vertical structures must account for non-static and emergent beaches, wind influence and broken waves.