Marine heat waves endanger the rebuilding of marine life

Abstract Marine heatwaves (MHWs) are increasing in frequency and intensity as a result of climate change, yet their implications for ecosystem recovery and habitat restoration remain poorly understood. Here, we document how an acute spring 2024 MHW likely contributed to the failure of a large-scale intertidal Zostera marina restoration effort in the Irish Sea. Across two neighbouring bays (∼50 km apart) in North Wales (UK), over 1.1 million seeds were sown in 2023–2024. Seagrass plantings in Bae Caergybi survived through 2024; however, all plantings in Bae Tremadog failed. Satellite-derived and in situ environmental data revealed that the Bae Tremadog failure coincided with a rapid-onset MHW, during which sea surface temperatures increased by ±3.5 °C in seven days and intertidal sediment temperatures exceeded 30 °C during low tides. This thermal extreme was followed by a sharp decline in salinity and an increase in wave exposure, creating a compounding multi-stressor event. By contrast, Bae Caergybi experienced cooler and more stable abiotic conditions, enabling seedling establishment and overwinter survival. Long-term records indicate increasing mean temperatures and high interannual variability at the failed sites, suggesting that suitable restoration windows are narrowing as the climate warms. Our findings provide one of the first documented cases of a MHW directly undermining marine habitat restoration in Northern Europe. Recognising and forecasting climate-driven extreme events and prioritising ‘refugia’ sites with greater abiotic stability will be critical for improving the resilience and success of future marine restoration initiatives.