Tidal phasing drives intertidal heat stress and climate vulnerability

Intertidal organisms are exposed to extreme and variable thermal conditions due to periodic aerial exposure by tides. For sessile species that cannot seek refuge, heat stress is strongly influenced by the timing of low tides, which in semidiurnal tidal systems is governed by the spring–neap tidal cycle. Here, we deployed biomimetic loggers, designed to replicate body temperature of the mussel Mytilus edulis, at two climatologically similar yet tidally distinct sites in Wales, UK, to examine how interactions between climate and tidal phasing regulate intertidal heat stress. Despite comparable weather conditions, South Wales mussels—exposed during midday spring low tides—experienced daily maximum body temperatures over 3 °C higher compared to North Wales, where exposure occurred during morning and evening spring low tides. Consequently, South Wales mussels exceed critical temperature thresholds more frequently. Based on the observations, we developed and validated a mussel body temperature model incorporating readily available tidal and climatic data, which accurately reproduced observed logger temperatures and successfully simulated a 2018 mass mortality event in the English Channel. Model experiments showed that tidal phasing can modulate 95th-percentile daily maxima by up to 5.5 °C, with midday spring low tides producing the warmest possible conditions. Long-term simulations (1990–2023) further revealed a 2 to 2.5-fold increase in extreme temperature exposure since 2020. These results demonstrate that tidal characteristics in semidiurnal systems can amplify or mitigate intertidal thermal stress as strongly as local climate, and that simple, process-based models can forecast heat exposure and ecological vulnerability under future climate change.