Simulated intertidal heat stress on the brown seaweed <i>Ascophyllum nodosum</i> demonstrates differential population sensitivity to future climate

Abstract Accurate forecasts of the biological impacts of climate change require a better understanding of how small‐scale temperature variability affects individual physiology and population dynamics. However, doing so for intertidal species with large distribution ranges while accounting for the effects of local adaptation presents numerous technical challenges. Historically, studies assessing the thermal thresholds of intertidal species have primarily focused on high‐tide conditions. However, neglecting the stress experienced during low tides may lead to misinterpretations of critical thermal limits, overestimation of ecological niches and underestimation of delayed mortality effects. Here, we assessed the macroecological consequences of thermal stress on the cold‐adapted brown seaweed Ascophyllum nodosum across its European distribution. We collected specimens from 10 populations spanning latitudes 41° N–60° N and subjected them to simulated intertidal heat stress using a novel, custom‐built experimental set‐up that replicated realistic conditions, including tidal cycles, light conditions and temperature trajectories based on in situ data. Results indicate that thermal stress is more closely associated with the magnitude of temperature change between high and low tides rather than the absolute maximum temperatures reached. Notwithstanding, algae exposed to warmer water temperatures (20.5°C) consistently outperformed those in colder water (15°C), suggesting that cold upwelled waters at the species' southern limit may not be essential for survival. By integrating empirical physiological observations with state‐of‐the‐art climate projections, we found that some populations, though more resilient to thermal stress, could still be overwhelmed by the pace of warming, leading to uneven impacts across the European distribution of the species. Synthesis. Our findings underscore the importance of replicating realistic tidal cycles and temperature trajectories to accurately assess thermal stress in intertidal species, as temperature fluctuations between high and low tides drive the stress response. Our projections not only align with observed local extinctions but also indicate future trends, emphasising that species' responses to climate change will depend on their population‐specific sensitivity and local climate conditions. Even heat‐tolerant populations may struggle to keep pace with the rapid rate of warming, highlighting the vulnerability of intertidal ecosystems.