Unravelling heat stress complexity in intertidal sediments: How rising sea water temperatures could expand thermal stress to the lower intertidal

Marine and atmospheric heatwaves are intensifying worldwide, exposing intertidal communities to increasing thermal stress. Yet the severity and ecological consequences of such stress are difficult to predict due to high spatial and temporal variability of temperatures in intertidal sediment. Using high-frequency temperature records, we quantified thermal exposure across emersion gradients at two contrasting intertidal flats within the Oosterschelde tidal inlet (the Netherlands). Threshold-based and cumulative metrics captured exposure to above optimal, sublethal and lethal temperature zones relevant for benthic infauna. Sediment temperatures frequently exceeded thresholds, but patterns differed sharply between locations. Under the same atmospheric conditions, a modest (∼2 °C) difference in daily mean water temperature produced markedly different heating patterns, amplifying sublethal stress in the lower intertidal at the location with warmer water. Here, elevated water temperatures prolonged infauna exposure during heatwaves by ∼6 h day -1 of sublethal and ∼15 h day -1 of above-optimal conditions. Lethal exposure, however, was confined to the upper intertidal and showed little variation between locations. The results demonstrate that thermal stress in intertidal sediments are determined from interactions between emersion duration and local environmental drivers, particularly water temperature, and that the areas most affected by cumulative sublethal stress may not align with areas experiencing lethal conditions. As coastal waters continue to warm, these drivers are likely to extend sublethal heatstress into lower intertidal zones, reshaping community composition and ecosystem function. Our findings highlight the need to account for both lethal and cumulative sublethal exposure, as well as location-specific conditions, to better predict vulnerability under climate change.