Old but cool: Historic masonry seawalls provide thermal benefits for marine wildlife along urbanised coasts

Crevices and overhangs are characteristic features of historic masonry in coastal structures including seawalls, harbours and breakwaters. By providing shelter from heat and desiccation, these features may act as important refuges for colonising wildlife during prolonged periods of aerial exposure. However, unlike natural rock and modern coastal infrastructure, the thermal characteristics of historic masonry structures and their associated microhabitats have not been quantitatively assessed. Temperature logging and infrared thermography were used to compare external (exposed surface) and internal (crevice) thermal regimes among historic masonry seawalls, modern concrete infrastructure and natural rock at four UK study sites. Differences in the richness and abundance of canopy-forming species with surface-cooling effects were also quantified to assess the potential contribution of biological cover to thermal conditions on each substrate. Natural rock exhibited the lowest mean external surface temperatures across the study sites. Historic masonry surfaces were consistently cooler than concrete, except at one location, where elevated surface temperatures were associated with extensive colonisation by the black lichen Hydropunctaria maura . Crevices in historic masonry and natural rock provided thermal buffering relative to exposed surfaces, creating greater spatial heterogeneity in thermal conditions than on topographically uniform concrete infrastructure. Historic masonry walls also supported a significantly greater abundance of canopy-forming organisms than the concrete walls at each site. Our findings indicate that historic masonry shares thermally beneficial characteristics with natural rock that are largely absent from modern concrete infrastructure, extending our understanding of the ecological benefits provided by heritage assets along urbanised coasts.