Hot rocks? Divergent rock-surface temperatures during extreme thermal events with implications for physiological stress in rocky shore organisms

Abstract

While rock–organism thermal interactions on rocky shores have known biogeomorphological relevance, the influences of rock thermal properties on the conditions experienced by rock-dwelling organisms (epiliths) remain understudied. This is a significant gap given the potential ecological and biogeomorphological consequences of changing average and extreme temperatures for coastal ecosystems. Using field block exposure trials in Southern England (including the 2023 September heatwave) alongside laboratory simulations, the thermal responses of four contrasting substrates (limestone, sandstone, basalt and concrete) were compared under the same heating conditions. Indicative organism temperatures were simultaneously obtained using biomimetic sensors (robolimpets [RLs] and robomussels [RMs]) attached to the substrate surfaces. Highly divergent thermal behaviours were observed, with peak substrate surface temperatures (T_max) differing by up to 13.2°C (basalt vs. limestone) under heatwave conditions in the field. Relative substrate temperatures were consistent between the field and laboratory (T_max limestone < sandstone < concrete < basalt), corresponding to key material properties such as density and colour; and hotter surfaces were always associated with higher biomimetic temperatures. The degree of association between surface and biomimetic temperatures differed between the two sensor types, attributed to more efficient conductive heat transfer (from substrate to organism) in the case of RLs. Thermal divergence between the two types of sensors was also mediated by rock type, with substrate porosity and evaporative cooling effects having a modulating effect. Biomimetic T_max also diverged under increasingly extreme scenarios depending on the substrates the sensors were attached to. These observations demonstrate how geomorphological approaches can contribute to thermal biology research (hinting at a new ‘thermal biogeomorphology’), with implications for patterns of physiological stress, the crossing of critical thermal limits, and resulting changes in the distribution and abundance of geomorphologically relevant species. Key challenges going forward, such as addressing sensor limitations and scale issues, are also identified.