Fine-scale surface complexity promotes temperature extremes but reduces the spatial extent of refugia on coastal rocks

The physical structure of microhabitats, especially orientation to direct solar radiation, can radically influence the body temperatures of individual organisms, their physiological performance, and survival. Using a numerical approach via finite element (FE) analysis to simulate the spatial and temporal temperature variations in rocky intertidal habitats, we systematically explored the role of substrate roughness in driving variability of surface temperatures at scales relevant to very small (cm) organisms. This approach accounts for three-dimensional heat exchange among fine-scale (mm-cm) surface features through radiation, convection, and conduction. Analyses were performed for a surface mapped using a terrestrial laser scanner at an intertidal site on the coast of Haifa, Israel. Simulation results provided comparable temperatures to those recorded in the field via infrared camera. A series of rough surfaces were generated numerically to explore relationships between the scale of surface roughness and microhabitat temperatures, and how these relationships changed both over a diurnal cycle and across seasons. Overall, increasing habitat complexity had little influence on the average temperature of a ∼1 m² surface, despite differences of up to 25 °C among microhabitats within that surface. Temperature magnitudes of the hottest and coolest microhabitats increased markedly with roughness, generally supporting the ‘habitat heterogeneity hypothesis’ where a range of thermal microenvironments is predicted to increase with surface roughness. Here, we attribute this pattern to the observation that the presence of cool, shaded “valley” microhabitats is invariably accompanied by the presence of “peaks” exposed to full, direct solar radiation.