Substrate heterogeneity outweighs colour in shaping thermal environment and intertidal barnacle recruitment on artificial surfaces

Understanding how physical attributes of artificial substrates influence ecological processes is essential for the design of habitat-enhancing structures in coastal ecosystems. Here, we examine how structural heterogeneity (flat vs. heterogeneous tiles) and colour (black vs. white) of 3D-printed substrates are associated with the thermal environment and recruitment of two intertidal barnacle species, Notochthamalus scabrosus and Jehlius cirratus. Surface temperature during low tide was quantified using infrared thermography. Black tiles heated significantly more than white tiles, with temperature increases of ∼6–12 °C and 1–4 °C, respectively. However, surface heterogeneity produced measurable thermal buffering, with crevices on black tiles remaining 3–6 °C cooler than adjacent ridges, generating fine-scale thermal refugia whose magnitude varied with substrate colour. Barnacle recruitment was primarily associated with habitat structure, with high recruitment on heterogeneous tiles, especially within crevices where recruitment was two orders of magnitude greater than ridges for both species. Species-specific responses to colour were limited: J. cirratus responded only to microhabitat identity, whereas N. scabrosus exhibited additional sensitivity to substrate colour. These results suggest that structural heterogeneity can mitigate thermal stress by providing stable microhabitats that may enhance post settlement survival and recruitment, even on thermally extreme artificial substrates. Our findings highlight the roles of substrate colour and microtopographic complexity in shaping thermal environments and recruitment patterns and provide a mechanistic basis for disentangling structural and functional components of thermal heterogeneity in artificial substrates under warming conditions.