Thermal patterns on eco-engineered coastal infrastructure depend on topographic complexity and spatial scale

Topographic complexity is a key driver of microhabitat formation, due to its critical role in providing refugia for organisms from environmental stressors. On marine infrastructure, low topographic complexity can lead to the homogenisation of associated thermal microclimates (‘thermal habitat complexity’), with potential impacts on species settlement and establishment, as well as long-term effects on biotic community composition. While marine eco-engineering techniques hold great potential for shaping thermally complex habitats through topography manipulation, effective design and implementation require a greater understanding of topography-temperature relationships. Here, we assessed in situ thermal patterns on a large (11 × 2 m) intertidal eco-engineering installation, using six panel topographies, five spatial scales (1, 3, 5, 10, 30 cm), two topography metrics (rugosity, fractal dimension), and two temporal factors (time since emersion, full-seawall shading). Thermal imaging, combined with 3D topographic analysis, revealed mean temperatures on topographically complex panels to be significantly lower than on flat controls across three natural air temperatures on separate days (mean air temperatures of 20, 27, 29 °C). Spatial temperature variability (i.e., thermal microhabitat range) was highest at intermediate or high topographic complexity, depending on spatial scale. Topography-driven thermal buffering increased disproportionately with air temperature. Our findings provide quantitative mechanistic insights and a proof-of-concept methodology for assessing topography-temperature relationships at high spatial resolution, with practical implications for creating complex thermal environments on urbanized shorelines.