Small‐scale thermal habitat variability may not determine seagrass resilience to climate change

Abstract Seagrass ecosystems are integral components of coastal marine environments, but they face global threats from climate change. The ability of seagrass to withstand these challenges depends on trait variation among individuals, which is influenced by genetic background and plasticity. In this study, we explore how small‐scale thermal habitat variability contributes to intraspecific trait variation in the eelgrass Zostera marina and how this variation affects resilience to climate change. We hypothesize that eelgrass meadows with higher mean and more variable summer temperatures exhibit greater resilience. Despite their proximity (maximum distance of 14 km), the 10 assessed meadows exhibited distinct thermal microclimates and were genetically differentiated from each other. We conducted a common garden experiment subjecting eelgrass shoots from these meadows to increased temperature and decreased salinity to represent a projected future climate. The treatment led to a significantly increased mortality, higher prevalence of the eelgrass wasting disease, reduced development of new leaves, and overall diminished growth of Z. marina . Responses varied among meadows, independent of their natural thermal summer microclimates. We discuss this mismatch between local microclimate and phenotypic response with a strong focus on the potential of microclimates (higher summer temperature, lower winter temperatures, pronounced variability) to impede differences in intraspecific trait variation among meadows. Overall, our study emphasizes the need for comprehensive habitat characterization paired with the assessment of (thermal) performance curves to unravel the complex effects of microclimates on intraspecific trait variation and, consequently, species resilience to climate change.