Changes in the community composition of beach wrack macrophytes along thermal and latitudinal gradients

Marine species and ecosystems are highly threatened by many anthropogenic drivers of biodiversity loss, among which the various components of climate change play a key role. Bending the curve of marine biodiversity loss requires the development of decision-making tools, such as indicators that provide information on community responses to climatic change. Although monitoring marine environments, such as benthic habitats, is highly challenging, beach wrack monitoring may provide an alternative and complementary approach to inform changes in proximate intertidal and subtidal habitats under anthropogenic threats. However, the relationship between macrophyte in beach wrack and benthic macrophyte is not fully understood. In particular, the composition of beach wrack macrophyte communities in relation with climate has not been explored yet, although such research is a prerequisite for investigating the ability of macrophyte communities in beach wrack to monitor composition changes of benthic macroalgal and seagrass communities in the face of climate change. Here, we assessed the thermal and spatial patterns of thermal affinity of macroalgae and seagrass communities (84 taxa) in beach wrack sampled at 172 sites (from Saint-Jean-de-Luz, latitude 43.39°N, to Calais, latitude 50.89°N) along the Channel and Atlantic French coast. We also investigated the contribution of taxa to these patterns, and evaluated the latitudinal patterns of abundance of the most and least contributing taxa. We found that thermal affinity of macrophyte communities in beach wrack increased with sea-surface temperature and decreased with latitude. Latitudinal patterns were also identified at smaller spatial scales. Our findings, that are consistent with previously documented macroecological patterns of benthic macrophytes, suggest that beach wrack might provide insights into proximate benthic macrophyte communities, especially their composition in light of climate warming. We recommend further investigations to ensure the relevance of developing indicators of benthic habitats based on beach wrack. • Thermal affinity of beach wrack macrophytes increases with sea-surface temperature • Thermal affinity of macrophyte communities in beach wrack decreases with latitude • The latitudinal pattern is identified from a national scale to a local scale • Patterns are consistent with earlier research on benthic macrophytes • We suggest further exploring beach wrack's ability to inform on benthic macrophytes