In coastal socio-ecosystems, the European Commission has proposed a set of indicators that could meet the objectives of the Marine Strategy Framework Directive. Most of them refer to the state of benthic habitats, a marine biodiversity compartment under major threat. However, biodiversity monitoring in the marine environment is logistically, technologically, and economically challenging. One possibility may be the monitoring of marine biodiversity “ex situ”, i.e. thanks to the material transported naturally along the sea to land continuum, but the relationship between benthic donor habitats and beach-casted macrophytes remains unclear. Only few studies have previously explored this relationship and they raised two questions: Could some wrack macrophyte species inform on the composition of proximate marine habitats? What is the contribution of local donor habitats in the composition of macrophyte wrack communities from one site to another? Here we provide answers to these questions from the sampling of macrophyte wracks on 131 beaches and the use of benthic habitat maps from the European Nature Information System. From linear regressions, we first show that the α-diversity of non-floating beach casted macrophytes reflects the heterogeneity of benthic habitats lying in straight marine buffers. Second, using non-linear modelling we show that this correlation peaks at 500 m offshore and maintains up to 1000 m far, which corresponds to the infralittoral seabed along our study coastline. Finally, regressions on distance matrices reveal that dissimilarities in wrack communities of non-floating macrophytes are more the result of differences in proximate benthic habitats rather than geographic patterns. While macrophyte wracks are already considered relevant indicators of geomorphological and ecological processes on coastal socio-ecosystem, this study advocates for their further consideration as ex situ indicators of benthic ecosystems.