Loss of fucoid algae along a gradient of urbanisation and relationships with the structure of benthic assemblages

Canopy-forming macroalgae of the genus Cystoseira are being lost in several areas of the Mediterranean Sea. Cystoseira amentacea var. stricta and C. compressa are common species in the Ligurian Sea; they are distributed in patches, but their abundance and distribution is locally variable. We investigated changes in relative cover, biomass and morphology of Cystoseira amentacea var. stricta and C. compressa with increasing urbanisation, and effects of the presence or absence of Cystoseira species on the composition and structure of understorey assemblages. C. amentacea var. stricta habitats were lost close to urban areas, while C. compressa slightly increased. The morphological characteristics of these 2 species were very variable in space and time, and did not vary with urbanisation. Assemblages lacking canopy differed markedly from Cystoseira-dominated assemblages, particularly assemblages dominated by Corallina elongata, the species most responsive to changes in habitat structure and urbanisation, which forms dense turfs in urban habitats lacking canopies. Marked species-specific differences between the assemblages dominated by the 2 Cystoseira species were detected. C. amentacea var. stricta is a key species maintaining habitat complexity and species diversity in Mediterranean rocky shores and we recommend additional conservation actions, such as habitat restoration by transplantation of this endangered species.