Mussel reefs promote taxonomic biodiversity and host a unique assemblage of mobile marine fauna in a coastal area of poor ecological status

Coastal ecosystems worldwide face increasing anthropogenic pressures, with implications for the local ecological status and biodiversity. Using a temperate coastal ecosystem in poor ecological condition, this study revealed the importance of blue mussel ( Mytilus spp.) reefs in supporting mobile marine fauna (e.g., fish). Using baited remote underwater video systems (BRUVS), we compared mobile marine fauna associated with A) high coverage mussel reefs in shallow and deeper parts of the area and B) corresponding low coverage soft-bottom sites. Our BRUVS findings demonstrate that blue mussel reefs harbor a higher taxonomic diversity of mobile fauna compared to the corresponding soft-bottom sites. Furthermore, the species community associated with mussel reefs exhibits distinct composition from the soft-bottom sites. Despite efforts to mitigate harmful eutrophication, the poor ecological status of the coastal ecosystem highlights ongoing challenges in coastal management. Our results have important implications for management of coastal resources and conservation of marine biodiversity. Despite its environmental challenges, the examined ecosystem serves as a representative case for many temperate waterbodies emphasizing the need for improved management outcomes. Incorporating mussel reef conservation and restoration into temperate water body management actions could enhance ecosystem resilience and local biodiversity in many coastal ecosystems subjected to anthropogenic pressures. • Mussel reefs boost mobile fauna diversity in degraded Helnæs Bay. • Unique fauna assemblages in shallow vs. deep mussel reefs identified. • Mussel reefs crucial for coastal conservation; need enhanced protection.