Maerl-associated macroalgae in the bay of Brest (Brittany, France)

International audienceMaerl beds are unique marine habitats hosting a great diversity of organisms while macroalgae are a major component of this diversity. The bay of Brest is one of the most studied coastal ecosystems in the world; in addition, it has a significant background concerning historical seaweeds check-lists associated to maerl beds. However, no recent work aimed at compiling and completing these data. In this study, a total of 7 subtidal and 3 intertidal maerl beds have been surveyed between 2020 and 2022. These data complete 4 previous inventories, giving the most accurate description of seaweeds colonizing maerl beds in the bay of Brest. The total number of macroalgal species reaches 170 among which 127 are Rhodophyta, 22 Phaeophyceae, and 21 Ulvophyceae, with 51 additions to the French and 19 additions to the European maerl beds check-list. A comparison with other maerl beds of the northeastern Atlantic coasts is compiled. It appears that the maerl beds from the bay of Brest are the most diverse in Atlantic France and among the most diverse in Europe. A description of the stratification of maerl beds and the associated macroalgal communities is proposed. It includes maerl species, encrusting species, maerl-entangling species, erect isolated species, and free-living species. In addition, specimens of the rarely recorded Rytiphlaea tinctoria, reaching its northernmost population in the bay, a possible introduced species/relict population, are described