The diversity of seaweeds on maerl in the NE Atlantic

Maerl beds worldwide face habitat destruction (e.g. from dredging, fishing gear and fish farms), the combined pressures of ocean warming and acidification, and the spread of invasive species. Maerl beds have high conservation status in European legislation, yet their associated flora is poorly known. Here, we evaluate the known macroalgal diversity of NE Atlantic maerl beds from Svalbard to Portugal. Maerl beds occur from the low intertidal down to 51 m in the clearest waters. To date, 349 macroalgal species have been recorded on maerl in the NE Atlantic (67 % are Rhodophyta), a remarkable 30 % of the total seaweed diversity in this region. Eleven non-native species have been recorded on Atlantic European maerl beds, the most widely distributed being phases of Bonnemaisonia hamifera (“Trailliella intricata”), Asparagopsis armata (“Falkenbergia rufolanosa”), Antithamnionella spirographidis and Heterosiphonia japonica. The flora of maerl beds off Iceland and Norway is poorly known, but maerl beds off Britain, Ireland, France and Spain have been surveyed extensively and support several species that are maerl specialists (i.e. Cruoria cruoriaeformis, Cladophora rhodolithicola, Gelidiella calcicola). Our observations of G. calcicola and Gelidium maggsiae are new records for Portugal. Maerl beds in the Algarve have many of the same macroalgal species as Mediterranean maerl beds, but they are not as floristically diverse as those in Galicia because they are confined to deeper water. Our census provides a baseline that can be used to assess changes to these habitats over the coming years.