Decline and local extinction of Fucales in the French Riviera: the harbinger of future extinctions?

The French Riviera is among the Mediterranean areas that have been subject to the most long-lasting anthropogenic influences with severe impact on the marine environment. Fucales are long-lived, habitat forming brown algae that constitute a good model for studying human impact on species diversity. We gathered all historical data (literature and herbarium vouchers), since the early 19th century, to reconstruct their past distribution. The present distribution was established on the basis of an extensive 7-year (2007-2013) survey of the 212-km shoreline (measured on a 1/2 500 map), by means of boating, snorkelling and scuba diving. A total of 15 taxa of Cystoseira and 3 taxa of Sargassum were reported. Upon comparison with historical data, 5 taxa were no longer observed (C. elegans, C. foeniculacea f. latiramosa, C. squarrosa, C. spinosa var. spinosa and S. hornschuchii) while C. jabukae, previously unrecorded, was observed. In addition to the 5, possibly extinct taxa locally, C. amentacea, C. barbata f. barbata, C. brachycarpa, C. crinita, C. sauvageauana and S. vulgare suffered a decline, while C. foeniculacea f. tenuiramosa, C. spinosa var. compressa and S. acinarium became nearly extinct. Cystoseira barbata f. barbata, C. brachycarpa, C. crinita and C. spinosa var. compressa that played significant functional roles in coastal communities in the past, can be considered as functionally extinct. A similar situation has already been reported, although on a smaller scale, in other Mediterranean localities. The following question, therefore, arises regarding the ecology of Fucales in the Mediterranean: are some species on the brink of extinction? Is their decline or possible extinction, as documented on the French Riviera, the harbinger of their extinction Mediterranean-wide?