Checklist of the macroalgae of Thau Lagoon (Herault, France), a hot spot of marine species introduction in Europe

A checklist of macroalgae in Thau Lagoon (Herault, France), based on literature records and new collections, is given. Using present-day taxonomy, 196 taxa and stadia of macroalgae have been identified to date. Sixty of these were no longer found in the samples, 36 were new to the lagoon, and the taxonomic identity of 15 species was amended. Twenty-five new introduced taxa were identified, giving a total of 45 introduced macroalgae for the flora (23% of total). Among these, 17 taxa and several genera (Acrorhrix, Chondrus, Dasysiphonia (?), Prionitis) are new to the Mediterranean Sea and 12 are new to Europe. The majority (43 taxa) may originate from the Pacific region, having arrived either directly or via other aquaculture sites. A highly probable vector of macroalgae introductions is the transfer of oysters, which appears, at present, to be the main vector of macrophyte introductions in the Mediterranean Sea, surpassing the Suez Canal. Thau Lagoon is revealed to be one of the major hot spots of marine species introduction in the Mediterranean Sea, Europe, but also in the world. This result is worrying as Thau is also an important exportation centre of living molluscs towards other French aquaculture sites and abroad. Therefore, the numerous exotic macroalgae, which have been acclimatized in the lagoon, have a high probability of being diffused throughout Europe and other Mediterranean countries. At present, the hard substrates of the Thau Lagoon are clearly dominated by the introduced species to the detriment of indigenous flora. The fate of these species in the Mediterranean Sea and Europe is unpredictable. It is probably not very realistic to hope that the accidental introductions linked to aquaculture activities will stop. A set of recommendations exists, however, that might contribute to limit the phenomenon. (C) 2001 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.