Non-indigenous seaweeds in the Northeast Atlantic Ocean, the Mediterranean Sea and Macaronesia: a critical synthesis of diversity, spatial and temporal patterns
Authors: van der Loos Luna M. , Bafort Quinten , Bosch Samuel , Ballesteros Enric , Bárbara Ignacio , Bercibar Estibaliz , Blanfuné Aurélie , Bogaert Kenny , Bouckenooghe Silke , Boudouresque Charles-François , Brodie Juliet , Cecere Ester , Díaz-Tapia Pilar , Engelen Aschwin H. , Gunnarson Karl , Hamdy Shabaka Soha , Hoffman Razy , Husa Vivian , Israel Álvaro , Karremans Mart , Knoop Jessica , Le Gall Line , Maggs Christine A. , Mineur Frédéric , Parente Manuela , Perk Frank , Petrocelli Antonella , Rodríguez-Prieto Conxi , Ruitton Sandrine , Sansón Marta , Serrão Ester A. , Sfriso Adriano , Sjøtun Kjersti , Stiger-Pouvreau Valérie , Surget Gwladys , Thibaut Thierry , Tsiamis Konstantinos , Van De Weghe Lotte , Verlaque Marc , Viard Frédérique , Vranken Sofie , Leliaert Frederik , De Clerck Olivier 
Published: June 2023

Effective monitoring and combatting the effect of non-indigenous seaweeds relies on a solid confirmation of the non-indigenous status of the species. We critically analysed the status of presumed non-indigenous seaweed species reported from the Mediterranean Sea, the Northeast Atlantic Ocean and Macaronesia, resulting in a list of 140 species whose non-indigenous nature is undisputed. For an additional 87 species it is unclear if they are native or non-indigenous (cryptogenic species) or their identity requires confirmation (data deficient species). We discuss the factors underlying both taxonomic and biogeographic uncertainties and outline recommendations to reduce uncertainty about the non-indigenous status of seaweeds. Our dataset consisted of over 19,000 distribution records, half of which can be attributed to only five species (Sargassum muticum,Bonnemaisonia hamifera,Asparagopsis armata,Caulerpa cylindraceaandColpomenia peregrina), while 56 species (40%) are recorded no more than once or twice. In addition, our analyses revealed considerable variation in the diversity of non-indigenous species between the geographic regions. The Eastern Mediterranean Sea is home to the largest fraction of non-indigenous seaweed species, the majority of which have a Red Sea or Indo-Pacific origin and have entered the Mediterranean Sea mostly via the Suez Canal. Non-indigenous seaweeds with native ranges situated in the Northwest Pacific make up a large fraction of the total in the Western Mediterranean Sea, Lusitania and Northern Europe, followed by non-indigenous species with a presumed Australasian origin. Uncertainty remains, however, regarding the native range of a substantial fraction of non-indigenous seaweeds in the study area. In so far as analyses of first detections can serve as a proxy for the introduction rate of non-indigenous seaweeds, these do not reveal a decrease in the introduction rate, indicating that the current measures and policies are insufficient to battle the introduction and spread of non-indigenous species in the study area.

Highlights

Non-indigenous seaweed species in the Northeast Atlantic Ocean, the Mediterranean Sea and Macaronesia are critically reanalysed.

>19,000 distribution records revealed considerable variation in diversity of non-indigenous seaweed species in the study area.

Taxonomic and biogeographic uncertainties hamper a critical evaluation of the non-indigenous status of many seaweed species.