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Warming as well as species introductions have increased over the past centuries, however a link between cause and effect of these two phenomena is still unclear. Here we use distribution records (1813-2023) to reconstruct the invasion histories of marine non-native macrophytes, macroalgae and seagrasses, in the Mediterranean Sea. We defined expansion as the maximum linear rate of spread (km year-1) and the accumulation of occupied grid cells (50 km2) over time and analyzed the relation between expansion rates and the species' thermal conditions at its native distribution range. Our database revealed a marked increase in the introductions and spread rates of non-native macrophytes in the Mediterranean Sea since the 1960s, notably intensifying after the 1990s. During the beginning of this century species velocity of invasion has increased to 26 ± 9 km2 year-1, with an acceleration in the velocity of invasion of tropical/subtropical species, exceeding those of temperate and cosmopolitan macrophytes. The highest spread rates since then were observed in macrophytes coming from native regions with minimum SSTs two to three degrees warmer than in the Mediterranean Sea. In addition, most non-native macrophytes in the Mediterranean (>80%) do not exceed the maximum temperature of their range of origin, whereas approximately half of the species are exposed to lower minimum SST in the Mediterranean than in their native range. This indicates that tropical/subtropical macrophytes might be able to expand as they are not limited by the colder Mediterranean SST due to the plasticity of their lower thermal limit. These results suggest that future warming will increase the thermal habitat available for thermophilic species in the Mediterranean Sea and continue to favor their expansion.

The dataset provides data of geographic occurrences of non-native marine macrophytes in the Mediterranean Sea over 200 years. It includes occurrences in their native biogeographical range, along with sea surface temperature data for both the Mediterranean Sea and the native range. Additionally, it provides measures of spread, such as least coastal distances and the number of invaded grid cells, aiding in the calculation of linear spread rates, accumulated, and new invaded areas by non-native macrophytes in the Mediterranean Sea, as outlined in Wesselmann et al., 2024.

Effective monitoring of non-indigenous seaweeds and combatting their effects relies on a solid confirmation of the non-indigenous status of the respective 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 cylindracea and Colpomenia 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

Introduction of non-indigenous species can alter marine communities and ecosystems. In shellfish farming, transfer of livestock, especially oysters, is a common practice and potentially constitutes a pathway for non-indigenous introductions. Many species of seaweeds are believed to have been accidentally introduced in association with these transfers, but there is little direct evidence. We experimentally simulated the transfer of oysters from the Thau Lagoon (France). These transfers involved increasing periods of aerial emersion and additional brine and hot water treatments. The brine and hot water treatments were evaluated as a means of reducing the probability of algal introductions with oyster transfers. Shells were cultured for 40 days in experimental tanks to identify the macroalgae likely to be introduced with any oyster transfer. A total of 57 macroalgal taxa, including 16 taxa not indigenous to the Thau Lagoon, were recorded across all treatments and experiments. The abundance of some species increased in several cases following aerial emersion. Elimination treatments (immersion in brine or hot water) significantly reduced algal diversity, with hot water treatments resulting in no species or only tubular Ulva spp. present. The results support the hypothesis that oyster transfers are effective as primary and secondary vectors of macroalgal introductions. Relatively simple changes to the transfer practice (particularly hot water treatments) are suggested as a means of reducing the risk of non-indigenous algal introductions.

L'aménagement du fleuve Charente constitue un enjeu particulièrement important tant au niveau des activités de la bande côtière que pour le bassin versant. Au niveau du littoral charentais, et en particulier pour la conchyliculture, le fleuve Charente représente le principal apport d'eaux douces, et donc d 'éléments nutritifs, essentiels au développement de la production primaire et des larves d'huîtres. Ainsi, les apports d'eau douce sont importants pour le développement du phytoplancton (printemps, été, automne), le développement des larves d'huîtres en période estivale qui requiert une dessalure, et la qualité des huîtres en fin d'année. Les suivis des caractéristiques de la Charente sont par conséquent particulièrement importants et ont déjà fait l'objet d'études spécifiques à des fins de modélisation. Sur le plan régional la station du réseau hydrologique " RAZLEC ", située en sortie d'estuaire, constitue l 'un des cinq sites suivis depuis 1977 (Soletchnik et al., 1998 ; Faury et al., 1999). Toutefois, la fréquence bimensuelle de prélèvements ne permet pas une analyse fine des processus mis en jeu au niveau de cet estuaire. L'acquisition en continue de la salinité et de la température est un début de réponse à la compréhension globale de l'environnement de l'estuaire, en particulier afin de caractériser la Charente en période critique de débit d'étiage estivale. L'objectif de ce suivi est par conséquent le développement d'une base de données pluriannuelle en la matière afin d'étudier l'évolution temporelle de l'estuaire ainsi que l'impact des aménagements présents et futurs sur le Bassin versant (e.g., barrage de La Trézence). L'objectif 1999 est donc la mise en place du réseau et le début d'acquisition en continu in-situ de ces paramètres hydrologiques. Cette estimation des tendances sera à corréler à l'avenir avec un Indice de recrutement du secteur afin de définir les conditions optimales de captage de C. gigas.