Biological invasions are a major driver of biodiversity loss, causing profound and often irreversible ecological changes. Among these invaders, the brown alga Rugulopteryx okamurae provides a striking example. Since its arrival in 2016 in Calanques National Park (Marseille, southern France), it has drastically transformed underwater landscapes by reshaping native algal communities. To better understand the mechanisms underlying its colonisation, we conducted an experimental study involving the complete removal of existing algae within experimental quadrats. Using the photoquadrat method, we monitored the spatio-temporal dynamics of algal succession. In parallel, we performed an independent field survey to quantify seasonal changes in biomass over time. Our results show that turf-forming algae, characterised by rapid growth, are the first to recolonise cleared surfaces. This initial colonisation phase is rapidly followed by the massive establishment of R. okamurae, which reached 100% cover within 1 year. This expansion was accompanied by a strong restructuring of benthic communities and a marked decline of native algae, whose relative abundances dropped below 2% over time, highlighting the species' ability to thrive after disturbance. In addition, we observed that the emergence of the invasive alga Asparagopsis armata in April significantly reduced R. okamurae cover, suggesting biotic interference and potential allelopathic competition. Overall, R. okamurae stands out as a highly impactful invader whose rapid dominance and interactions with other invasive species emphasise the urgent need to improve our understanding and management of marine invasions to mitigate biodiversity loss.

Coralligenous reefs are among the most diverse Mediterranean ecosystems, particularly in the circalittoral zone. Shaped by calcified red algae, sponges, cnidarians, and bryozoans, they create a complex three-dimensional structure providing shelter for diverse fauna. These reefs develop either on steep rocky walls or as bioherms when calcified algae are the dominant organisms. Their structure and composition vary with location, depth, substrate, and environmental conditions. Assessing the status of such a complex ecosystem poses significant challenges. Ecosystem-based quality indices (EBQI) have already been applied successfully to various coastal Mediterranean habitats. Using a similar methodology, a new index, the Coralligenous Ecosystem-Based Quality Index (Cor-EBQI), was developed to assess the ecological status of the coralligenous ecosystem. The index incorporates the main functional compartments of these reefs, with each compartment weighted according to its importance in ecosystem functioning. Suitable descriptors were then selected to define five status classes for each compartment. A confidence index was also created to estimate data quality based on criteria such as methodology and expert judgement. Data from 63 sites along the French Mediterranean coast, including the Gulf of Lions, Provence, the French Riviera, Corsica, and Monaco, were analyzed. The ecological status ranged from bad to high, influenced by environmental conditions, geomorphology, anthropogenic pressures, and management practices. The Cor-EBQI is designed to meet the objectives of both the Habitats and Marine Strategy Framework Directives of the European Union. As such, it offers a practical tool for future monitoring networks across the Mediterranean Sea.

Monitoring marine biodiversity requires approaches that capture its full complexity through space and time. DNA metabarcoding coupled with Autonomous Reef Monitoring Structures (ARMS) is increasingly used for this purpose, yet most applications still pool all sessile fractions and rarely benchmark molecular ouputs against photographic observations. Here, we combined photographic analysis with cytochrome c oxidase I (COI) metabarcoding across ten north-western Mediterranean sites to test, compare, and refine ARMS-based monitoring protocols. We first optimized laboratory procedures (DNA extraction and polymerase choice) and applied the control-driven, replicate-aware VTAM pipeline to minimize false positives and ensure full traceability. We then conducted the first face-by-face comparison of α- and β-diversity between imaging and eDNA in which each individual ARMS face was metabarcoded separately rather than pooled. Metabarcoding detected ∼15× higher site-level richness and revealed stronger correlations with geographic distance and environmental gradients, whereas photography provided complementary information on macro-taxa and surface cover. For metabarcoding, processing each face separately yielded much higher richness and markedly stronger β-diversity–distance correlations than with the NOAA pooling protocol, demonstrating that pooling inflates sampling variance resulting in a loss of the ecological signal. Grouping faces into five structural categories offered a more operational alternative while further increasing α-diversity and strengthening β-diversity correlations. Overall, our results show that retaining ARMS microhabitat structure is critical for maximizing metabarcoding performance. Using five structural sessile fractions per ARMS combined with a control-driven bioinformatic workflow provides a reproducible, scalable framework for long-term eDNA monitoring and early detection of biodiversity change. ### Competing Interest Statement The authors have declared no competing interest. EMBRC France, OOB\_EMBRC FR\_AAP2018_n°2179 Agence Nationale de la Recherche, ANR-17-MART0001-01, ANR-17-MART0001-02, ANR-17-MART0001-03 ERA-Net Mar-TERA, id. 145 European Space Agency, No.4000141547/23/I-DT FEDER, under project 1166-39417

The Mediterranean Sea, a biodiversity hotspot, faces significant threats from non-indigenous species (NIS), which drive biodiversity changes. Over the past century, the introduction of NIS has accelerated due to maritime traffic, aquaculture, and interoceanic canals, fostering biological invasions. Marine protected areas (MPAs), established to preserve biodiversity, are increasingly impacted. This review quantified and characterized French Mediterranean MPAs, analyzing non-indigenous macroalgae distribution based on the existing literature and the authors’ observations. Results revealed widespread occurrence, with the highest NIS richness in strictly regulated MPAs; their proximity to large harbors highlights the paramount importance of the introduction pathways. In addition, there is a significant knowledge gap regarding the distribution of NIS within MPAs, complicating efforts to monitor and study these species effectively. These findings highlight the challenges in monitoring and managing invasions and the urgent need for controlling primary and secondary invasion pathways, within and outside the MPAs, international collaboration to control them, and enhanced funding for NIS monitoring. Without adaptive management, even strictly protected MPAs are vulnerable to the escalating impacts of invasive species.

Marine Spatial Planning (MSP) is fundamental to the management of marine resourcesand space. Many countries have drawn up national marine spatial plans that, as withland spatial planning, need to be transposed and adapted at local level, particularly inareas of significant socio-economic interest. Major coastal cities and adjacent marineareas are critical places for local MSP. However, there has been little investigation ofMSP implementation in these priority contexts. To narrow this gap, this study analyseshow the local authorities of seven major coastal cities of the NorthwesternMediterranean, located in France, Italy and Spain, are including the sea in theirplanning strategies, and what they are doing in terms of MSP. Examining their publicpolicies and spatial planning strategies sheds light on whether and how they aretransposing MSP, and with what objectives. Results show that local implementation ofMSP in these cities is highly heterogeneous, influenced both by national policies andlocal initiatives, and still rare. Among the seven cities, only Barcelona and Marseillestand out for their initiatives to plan and manage their marine area. This calls intoquestion the impact of the European and national maritime policies enacted over thepast decades to promote MSP.

The warming trend of the Mediterranean Sea is a long-term process. It has resulted in a northwards and westwards range expansion and abundance increase of thermophilic species, both native and non-indigenous, and in a shrinking of the range of cold-affinity species. Marine heatwaves (MHWs) are relatively short-term extreme episodes that are responsible for spectacular mortality events in some species and have been extensively reported in the literature. In contrast, the species that benefit from MHWs (the ‘winners’) have been much less studied. A record-breaking MHW occurred in 2022 in the north-western Mediterranean Sea. We focus on three ‘winner’ species, the thermophilic green macroalgae Penicillus capitatus and Microdictyon umbilicatum and the endemic seagrass Posidonia oceanica. Penicillus capitatus, which is mainly present in the area as an inconspicuous turf of entangled filaments (espera stage), produced the erect paintbrush-like stage where sexual reproduction takes place. Microdictyon umbilicatum, usually uncommon, bloomed to the point of clogging fishing nets. Finally, a mass flowering of P. oceanica occurred in late August–September, followed the following year (April–May 2023) by the extensive production and dissemination of fruits and seeds. Both processes, the long-term warming trend and one-off heatwaves, both ‘losers’ and ‘winners’, shape the change in structure and functioning of Mediterranean ecosystems.

Early-life stages play a key role in the dynamics of bipartite life cycle marine fish populations. Difficult to monitor, observations of these stages are often scattered in space and time. While Mediterranean coastlines have often been surveyed, no effort has been made to assemble historical observations. Here we build an exhaustive compilation of dispersal traits for coastal fish species, considering in situ observations and growth models (Di Stefano et al., 2023; https://doi.org/10.17882/91148). Our database contains over 110 000 entries collected from 1993 to 2021 in various subregions. All observations are harmonized to provide information on dates and geolocations of both spawning and settlement, along with pelagic larval durations. When applicable, missing data and associated confidence intervals are reconstructed from dynamic energy budget theory. Statistical analyses allow traits’ variability to be revisited and sampling biases to be revealed across taxa, space and time, hence providing recommendations for future studies and sampling. Comparison of observed and modelled entries provides suggestions to improve the feed of observations into models. Overall, this extensive database is a crucial step to investigate how marine fish populations respond to global changes across environmental gradients.

Infralittoral rocky reefs (IRR) are a coastal marine habitat subject to various anthropogenic pres-sures in the Mediterranean. An ecological assessment using an ecosystem-based approach appears essential to guide sustainable environmental management practices and meet the requirements of the European Union Marine Strategy and Habitats Directive. For this, the reef-EBQI has been developed at 96 sites along the French Mediterranean coast. We show an overall 'good' and 'moderate' IRR status based on the reef-EBQI. The status appears sensitive to the level of pressure, but more to local specificities. Management and geomorphology did not significantly impact the status. Comparing reef-EBQI scores with another ecosystem-based index, ECOfast, revealed a positive correlation but dissimilarities. Thus, the reef-EBQI is suitable for long-term monitoring in the Mediterranean but interpreting ecological status requires careful handling. Further work should include abiotic factors such as geomorphology and a suitable quantitative pressure index.

Among marine primary producers, macroalgae support complex and productive coastal food webs, but coastal primary production relies on terrigenous inputs and remineralized organic matter which both vary seasonally. An approach combining stable isotope and biochemical analyses enables a better characterization of macroalgae specificities and highlights environmental influences on their chemical signature. This study compared the isotopic signature and biochemical composition of 22 Mediterranean macroalgae belonging to Rhodophyta (red algae), Phaeophyceae (brown algae) and Chlorophyta (green algae) between March and November 2010 to capture the differences in species chemical signatures potentially driven by metabolic traits or environmental drivers. Carbon stable isotope values were evidenced as a good proxy of specific carbon metabolism: low values observed in red algae could be related to the reported absence of carbon concentrating mechanisms (CCMs) in this group while higher values were driven by strong CCM activity in green algae. Biochemical patterns also differed between groups: soluble carbohydrates were a major component for red algae, while lipids and proteins dominated in brown algae, and insoluble carbohydrate concentrations were high in green algae. Variation within species across two collection times could be related to environmental changes and algal metabolism. delta N-15 values confirm the efficiency of this parameter as a proxy of the impact of human influence in the Bay of Marseille.

Conservation of ecosystems is an important tool for climate change mitigation. Seagrasses, mangroves, saltmarshes and other marine ecosystems have particularly high capacities to sequester and store organic carbon (blue carbon), and are being impacted by human activities. Calls have been made to mainstream blue carbon into policies, including carbon markets. Building on the scientific literature and the French voluntary carbon standard, the 'Label Bas-Carbone', we develop the first method for the conservation of Posidonia oceanica seagrasses using carbon finance. This methodology assesses the emission reduction potential of projects that reduce physical impacts from boating and anchoring. We show how this methodology was institutionalized thanks to a tiered approach on key parameters including carbon stocks, degradation rates, and decomposition rates. We discuss future needs regarding (i) how to strengthen the robustness of the method, and (ii) the expansion of the method to restoration of seagrasses and to other blue carbon ecosystems.

The Mediterranean Sea is worldwide the area most affected by introduced species. The Suez Canal (Lessepsian species) and shellfish aquaculture are the main routes for introduction, in addition to shipping (fouling, clinging and ballast waters). Two non-indigenous Lophocladia (Montagne) F.Schmitz species have been recorded from the Mediterranean Sea: the Red Sea and Indian Ocean L. lallemandii (Montagne) F.Schmitz and the western tropical Atlantic Ocean L. trichoclados (C.Agardh) F.Schmitz. On the basis of molecular data, Golo et al. (2023, 2024) concluded that the species which invaded the Mediterranean only belongs to the western Atlantic L. trichoclados. Until the mid-2010s, the north of the western basin was the only Mediterranean area not invaded by this species. In the present paper, we report for the first time the presence of L. trichoclados from this region in Provence and Corsica. Interestingly, L. trichoclados does not occur and bear reproductive organs in spring but in autumn and winter, although it is a species of tropical affinity.

La posidonie Posidonia oceanica est endémique de la Méditerranée. C’est une plante à fleurs issue d’ancêtres terrestres qui sont retournés au milieu marin, comme l’on fait plus tard les ancêtres des cétacés. Elle est l’espèce ingénieure d’un écosystème que ses caractéristiques rapprochent des forêts terrestres. Cet écosystème fournit à l’Homme des services considérables : pêche, production de sable alimentant les plages, protection des plages contre l’érosion, séquestration du CO2, etc. Les principales menaces qui pesaient, au xxe siècle, sur la posidonie étaient la pollution, l’urbanisation littorale et la surpêche. Aujourd’hui, tout au moins au sein de l’Union européenne, ce sont l’ancrage, le chalutage, et la surpêche. En outre, la destruction des banquettes de feuilles mortes de posidonies sur les plages, pour le supposé confort des baigneurs, est une très grave erreur écologique et économique. Quant au réchauffement climatique, il ne constitue pas une réelle menace pour la posidonie.

Since 2011, the Caribbean coasts have been subject to episodic influxes of floating Sargassum seaweed of unprecedented magnitude originating from a new area "the Great Atlantic Sargassum Belt" (GASB), leading in episodic influxes and mass strandings of floating Sargassum. For the biofilm of both holopelagic and benthic Sargassum as well as in the surrounding waters, we characterized the main functional groups involved in the microbial nitrogen cycle. The abundance of genes representing nitrogen fixation (nifH), nitrification (amoA), and denitrification (nosZ) showed the predominance of diazotrophs, particularly within the GASB and the Sargasso Sea. In both location, the biofilm associated with holopelagic Sargassum harboured a more abundant proportion of diazotrophs than the surrounding water. The mean δ15N value of the GASB seaweed was very negative (-2.04‰), and lower than previously reported, reinforcing the hypothesis that the source of nitrogen comes from the nitrogen-fixing activity of diazotrophs within this new area of proliferation. Analysis of the diversity of diazotrophic communities revealed for the first time the predominance of heterotrophic diazotrophic bacteria belonging to the phylum Proteobacteria in holopelagic Sargassum biofilms. The nifH sequences belonging to Vibrio genus (Gammaproteobacteria) and Filomicrobium sp. (Alphaproteobacteria) were the most abundant and reached, respectively, up to 46.0% and 33.2% of the community. We highlighted the atmospheric origin of the nitrogen used during the growth of holopelagic Sargassum within the GASB and a contribution of heterotrophic nitrogen-fixing bacteria to a part of the Sargassum proliferation.

Over the centuries, the Mediterranean Sea has been significantly impacted by biological invasions, hosting more than 1000 non-indigenous species. In the early 2000s, the brown alga Rugulopteryx okamurae, native to the northwestern Pacific Ocean, was introduced to the Thau Lagoon (Occitania, France) due to extensive shellfish farming activities. Since 2016, this species has proliferated, completely covering the rocky habitats along the Marseille coasts (Provence, France) and becoming the dominant alga. The impact of R. okamurae on flora and fauna was investigated by comparing rocky benthic assemblages of invaded and non-invaded sites. Results showed a change in the communities, with a significant decrease in species diversity and a homogenization of algal and invertebrate populations in invaded sites. Rugulopteryx okamurae is a habitat transformer species inducing a significant habitat shift. Rugulopteryx okamurae also seems to affect the different algal strata of the benthic community, thus impacting the entire canopy. However, a weak effect of R. okamurae was observed on the functional structure of native algal communities, comparing invaded and non-invaded areas. This lack of structural variation between these zones could be explained by the degradation of habitats within the Calanques National Park, due to decades of intense overgrazing by Sarpa salpa and sea urchins. But an impact on the ecosystem functioning should not be excluded. It is noteworthy that R. okamurae, a species presumed to prefer colder waters, has shown resistance to successive marine heat waves, allowing it to continue spreading and dominating without impairment.

A significant challenge in the integration of ecosystem services into decision-making processes lies in effectively capturing the dynamics of marine socio-ecological systems, including their evolutionary pathways, equilibrium states, and tipping points. This paper explores the evolutionary trajectories of a vital marine ecosystem endemic to the Mediterranean Sea: the Posidonia oceanica seagrass meadows, in response to various drivers of change. A state-and-transition model is employed to assess the ecosystem services provided by P. oceanica across different states defined by selected transitions, such as overfishing, fragmentation, pollution, and invasion by non-native species. To apply this model, scientific expertise is combined with field data generated using the Ecosystem-Based Quality Index to evaluate the conservation status of P. oceanica. This integrated approach allows for the representation of the ecosystem services offered by the meadows across different states, leveraging ecological data. The findings highlight the disproportionate impact on provisioning services, particularly sea urchins and commercial fish production, which suffer the most under various stressors. Notably, when these services decline to critical levels, the meadows cease to provide significant benefits. Finally, a synthesized representation is presented, merging ecological insights with monitoring data, offering a framework that is more accessible to stakeholders and decision-makers.

Biological invasions are one of the main global threats to biodiversity in terrestrial, freshwater and marine ecosystems worldwide, requiring effective inventorying and monitoring programs. Here, we present an updated list of non-indigenous species in French marine and transitional waters. Focused on eukaryote pluricellular species found throughout the three metropolitan French marine regions (Western Mediterranean Sea, Bay of Biscay and the Northern Seas), a total of 342 non-indigenous, including 42 cryptogenic, species are listed as having been introduced since the 13th century. The majority of the species originated from the temperate Northern Pacific. They mainly arrived through both ballast and hull fouling and also are associated with shellfish farming activities. Most of them have been introduced since the 1970s, a time when maritime and aquaculture trade intensified. Despite important human-aided opportunities for species transfer between the three marine regions (for instance, via recreational boating or aquaculture transfers), only a third of these NIS are common to all regions, as expected due to their environmental specificities.

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 insufficient to battle the introduction and spread of non-indigenous species in the study area.