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.

Marine forests play a vital role in shaping seascape complexity worldwide. In the Mediterranean Sea, they are mainly formed by macroalgal species belonging to the Cystoseira sensu lato group (i.e. Cystoseira, Ericaria and Gongolaria). These species are declining throughout Mediterranean coastlines due to multiple, often synergistic stressors. Among the principal biotic drivers of this regression is grazing by herbivores (i.e. the sea urchins Paracentrotus lividus and Arbacia lixula and the teleost Sarpa salpa), which can transform luxuriant marine forests into urchin barrens-ecosystems of reduced complexity, lower species richness and limited ecological function. A widely used management tool to preserve marine ecosystems is the implementation of Marine Protected Areas (MPAs), often including no-take zones (NTZs). In this context, we assessed the effectiveness of MPAs and NTZs in preserving Ericaria brachycarpa forests. To this end, two large-scale monitoring campaigns were conducted in the Nature Reserve of the Strait of Bonifacio (Corsica, France), the first in 2011 and the second in 2025. Sea urchin populations, algal cover and fish communities (in 2025 only) were surveyed using SCUBA diving. Our results revealed a drastic decline in E. brachycarpa cover, accompanied by a steady increase in urchin barrens and sea urchin abundance in NTZs between the two campaigns. Conversely, in fishing zones, E. brachycarpa populations remained stable while both urchin barrens and sea urchin abundance decreased. Synthesis and applications. This study demonstrates that NTZs can be counterproductive, as they may exacerbate the decline of marine forests by protecting sea urchins that are not effectively controlled by predators. We therefore call for a reassessment of management policies, including the targeted harvesting of herbivores where natural top-down control is absent.

In the Mediterranean Sea, seaweed marine forests, one of the most important benthic assemblages for the coastal ecosystem functioning, are shaping the rocky reefs. Among the Fucales, Gongolaria barbata (syn. Cystoseira barbata) is a several years living species usually growing in very shallow, sheltered and well-lit marine reef habitats and in coastal lagoons. The long-term change in its distribution in relation to a variety of disturbances has been assessed along the French Mediterranean coast, including Corsica and brackish lagoons, thanks to historical data dating back to the 19th century. The current distribution was established through an extensive survey conducted by snorkelling, encompassing approximately 3 000 km of coastline. A GIS analysis indicates that Gongolaria barbata can currently be considered as regionally extinct in French Catalonia and Western Provence and functionally extinct in the French Riviera. In Languedoc, the species is extinct in the open sea and only present in certain brackish lagoons despite severe repeated anoxic crises (malaïgues) and competition with a rich exotic flora introduced from the NE Atlantic and NW Pacific, especially the invasive Sargassum muticum. In contrast, the populations of G. barbata have remained stable in Eastern Provence and Corsica. The main possible causes of decline are uprooting, overgrazing by herbivores, habitat destruction and competition with introduced seaweeds. The relevance of ecological restoration of G. barbata populations was assessed.

Canopy-forming seaweeds, especially fucoids (Fucales, Phaeophyceae), constitute marine forests in the Mediterranean Sea that deliver key ecosystem services. However, escalating human pressures in coastal areas have led to habitat fragmentation, significantly impacting the dynamics of gene flow and evolutionary trajectories. In this study, we investigated population connectivity among 43 sites with Ericaria amentacea in the northwestern Mediterranean Sea. By integrating microsatellite genotyping and biophysical modelling, we aimed to evaluate the hypothesis that population connectivity is approximated by oceanographic connectivity, rather than coastal distance. While traditional approaches to oceanographic connectivity had focused on single-generation dispersal models, we adopted a multi-generation perspective. This involves employing a biophysical model that considers gene flow through multiple stepping-stone populations across the E. amentacea distribution and over distinct generations of dispersal. Using distance-based redundancy analysis (dbRDA), we found that multi-generation dispersal significantly contributes to genetic differentiation, surpassing the influence of coastal distance. Even so, genetic differentiation remained significant among all population pairs, suggesting that gene flow may be hindered by differential selection acting against migrants and/or obscured by the effects of local genetic drift. The latter is likely, given the species' low dispersal potential and self-compatibility, both of which promote small, spatially restricted breeding units. In addition, our results emphasized that oceanographic connectivity promoted long-distance dispersal across northern Corsica and Eastern Provence over a single generation through the drifting of fertile thalli, which might have contributed to moderate differentiation between local populations. Overall, this framework highlights the value of considering multi-generation dispersal across numerous intermediate stepping-stones for informing management strategies aimed at enhancing population connectivity and safeguarding genetic diversity in seaweeds.

In the Mediterranean Sea, overgrazing is a major factor in the degradation of algal communities. Marine protected areas (MPAs) are intended to ensure the conservation of biodiversity, but most Mediterranean MPAs are 'paper parks' (i.e. protected in theory, but with no enforcement) with inadequate protection. Along the French Mediterranean coast, less than 1% surface area of MPAs is actually protected by means of no-take zones (NTZs). This study examines the impact of different protection levels in the Calanques National Park, Marseilles, France, on the relationships among algal communities, herbivores, and predators. Using an ecosystem-based approach, we compared algal communities, sea urchin populations (Arbacia lixula and Paracentrotus lividus), and fish communities between NTZs and fishing zones. Our findings reveal that algal communities subjected to higher levels of protection (NTZs) are severely degraded, leading to a loss of habitat complexity. Fucoid marine forests are completely absent, replaced by encrusting algae (barren ground) or turf algae. Herbivore biomass (sea urchins and fish Sarpa salpa) is higher in the NTZs, and the presence of their predators does not cascade down to influence algal communities. We emphasize the importance of substrate topography as a key driver of sea urchin abundance. This MPA is insufficient to rebalance the trophic network and control herbivore populations. We strongly recommend implementing herbivore management measures to counterbalance the lack of natural predator-prey interactions. Moreover, the French policy of reinforcing sea urchin protection is counterproductive to preserving and restoring habitat complexity. Additionally, the practice of releasing thousands of juvenile sea urchins to restore populations should be discontinued.

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.

In the Mediterranean, the dune–beach ecosystem is characterized by the presence of thick deposits of dead leaves of the endemic seagrass Posidonia oceanica, called banquettes (Dune–Beach–Banquette ecosystem—DBB). This ecosystem plays an important role in the coupling between sea and land. The banquettes provide important ecosystem services: protection of beaches against erosion, contribution to the building of the dune, and a source of nitrogen for coastal vegetation. They are home to a rich and diverse invertebrate fauna that are consumed by other predatory invertebrates and seabirds. A conceptual model of the functioning of the DBB ecosystem and its relation with adjacent ecosystems has been outlined. When dead P. oceanica leaves return to the sea, which is the fate of most of the banquette, they constitute an important source of carbon and nutrients for coastal ecosystems and fisheries. Beach management, with the removal of banquettes and driftwood to meet the supposed requirements of beach users and tourists, is an ecological disaster, in addition to being an economic burden for coastal municipalities. Beach management methods that respect the interactions between the marine and terrestrial realms, which preserve the beaches from erosion and allow the return of the banquettes to the sea, and which take into account the real perceptions of beach users are feasible in the framework of the concept of the ‘ecological beach’.

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.

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.

The specific identification of three major morphotypes of the tropical holopelagic Sargassum species causing massive strandings on the African and Caribbean coastlines was attempted by morphological characterisation as well as quantitative and qualitative analyses of several metabolites. Of the 25 morphological variables studied on 208 samples from the North Atlantic Ocean, 22 were used to establish a dichotomous identification key, allowing without any doubt the identification of each morphotype based on their morphological criteria alone. We also attempted to differentiate morphotypes using chemical fingerprintings (HR-MAS NMR) and markers by analysing pigment level and composition using High Pressure Liquid Chromatography, terpene profiles by Thin Layer Chromatography, phenolic compound levels by the Folin-Ciocalteu assay and structures by 2D Nuclear Magnetic Resonance spectroscopy, and fatty acid composition by Gas Chromatography. While pigment level and composition, terpene profiles, and phenolic contents were not discriminating, quantification of eight fatty acids enabled the differentiation of the three morphotypes. Furthermore, phlorotannin purification permitted their structural characterisation allowing discrimination between the three morphotypes. Our study highlights the potential of the free fatty acid profile and phlorotannin structure as good chemomarkers in order to discriminate between the three morphotypes of holopelagic Sargassum.

Understanding connectivity patterns exhibited by endangered species living in fragmented habitats is fundamental to improving management and conservation actions. Such improvements can be particularly pressing at the trailing edges of these habitats, where populations are facing the greatest challenges from climate change, and appear even more crucial if the species is commercially harvested. Seascape genetics have been increasingly used to meet these needs. In this study, we examined connectivity patterns among 32 populations of the oarweed kelp Laminaria digitata located at the species' southern range limit. The distance (or sampling gap) between neighboring populations ranged from a few km to a few 100s of km. By genotyping 11 microsatellite markers, we aimed to (1) refine analyses of population structure; (2) test whether on-shelf islands are genetically more differentiated than mainland populations; (3) evaluate the relative importance of various abiotic conditions in shaping the genetic structure; and (4) evaluate if the relative importance of each environmental factor varied according to sampling schemes. Our analyses revealed a positive relationship between connectivity links and genetic diversity: populations with high levels of connectivity were genetically enriched while isolated populations showed signs of genetic erosion. The genetically impoverished populations corresponded to the southernmost populations as well as populations along the northern coast of Brittany (Locquirec, Saint-Malo Bay) and the northernmost population in Pas-de-Calais. By performing distance-based redundancy analysis on various sampling schemes, geographic distance appeared as the dominant factor influencing connectivity between populations separated by great distances, while hydrodynamic processes were the main factor when analyzing at a final spatial resolution.

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.

Since 2011, massive new strandings of holopelagic Sargassum have been reported on the coasts of the Caribbean, northern Brazil, Guiana, and West Africa, causing severe economic and ecological damage. Three common morphotypes (S. fluitans III, S. natans I, and S. natans VIII) were identified as responsible for these catastrophic events, with dominance shifts between them over time. However, the taxonomic status of these holopelagic Sargassum morphotypes remains unclear. Using an integrative taxonomy framework, combining a morphological study and molecular analyses, this study aimed to clarify their taxonomic status. Morphological analyses of 54 characters revealed no intermediate form between the three morphotypes, with the overall shape, nature of the axis, and size and shape of blades and vesicles being the most discriminating. An analysis of mitochondrial (IGS, cox2, cox3, mt16S rRNA, and nad6) and plastid (rbcL) markers confirmed the genetic divergence among the three morphotypes, with a lower level of divergence between the two S. natans morphotypes. Without additional molecular characterization, these morphotypes cannot be classified as three distinct species. However, due to their distinct morphological characteristics and sympatry within drifting aggregations, a revision of holopelagic species names is proposed, with Sargassum fluitans var. fluitans (for S. fluitans III), Sargassum natans var. natans (for S. natans I), and S. natans var. wingei (for S. natans VIII). This revision provides necessary clarity on the species involved in inundations of the tropical Atlantic.

ABSTRACTUnderstanding the environmental processes shaping connectivity can greatly improve management and conservation actions which are essential in the trailing edge of species’ distributions. In this study, we used a dataset built from 32 populations situated in the southern limit of the kelp speciesLaminaria digitata. By extracting data from 11 microsatellite markers, our aim was to (1) refine the analyses of population structure, (2) compare connectivity patterns and genetic diversity between island and mainland populations and (3) evaluate the influence of sampling year, hydrodynamic processes, habitat discontinuity, spatial distance and sea surface temperature on the genetic structure using a distance-based redundancy analysis (db-RDA). Analyses of population structure enabled to identify well connected populations associated to high genetic diversity, and others which appeared genetically isolated from neighboring populations and showing signs of genetic erosion verifying contrasting ecological (and demographic) status in Brittany and the English Channel. By performing db-RDA analyses on various sampling sizes, geographic distance appeared as the dominant factor influencing connectivity between populations separated by great distances, while hydrodynamic processes were the main factor at smaller scale. Finally, Lagrangian simulations enabled to study the directionality of gene flow which has implications on source-sink dynamics. Overall, our results have important significance in regard to the management of kelp populations facing pressures both from global warming and their exploitation for commercial use.

The worldwide decline of macroalgal forests is raising major concerns for the potentially negative consequences on biodiversity and ecosystem functions, pushing for the definition of specific conservation and restoration measures. Protecting and restoring these habitats requires detailed information on their distribution, ecological status, and drivers of decline. Here, we provide the most updated available information on the distribution of Mediterranean Cystoseira s.l. forests by conducting a comprehensive bibliographic survey of literature published from 2009 to 2021, complemented by unpublished data. We also provide insights into the ecological status of these forests and the stressors affecting them across the Mediterranean basin. Our results show that most Mediterranean coasts remain un(der)studied and that the available information is concentrated in spatially limited coastal areas, restricted to very few species. When the ecological status is reported, data is highly heterogeneous, making any comparisons problematic, what claims for the description and use of easy and standardized monitoring methods for comparative purposes. Drivers of decline of Cystoseira s.l. forest have been even less investigated and, therefore, still poorly characterized. Our results highlight that our current knowledge is still insufficient to implement effective conservation and restoration strategies at the basin scale but also regionally. We call for the urgent need for mapping and standardized monitoring of Cystoseira s.l. forests to obtain baseline information for future management strategies involving their conservation, the mitigation of the stressors threatening them and the restoration of the degraded forests.

The analysis of biological and ecological traits has a long history in evolutionary and ecological research. However, trait data are often scattered and standardised terminology that transcends taxonomic and biogeographical context are generally missing. As part of the development of a global trait database of marine species, we collated trait information for European seaweeds and structured the data within the standardised framework of the World Register of Marine Species (WoRMS). We collected 45 175 trait records for 21 biologically and ecologically relevant traits of seaweeds. This resulted in a trait database for 1745 European seaweed species of which more than half (56 %) of the records were documented at the species level, while the remaining 44 % were documented at a higher taxonomic level and subsequently inherited at lower levels. The trait database for European seaweeds will serve as a foundation for future research on diversity and evolution of seaweeds and their responses to global changes. The data will contribute to developing detailed trait-based ecosystem models and will be an important tool to inform marine conservation policies. The data are publicly accessible through the AlgaeTraits portal, https://doi.org/10.14284/574 (AlgaeTraits, 2022).

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.

The calcified red macroalga Lithophyllum byssoides, a very common midlittoral species in the western Mediterranean Sea, is a significant ecosystem engineer capable, under exposed and dim light conditions, of building wide and solid endemic bioconstructions near the mean sea level: the L. byssoides rims or 'trottoirs à L. byssoides'. Although the growth of the species is relatively rapid for a calcified alga, the construction of a large rim requires several centuries of near stable or slowly rising sea level. As the time scale of their formation is measured in centuries, L. byssoides bioconstructions constitute valuable and sensitive sea level markers. The health status of L. byssoides rims has been studied at two sites located far apart from each other (Marseille and Corsica), both in areas heavily impacted by humans and in areas with little impact (MPAs and unprotected areas). A health index is proposed: Lithophylum byssoides Rims Health Index. The main and inevitable threat is the rise in the sea level. This ecosystem would be the first case worldwide of marine ecosystem collapse resulting, indirectly, from man-induced global change.