Dead Matte Research Articles

Evaluating the extent and impact of the extreme Storm Gloria on Posidonia oceanica seagrass meadows

Extreme storms can trigger abrupt and often lasting changes in ecosystems by affecting foundational (habitat-forming) species. While the frequency and intensity of extreme events are projected to increase under climate change, its impacts on seagrass ecosystems remain poorly documented. In January 2020, the Spanish Mediterranean coast was hit by Storm Gloria, one of the most devastating recent climate events in terms of intensity and duration. We conducted rapid surveys of 42 Posidonia oceanica meadows across the region to evaluate the extent and type of impact (burial, unburial and uprooting). We investigated the significance of oceanographic (wave impact model), geomorphological (latitude, depth, exposure), and structural (patchiness) factors in predicting impact extent and intensity. The predominant impact of Storm Gloria was shoot unburial. More than half of the surveyed sites revealed recent unburial, with up to 40 cm of sediment removed, affecting over 50 % of the meadow. Burial, although less extensive, was still significant, with 10–80 % of meadow cover being buried under 7 cm of sediment, which is considered a survival threshold for P. oceanica. In addition, we observed evident signs of recently dead matte in some meadows and large amounts of detached drifting shoots on the sea bottom or accumulated as debris on the beaches. Crucially, exposed and patchy meadows were much more vulnerable to the overall impact than sheltered or continuous meadows. Given how slow P. oceanica is able to recover after disturbances, we state that it could take from decades to centuries for it to recoup its losses. Seagrass ecosystems play a vital role as coastal ecological infrastructure. Protecting vulnerable meadows from anthropogenic fragmentation is crucial for ensuring the resilience of these ecosystems in the face of the climate crisis.

Functional traits of polychaetes change between different types of Posidonia oceanica habitats

Meadows of Posidonia oceanica harbor rich biodiversity and ecosystem functions, yet Biological Traits Analysis on the resident benthic communities are lacking. This study aims to provide insight on the functional diversity of polychaetes communities, a dominant benthic group, between the different habitat types of P. oceanica (plain meadow, strips/patches and dead matte), as well as pilot indicators of habitat modification. The results showed how specific traits relate to the different habitat types. Plain meadow was different to strips/patches and dead matte in functional composition, diversity and thus, the ecosystem functions involved. However, an overlap was observed in functional composition between dead matte and living P. oceanica, due to the remaining matte structure. This highlights the importance of the former on ecosystem functioning and the serious consequences of its current exclusion from conservational legislation. In addition, the classification of species to ecosystem engineering types showed interesting potential as an indicator.

The Importance of Dead Seagrass (Posidonia oceanica) Matte as a Biogeochemical Sink

We assessed the potential of dead seagrass Posidonia oceanica matte to act as a biogeochemical sink and provide a coherent archive of environmental change in a degraded area of the Mediterranean Sea (Augusta Bay, Italy). Change in sediment properties (dry bulk density, grain size), concentration of elements (Corg, Cinorg, N, Hg) and stable isotope ratios (δ13C, δ 15N) with sediment depth were measured in dead P. oceanica matte and unvegetated (bare) sediments in the polluted area, and an adjacent P. oceanica meadow. Principal Component Analysis (PCA) revealed a clear clustering by habitat, which explained 72% of variability in our samples and was driven mainly by the accumulation of N and Hg in finer sediments of the dead matte. Assessment of the temporal trends of Corg, N and Hg concentrations in the dead matte revealed changes in the accumulation of these elements over the last 120 years, with an increase following the onset of industrial activities 65 y BP (i.e., yr. 1950) that was sustained even after seagrass loss around 35 y BP. Despite a decrease in Hg concentrations in the early 1980s following the onset of pollution abatement, overall Hg levels were 2-fold higher in the local post-industrial period, with a Hg enrichment factor of 3.5 in the dead matte. Mean stocks of Corg, N and Hg in 25 cm thick sediment deposits (4.08 ± 2.10 kg Corg m-2, 0.14 ± 0.04 kg N m-2, 0.19 ± 0.04 g Hg m-2) and accumulation in the last 120 yr (35.3 ± 19.6 g Corg m-2 y-1, 1.2 ± 0.4 g N m-2 y-1, 0.0017 ± 0.0004 g Hg m-2 y-1) were higher in the dead matte than bare sediment or adjacent P. oceanica meadow. Our results indicate that dead P. oceanica matte maintained its potential as a biogeochemical sink and, like its living counterpart, dead matte can serve as an effective archive to allow for reconstructing environmental change in coastal areas of the Mediterranean where severe perturbations have led to P. oceanica loss. Appropriate management for contaminated areas should be prioritized to prevent release of pollutants and carbon from dead mattes.

Spatiotemporal Trends Observed in 20 Years of Posidonia oceanica Monitoring along the Alicante Coast, Spain

Posidonia oceanica meadows, known to be valuable marine ecosystems, have been reported to be in decline as a result of human activities in recent decades. However, it is still controversial if this decline is a global phenomenon or it is caused by specific disturbances related to human development at a local scale. In order to evaluate changes in P. oceanica meadows, in this study, monitoring data obtained at 14 stations along the Mediterranean coast near Alicante, Spain, over a 20-year period were analyzed. Field data were obtained through the citizen science project POSIMED, which had the aim of carrying out annual monitoring of both shallow and deep P. oceanica meadows along the coast near Alicante and determining whether their ecological status was changing over time. The percentage cover of living P. oceanica and dead matte and shoot density data were used to assess the ecosystem status and to determine whether there had been an overall regional decline in seagrass over the 20-year period. Both cover and density data showed a significant positive trend at most locations. However, the amount of dead matte was noted to slightly increase with time while six shallow and one deep station showed a negative P. oceanica cover trend, indicating that in certain locations meadow regression might be taking place. Shoot density decreased with depth and increased with the amount of rock cover; its correlation with the dead matte percentage was unclear, which probably means that a range of different factors can result in the presence of dead plants. These results support the idea that local disturbances are the cause of seagrass decline in the Mediterranean, thus demonstrating the need for management plans that focus on local stressors of P. oceanica meadows at specific locations. Long-term, large-scale monitoring allows the ecosystem status in the western Mediterranean to be assessed; however, local disturbances can also affect specific locations.

Posidonia oceanica L. (Delile) meadows regression: Long-term affection may be induced by multiple impacts

Coastal development has an undeniable impact on marine ecosystems resulting in the detriment of the more sensible communities. Posidonia oceanica meadows are climax communities which offer a wide variety of ecosystem services both ecological and socio-economic. Human-derived impact on these habitats has been widely assessed although conclusions may vary depending on the area. P. oceanica meadow regression next to the city of Alicante (SE Spain) was analyzed on the long term (1984–2014) using bionomic cartographies and side-scan sonar images and, during the last two decades (2003–2021), using cover percentage and shoot density descriptors in the remaining meadow. Results showed a 25% colonized area reduction since 1984, this process being more rapid during the 1984–1994 period and decreasing with time. Cover and density have suffered a significant decrease in the last 20 years, mainly in the upper limit of the meadow. Dead matte cover was also assessed and have shown a significant increase in the same period following an inverse trend with the other metrics. There are several coastal impacts which have co-occurred in the area in the last few decades (port enlargement, brine and sewage discharges, industrial activity) thus resulting in the regression of the meadow. The existing negative trend of the measured descriptors indicate the necessity of implementing management actions which focus on the present sources of impact and actively reduce their effect on P. oceanica beds.

Distribution of the seagrass Halophila stipulacea: A big jump to the northwestern Mediterranean Sea

The Red Sea and Indian Ocean seagrass Halophila stipulacea entered the Mediterranean in the late 19th century via the Suez Canal. Here, we report on the discovery of a population of H. stipulacea covering 16.5 ha off the harbour of Cannes, French Riviera, France. This represents the northernmost locality of the species and a jump of 350 km to the north from its closest western Mediterranean locality (Razza di Juncu in Sardinia). At Cannes, it was found dwelling between 11 and 17 m depth on a dead matte of Posidonia oceanica. The species has most probably been introduced through mega-yacht or cruise ship anchoring since the Bay of Cannes is among the most highly-frequented places for luxury yachting and tourism. The lack of effective regulation to prevent the introduction of non-indigenous species in France and Europe is highlighted.

Ocean Acidification and Mollusc Settlement in Posidonia oceanica Meadows: Does the Seagrass Buffer Lower pH Effects at CO2 Vents?

Ocean acidification has been broadly recognised to have effects on the structure and functioning of marine benthic communities. The selection of tolerant or vulnerable species can also occur during settlement phases, especially for calcifying organisms which are more vulnerable to low pH–high pCO2 conditions. Here, we use three natural CO2 vents (Castello Aragonese north and south sides, and Vullatura, Ischia, Italy) to assess the effect of a decrease of seawater pH on the settlement of Mollusca in Posidonia oceanica meadows, and to test the possible buffering effect provided by the seagrass. Artificial collectors were installed and collected after 33 days, during April–May 2019, in three different microhabitats within the meadow (canopy, bottom/rhizome level, and dead matte without plant cover), following a pH decreasing gradient from an extremely low pH zone (pH < 7.4), to ambient pH conditions (pH = 8.10). A total of 4659 specimens of Mollusca, belonging to 57 different taxa, were collected. The number of taxa was lower in low and extremely low pH conditions. Reduced mollusc assemblages were reported at the acidified stations, where few taxa accounted for a high number of individuals. Multivariate analyses revealed significant differences in mollusc assemblages among pH conditions, microhabitat, and the interaction of these two factors. Acanthocardia echinata, Alvania lineata, Alvania sp. juv, Eatonina fulgida, Hiatella arctica, Mytilys galloprovincialis, Musculus subpictus, Phorcus sp. juv, and Rissoa variabilis were the species mostly found in low and extremely low pH stations, and were all relatively robust to acidified conditions. Samples placed on the dead matte under acidified conditions at the Vullatura vent showed lower diversity and abundances if compared to canopy and bottom/rhizome samples, suggesting a possible buffering role of the Posidonia on mollusc settlement. Our study provides new evidence of shifts in marine benthic communities due to ocean acidification and evidence of how P. oceanica meadows could mitigate its effects on associated biota in light of future climate change.

Poor success of seagrass Posidonia oceanica transplanting in a meadow disturbed by power line burial

Local disturbances drive the decrease of the area covered by Posidonia oceanica in the Mediterranean. Mechanical impacts during the development of coastal infrastructures alter sea floor and the recolonization of benthic community will depend on the recovery of pre-disturbance environmental conditions and on the intrinsic characteristics of the local community that was disturbed. We transplanted 468 rhizome fragments and 450 seedlings of P. oceanica in a meadow disturbed by the trenching and deployment of a power line to evaluate the suitability of the disturbed sea floor for rehabilitating P. oceanica meadows. We quantify and compare the survivorship and vegetative development of the transplanted/planted (i.e. fragments/seedlings) material in the two types of the unconsolidated substrata left after infrastructure deployment works finished: sand and burlap bags filled with coarse gravel. The latter was used as a corrective measure for topographic restoration. Three experimental plots with sixteen transplanted fragments or twenty-five seedlings were placed at each substratum type at three different depths (i.e. 15, 20 and 25 m). Our results show that the transplanting of P. oceanica rhizome fragments in the disturbed substrata had low survival rates (0–31%) after 40–48 months. The survivorship of seedlings was lower than that of fragments. Our results highlight the importance of substratum for P. oceanica recovery after mechanical impact; disturbed, non-consolidated substrata will preclude P. oceanica rehabilitation through planting. Preservation of meadow substratum (i.e. dead matte) is a critical element that coastal infrastructure projects should consider to enable future recovery of P. oceanica meadows.

Performance Assessment of Posidonia oceanica (L.) Delile Restoration Experiment on Dead matte Twelve Years after Planting—Structural and Functional Meadow Features

Following the restoration of natural conditions by reducing human pressures, reforestation is currently considered a possible option to accelerate the recovery of seagrass habitats. Long-term monitoring programs theoretically represent an ideal solution to assess whether a reforestation plan has produced the desired results. Here, we report on the performance of a 20 m2 patch of Posidonia oceanica transplanted on dead matte twelve years after transplantation in the Gulf of Palermo, northwestern Sicily. Photo mosaic performed in the area allowed us to detect 23 transplanted patches of both regular and irregular shape, ranging from 0.1 to 2.7 m2 and an overall surface close to 19 m2. Meadow density was 331.6 ± 17.7 shoot m−2 (currently five times higher than the initial value of 66 shoots m−2), and it did not show statistical differences from a close by natural meadow (331.2 ± 14.9). Total primary production, estimated by lepidochronology, varied from 333.0 to 332.7 g dw m2/year, at the transplanted and natural stand, respectively. These results suggest that complete restoration of P. oceanica on dead matte is possible in a relatively short time (a decade), thus representing a good starting point for upscaling the recovery of the degraded meadows in the area.

Structural Changes of Seagrass Seascapes Driven by Natural and Anthropogenic Factors: A Multidisciplinary Approach

Seascape ecology has been widely applied to marine habitats, including seagrass meadows, through various approaches all over the world for the past 30 years. However, these methods mainly study seagrass meadows on a single spatial scale and monitor a single driver of heterogeneity. In addition, few assess the seascape’s structural evolution. This creates gaps between the scientific data provided and those required by environmental managers and stakeholders in charge of seagrass meadow conservation. To meet their expectations, we developed in this paper a new multidisciplinary approach based on the coupling of mapping techniques, particle flux and biometric investigations in a Mediterranean Bay, the Calvi Bay (Corsica, France), to assess the structural evolution of Posidonia oceanica (L.) Delile meadows subject to disturbances. We focused our investigations on the structural characteristics, the spatial dynamics and the particle fluxes of natural sand areas generated by bottom currents and dead matte patches ensued from anchoring damages at 10, 15 and 20 m depth. Natural sand patches and anchoring patches differed in size, the first the largest. They also displayed different erosion-colonization dynamics. Natural sand patches were eroded at a mean speed of 12 cm.a-1 and colonized at a rate of 7 cm.a-1. Anchoring patches showed a mean erosion speed of 3.5 cm.a-1 and a colonization rate of 6.5 cm.a-1. Regarding particle fluxes, continuous meadow and natural patch sedimentation and resuspension rates were 3.7 gDW.m-2.d-1 and 4.1 gDW.m-2.d-1 in average, respectively. In contrast, anchoring patches at 20 m depth acted as sediment traps (112.60 gDW.m-2.d-1 in winter) and showed a higher particle resuspension rate. Our results highlighted the dichotomous dynamics of seagrass seascapes influenced by natural and anthropogenic factors (here anchoring). With an upscaling approach, together with the newest mapping tools of marine habitats, we suggest a new method to study the evolution of seagrass meadows at a large spatial scale. Due to the slow growth of P. oceanica and the centimetric annual evolution of patches revealed in the present study, it is obvious that a metric positioning accuracy is not sufficient to properly investigate the spatial evolution of P. oceanica seascapes.

A predictive model based on multiple coastal anthropogenic pressures explains the degradation status of a marine ecosystem: Implications for management and conservation

During the last fifty years, there has been a dramatic increase in the development of anthropogenic activities, and this is particularly threatening to marine coastal ecosystems. The management of these multiple and simultaneous anthropogenic pressures requires reliable and precise data on their distribution, as well as information (data, modelling) on their potential effects on sensitive ecosystems. Focusing on Posidonia oceanica beds, a threatened habitat-forming seagrass species endemic to the Mediterranean, we developed a statistical approach to study the complex relationship between human multiple activities and ecosystem status. We used Random Forest modelling to explain the degradation status of P. oceanica (defined herein as the shift from seagrass bed to dead matte) as a function of depth and 10 anthropogenic pressures along the French Mediterranean coast (1700 km of coastline including Corsica). Using a 50 × 50 m grid cells dataset, we obtained a particularly accurate model explaining 71.3% of the variance, with a Pearson correlation of 0.84 between predicted and observed values. Human-made coastline, depth, coastal population, urbanization, and agriculture were the best global predictors of P. oceanica's degradation status. Aquaculture was the least important predictor, although its local individual influence was among the highest. Non-linear relationship between predictors and seagrass beds status was detected with tipping points (i.e. thresholds) for all variables except agriculture and industrial effluents. Using these tipping points, we built a map representing the coastal seagrass beds classified into four categories according to an increasing pressure gradient and its risk of phase shift. Our approach provides important information that can be used to help managers preserve this essential and endangered ecosystem.

ALEX index enables detection of alien macroalgae invasions across habitats within a marine protected area

A modified version of the ALien Biotic IndEX (ALEX) has been recently proposed to evaluate biological invasions in macroalgal assemblages. ALEX was applied in a Marine Protected Area where a recreational-fishing port is present testing the following hypotheses: ALEX increases with the distance from the port, it changes between the two directions off the port and it changes among three different habitats: Cystoseira beds, algal turf and dead matte of the seagrass Posidonia oceanica. A total of 78 native macroalgal taxa and 4 introduced species were found, the Chlorophyta Caulerpa cylindracea and the Rhodophyta Apoglossum gregarium, Acrothamnion preissii and Womersleyella setacea. All study sites were in high quality status highlighting that the assemblages investigated were at an early stage of NIS invasion. However, ALEX detected different values among conditions and habitats within the MPA, suggesting a local dynamics of NIS spread and different resistance to invasion of the investigated habitats.

Use of Posidonia oceanica seedlings from beach-cast fruits for seagrass planting

Abstract Posidonia oceanica seedlings produced from beach-cast fruits were planted in an area where this Mediterranean seagrass was lost as a consequence of fish-farming. The effects of substratum type (dead matte vs. meadow) and planting level (above vs. below bottom surface) on seedling survivorship and leaf development were evaluated after 3 years. Seedling capacity to resist uprooting was also tested by comparing survivorship and leaf development after 2 years in seedlings firmly anchored in dead matte by artificial means (mesh-pot) with those planted without any anchoring other than their roots. Survivorship in dead matte was 44% after 3 years and was not affected by planting level. Seedling planting inside the meadow led to complete mortality after 3 years. Seedlings planted below the bottom surface branched during the second year and 50% had produced from one to five additional shoots after 3 years. After 2 years, survivorship and leaf development of seedlings planted with mesh-pot anchoring was similar to that of seedlings planted without anchoring. Current environmental conditions in the impacted area allow P. oceanica seedlings to survive, but natural recovery might be limited by propagule availability. Planting of seedlings in dead matte without anchoring does not affect survivorship and leaf development and might be a feasible technique for restoration of P. oceanica meadows.

Determinants of Caulerpa racemosa distribution in the north-western Mediterranean

Predicting community susceptibility to invasion has become a priority for preserving biodiversity. We tested the hypothesis that the occurrence and abundance of the seaweed Caulerpa racemosa in the north-western (NW) Mediterranean would increase with increasing levels of human disturbance. Data from a survey encompassing areas subjected to different human influences (i.e. from urbanized to protected areas) were fitted by means of generalized linear mixed models, including descriptors of habitats and communities. The incidence of occurrence of C. racemosa was greater on urban than extra-urban or protected reefs, along the coast of Tuscany and NW Sardinia, respectively. Within the Marine Protected Area of Capraia Island (Tuscan Archipelago), the probability of detecting C. racemosa did not vary according to the degree of protection (partial versus total). Human influence was, however, a poor predictor of the seaweed cover. At the seascape level, C. racemosa was more widely spread within degraded (i.e. Posidonia oceanica dead matte or algal turfs) than in better preserved habitats (i.e. canopy-forming macroalgae or P. oceanica seagrass meadows). At a smaller spatial scale, the presence of the seaweed was positively correlated to the diversity of macroalgae and negatively to that of sessile invertebrates. These results suggest that C. racemosa can take advantage of habitat degradation. Thus, predicting invasion scenarios requires a thorough knowledge of ecosystem structure, at a hierarchy of levels of biological organization (from the landscape to the assemblage) and detailed information on the nature and intensity of sources of disturbance and spatial scales at which they operate.

Experimental Evaluation of the Restoration Capacity of a Fish‐Farm Impacted Area with Posidonia oceanica (L.) Delile Seedlings

Marine aquaculture is an activity that has induced severe local losses of seagrass meadows along the coastal areas. The purpose of this study was to evaluate the capacity of an area degraded by fish‐farm activities to support Posidonia oceanica seedlings. In the study site, a bay in the southeast coast of Spain where part of a meadow disappeared by fish‐farm activities, seedlings inside mesh‐pots were planted in three areas. Two plots were established in each area, one in P. oceanica dead matte and another inside a P. oceanica meadow. To evaluate if sediment conditions were adequate for the life of the seedlings, half of them were planted in direct contact with the sediment and the other half were planted above the surface of the sediment in each plot. Monitoring during 1 year showed that there were large differences in seedling survival between the dead matte and the P. oceanica meadow. While seedlings planted in dead matte had a high survivorship after 1 year (75%), seedlings planted in P. oceanica progressively died (survivorship of 20% after 1 year). The average leaf length of the seedlings surviving in the two substrata was not different, but the leaf area per seedling was lower in the seedlings growing inside the P. oceanica meadow during most part of the year. Seedling survivorship and vegetative development were not affected by the level of planting and suggest that the sediment conditions are adequate for the life of P. oceanica seedlings.

Assessment of substratum effect on the distribution of two invasive Caulerpa (Chlorophyta) species

Two-year monitoring of the invasive marine Chlorophyta Caulerpa taxifolia and Caulerpa racemosa var. cylindracea shows the great influence of substratum on their spatial distribution. The cover of C. taxifolia and C. racemosa was measured in shallow (<8 m) areas indicating that these species are more abundant in rocks with photophilic algae and in the dead matte of the seagrass Posidonia oceanica than in sand or inside the P. oceanica meadow. A short-term experiment comparing the persistence of C. taxifolia and C. racemosa planted either in a model of dead matte of P. oceanica or in sand shows that the persistence of these species was higher in the dead matte model than in sand. Correlative evidence suggests that C. taxifolia and C. racemosa tolerate near-bottom orbital velocities below 15 cm s −1 and that C. taxifolia cover declines at velocities above that value. These results contribute to understand the process of invasion of these Caulerpa species predicting which substrata would be more susceptible to be invaded and to the adoption of appropriate management strategies.

Density and distribution patterns of the endangered species Pinna nobilis within a Posidonia oceanica meadow in the Gulf of Oristano (Italy)

Pinna nobilis is an endemic bivalve of the Mediterranean Sea. In the last decades P. nobilis populations have declined drastically due to increasing anthropogenic pressure and it has been declared a protected species since 1992. Despite the need for conservation, knowledge of the ecology and monitoring of the main populations of P. nobilis are limited. This study considered a population living within a Posidonia oceanica meadow in the Gulf of Oristano (western Mediterranean, Italy). The study area, about 150 hectares, part of which is included within a Marine Protected Area and a Site of Community Importance, was subdivided in 3 sub-areas. The percentage cover of different habitat types (P. oceanica, dead matte and sand) in each sub-area was measured and meadow features (substrate coverage, matte compactness and shoot density) characterized. The hypotheses of differences in density, percentage of dead individuals, population structure, shell burial level and orientation of P. nobilis, were investigated according to sub-areas and to habitat type. The spatial distribution was patchy, and the habitat type resulted a key factor in determining both density and distribution. A strong edge effect was demonstrated: more than half of the observed individuals colonized the P. oceanica border. Matte compactness and shoot density were found to affect the density and distribution of P. nobilis. Shell burial level and percentage of dead individuals varied with sub-areas and habitat types. Size distribution was bimodal and common shell orientation was observed in two sub-areas. These results contribute to increase the knowledge of population ecology of this species and to provide useful information for implementing conservation policies.

Detection and mapping of Posidonia oceanica dead matte by high-resolution acoustic imaging

A high-resolution echosounder was used to acquire seismo-acoustic records of P. oceanica dead matte patches in the Gulf of Palermo. Seismo-acoustic profiling enabled detection of dead matte structures that are invisible to visual inspection, being hidden by variable layers of sandy sediment. The thickness of the dead matte measured reaches a maximum of 2.2 m. The mean volume of the matte per unit area of seabed surface (MEIx) varies from 0.2 to 2.2 m 3 /m 2 , with an average value of 1.6 m 3 /m 2 and a total volume estimated at 73,000 m 3 . From literature data and from the volume of dead matte calculated in the present work we estimated that total carbon accumulated in the matte is about 2,484 tons. Seismo-acoustic technologies proves to be a powerful, non-destructive method to measure the thickness, extension, volume of dead matte and its potential carbon content.

New record of the alien seagrass Halophila stipulacea (Hydrocharitaceae) in the western Mediterranean: a further clue to changing Mediterranean Sea biogeography

The occurrence of the tropical seagrass species Halophila stipulacea (Hydrocharitaceae) is reported within the harbour of Palinuro (Salerno, central Tyrrhenian Sea, Italy). The bottom covered by the seagrass has been evaluated in about 16 m2, fragmented in various small patches distributed in a narrow belt between 0.5 and 5 m depth, and all settled on dead matte of the endemic Mediterranean seagrass Posidonia oceanica. Mean density was 10,500 shoots m−2. This record documents a displacement of about 180 km north of the previously documented limit of Halophila stipulacea in the western Mediterranean, likely mediated by pleasure boat traffic and anchoring, and favoured by climate change. This record illustrates a further example of the changing Mediterranean Sea biogeography: north–south patterns in biotic ranges will probably replace the presently well established west–east patterns in the Mediterranean Sea of tomorrow.

Legal protection is not enough: Posidoniaoceanica meadows in marine protected areas are not healthier than those in unprotected areas of the northwest Mediterranean Sea

Using the Conservation Index, which measures the proportional amount of dead matte relative to live Posidonia oceanica, we assessed the health of 15 P. oceanica meadows at a regional scale along the coast of Liguria (NW Mediterranean). These areas were characterized by different degrees of anthropization, from highly urbanized sites to marine protected areas. Two different scenarios were identified according to depth: in shallow zones, the health of P. oceanica meadows was related to the degree of anthropization along the coastline. In contrast, in deep zones, most meadows exhibited poor health, independent of both the degree of disturbance and the legal measures protecting the area. Working synergistically with the regional impact of increased water turbidity, local impacts from the coast were recognized as the main causes of the severe regression of most Ligurian P. oceanica meadows. We conclude that marine protected areas alone are not sufficient to guarantee the protection of P. oceanica meadows. We emphasize the need for a management network involving the Sites of Community Interest (SCIs) containing P. oceanica meadows.