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This paper describes an advanced methodology to monitor and assess, in temporal series, meadows of the seagrass Posidonia oceanica. The process includes, the following steps: (a) exploring marine regions of certain biological interest with Autonomous Underwater Vehicles equipped with cameras looking downwards, (b) taking images continuously during missions of preprogrammed trajectories, (c) processing the images off-line to build colour photomosaics, (d) segmenting seagrass from the background in every image using a pretrained Neural Network, (e) computing the same photomosaics but using the segmented images, and (f) computing automatically the bottom coverage of the seagrass counting the proportion of pixels labelled positively. This procedure avoids the involvement of divers, allows increasing depth, extension and duration of missions and offers 2D maps of the whole inspected areas in a single view, which allows us to get more accurate coverage measurements than those obtained with traditional techniques. Experiments have been performed with datasets collected in areas of the Balearic Islands colonized with P. oceanica seagrass and subject to low and high touristic and anchoring pressure during high season, repeating the same transects in consecutive years in order to obtain interannual results. Data obtained with this methodology permit a direct biological qualitative, quantitative and temporal analysis and interpretation, such as the percentage of temporal decline of seagrass coverage in some of the surveyed areas and the annual increase of the meadows extension in others. • Classical methods to explore and obtain data from submarine habitats are based, either on divers, remote sensing or ROVs. • Diver-based data is limited in depth, temporal and espatial range, and the post-processed information is obtained through extrapolations.. • Visual remote sensing is limited by the capacity of cameras to perceive the bottom as the depths increases.. • ROVs permit "in situ" observations, but they are costly, need to be attached to a support vessel and cannot perform pre-programmed trajectories. • Here, interannual submarine data is obtained with an AUV and seagrass coverage is computed using advanced image processing techniques and photo-mosaics. [ABSTRACT FROM AUTHOR]

Barrier reefs are peculiar structures of high ecological importance formed by the Mediterranean seagrass Posidonia oceanica in very shallow areas through the accretion of biogenic matte. This study reports for the first time the multiple discovery of several matte blocks of P. oceanica stranded on the southern coast of Sicily (central Mediterranean) in front of a barrier reef. Unmanned Aerial Vehicle (UAV) mapping of the barrier reef and morphological analysis of the blocks made it possible to hypothesise that the original substrate of the stranded blocks was rocky, and that the colonisation of this substrate may have been facilitated by boring bivalve molluscs through a so far unexplored plant-animal interaction. Furthermore, the indirect estimation of the theoretical bathymetric origin of the blocks through a shoot density depth model, combined with the retrospective analysis of the time series of the wave regime prior to stranding, suggests that a severe storm caused this collective uprooting and stranding event from very shallow depths, raising new questions about the vulnerability of these important ecosystems to the increase in frequency and intensity of storms expected in the climate change scenarios. • First discovery of stranded Posidonia oceanica matte blocks from barrier reef. • Estimated origin of the matte blocks from very shallow rocky substrate. • Roots anchorage to rock potentially facilitated by interaction with boring bivalves. • Severe storms in the previous days caused uprooting and stranding of matte blocks. • Shallow barrier reefs are particularly vulnerable to extreme climate events. [ABSTRACT FROM AUTHOR]

The highly efficient carbon storage capacity of beds formed by Posidonia oceanica , an endemic Mediterranean seagrass species, has been widely recognized. Recently, the supra-littoral deposits of leaf litter (i.e., banquettes) have been investigated in terms of their nutrients, biomass and associated community. Nevertheless, an overlooked fraction of the P. oceanica detritus never reaches the shore and sinks far away to deeper seafloor. Additionally, part of the supra-littoral deposit goes back to the sea during winter swells. This deep detrital compartment, mainly composed of sediment mixed with dead leaves and rhizomes coming from P. oceanica beds, has only been described once by Pères in 1953, who focused on the macrofaunal component. Here, we investigated for the first time the meiofaunal community inhabiting sediments characterized by P. oceanica detritus in a deposit located at 65−80 m depth off the Ischia Island (Gulf of Naples, Italy, Tyrrhenian Sea). Our results show that the meiofaunal community appears highly diversified and strongly dominated by nematodes (from 85% to 93%). Differences in meiobenthic assemblage structures were significant only when rare taxa were considered (i.e. taxa found in low abundances and characterized by a sporadic distribution in the study area). The nematode community revealed a very high biodiversity (number of families and genera: 31 and 104, respectively), with a clear prevalence of selective and non-selective deposit feeders that suggest the key role of this habitat in the benthic detrital food web. The richness of meiofauna and the taxonomic and functional diversity of the nematode assemblages account for a "good" to "moderate" ecological quality status. These findings support the high ecological value of the macerating seagrass bottoms, an overlooked component of the blue carbon cycle that deserves to be further investigated. • Re-discovering the overlooked deep macerating Posidonia oceanica bottoms. • First description of the meiofaunal community inhabiting this detrital habitat. • Meiofaunal community is highly diversified and strongly dominated by nematodes. • Dominance of deposit feeders suggests the habitat role in the detrital food web. • Taxonomic/functional diversity supports the high ecological value of this habitat. [ABSTRACT FROM AUTHOR]

We report estimates of community gross primary production (GPP), community respiration (CR), and net community production (NCP) based on the change of dissolved O 2 during incubations over epilithic turf-forming macroalgae (Halopteris scoparia , Padina pavonica , and Dictyota dichotoma) on 7 occasions and in accumulations of Posidonia oceanica macrophytodetritus (i.e. litter) on 8 occasions in the Bay of Revellata (Corsica) from March 2009 to May 2011. In the epilithic macroalgae community, GPP ranged between 7.8 and 82.2 mmol O 2 m−2 d−1, CR ranged between −108.5 and −13.6 mmol O 2 m−2 d−1, and NCP ranged between −53.2 and −5.7 mmol O 2 m−2 d−1. In the P. oceanica macrophytodetritus accumulation, GPP ranged between 5.7 and 91.6 mmol O 2 m−2 d−1, CR ranged between −112.8 and −27.2 mmol O 2 m−2 d−1, and NCP ranged between −46.8 and −9.9 mmol O 2 m−2 d−1. GPP in both the epilithic macroalgae community and the P. oceanica macrophytodetritus accumulation peaked in summer and was lowest in fall, following the seasonal variation of incoming light. GPP correlated to macroalgal biomass but was unrelated to the biomass of living macroscopic plant material in the P. oceanica macrophytodetritus accumulation. The annual average of GPP was equivalent in the epilithic macroalgae and P. oceanica macrophytodetritus accumulation communities (17.6 and 19.4mol O 2 m−2 yr−1). Both the epilithic macroalgae community and the P. oceanica macrophytodetritus accumulation were net heterotrophic with an annual average NCP of −6.1 and −8.8mol O 2 m−2 yr−1, respectively. The NCP of the adjacent P. oceanica meadow at 10 m depth based on simultaneous measurements based on the open water O 2 mass balance from moored O 2 probes (optodes) was 28.9mol O 2 m−2 yr−1. The potential export of dissolved organic carbon from the P. oceanica meadow could quantitatively meet the carbon demand to sustain the net heterotrophy of the adjacent epilithic macroalgae community in the Bay of Revellata. We also show the limitation and possibly over-estimation of extrapolating decay rates based on litter bag experiments with small quantities of material to "real" macrophytodetritus biomass densities. • Net heterotrophy of epilithic macroalgae adjacent to P. oceanica meadow. • Net heterotrophy of P. oceanica macrophytodetritus accumulation. • Over-estimation of decay rates by litter bag experiments. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • Different anthropogenic pressures drive P. oceanica meadows into different degraded states. • Provisioning services are the most affected services in degraded states. • In the dead matte, the habitat becomes unable to provide any significant ES. [ABSTRACT FROM AUTHOR]

Despite the high potential of seagrass restoration to reverse the trend of marine ecosystem degradation, there are still many limitations, especially when ecosystems are severely degraded. In particular, it is not known whether restoring polluted ecosystems can lead to potentially harmful effects associated with contaminant remobilisation. Here, we aimed to investigate the role of P. oceanica transplanted from a pristine meadow to a polluted site (Augusta Bay, Italy, Mediterranean Sea) in two seasons of the year, as a sink or source of trace elements to the environment. The main results showed i) higher accumulation of chromium (Cr), copper (Cu) and total mercury (THg) in plants transplanted in summer than in winter, as well as an increase in Cr and THg in plants from sites with higher trace element loads; ii) an increase in leaf phenolics and a decrease in rhizome soluble carbohydrates associated with As and THg accumulation, suggesting the occurrence of defence strategies to cope with pollution stress; iii) a different partitioning of trace elements between below- and above-ground tissues, with arsenic (As) and Cr accumulating in roots, whereas Cu and THg in both roots and leaves. These results suggest that P. oceanica transplanted to polluted sites can act as both a sink and a source, sequestering trace elements in the below-ground tissues thus reducing their bioavailability, but also potentially remobilising them. However, the amount of trace elements potentially exported from P. oceanica to the environment through transfer into food webs via leaves and detritus appeared to be low under the specific conditions of the study site. Although further research into seagrass restoration of polluted sites would improve current knowledge to support effective ecosystem-based coastal management, the benefits of restoring polluted sites through seagrass transplantation appear to outweigh the potential costs of inaction over time. [Display omitted] • Posidonia oceanica transplanted to polluted sites was monitored for 5 months. • Seagrasses accumulated more trace elements in summer than winter. • Trace element accumulation was higher in leaves and roots than in rhizomes. • Seagrass acted as a sink rather than a source of trace elements in the environment. • Active restoration of polluted sites provides more benefits over time than inaction. [ABSTRACT FROM AUTHOR]

Restoration of coastal ecosystems, particularly those dominated by seagrasses, has become a priority to recover the important ecosystem services they provide. However, assessing restoration outcomes as a success or failure remains still difficult, probably due to the unique features of seagrass species and the wide portfolio of practices used on transplanting actions. Here, several traits (maximum leaf length, number of leaves, leaf growth rate per shoot, and leaf elemental carbon and nitrogen contents) of transplanted seagrass Posidonia oceanica were compared to reference meadows in five sites of Western Mediterranean Sea in which restoration were completed in different times. Results have evidenced the resilience of transplanted P. oceanica shoots within a few years since restoration, as traits between treatments changed depending on the elapsed time since settlement. The highlighted stability of the restoration time effect suggests that the recovery of the plants is expected in four years after transplanting. [Display omitted] • Estimating seagrass restoration success is critical to improve best practices. • Transplanted Posidonia oceanica traits were compared to those of reference meadows. • Restoration time reduced the difference between transplanted and reference shoots. • Resilience of restored plants occurs after four years since restoration set-up. [ABSTRACT FROM AUTHOR]

• Secondary dispersal is an important process in seagrass meadows. • Microtopography could affect secondary seed dispersal distances. • A theory relating secondary dispersal with the microtopography is formulated. • The formulation is validated with computational fluid dynamics for two seagrasses. • These results can inform seed-based restoration to assess secondary dispersal. Motivated by observational and experimental evidence, a theoretical model is proposed to relate the secondary dispersal of seagrass seeds with the complexity of microtopography in natural environments. Complexity is encoded in terms of the Hurst exponent of a fractal description of the microtopographical geometry. The percentage of a seafloor transect where secondary dispersal of seagrass seeds occurs, is quantified in terms of the mainstream velocity, bottom complexity and properties of the seeds. Theoretical expressions are validated considering the cases of Zostera marina and Posidonia oceanica seeds and using computational fluid dynamics (CFD). A total of 200 CFD simulations with different bottom complexities and flow conditions, were done for each seagrass genus to validate the theoretical model. Numerical results agree with theoretical predictions. This finding provides a management tool to assess the degree of seed retention in seed-based restoration areas. [ABSTRACT FROM AUTHOR]

Despite the effects of ocean acidification (OA) on seagrasses have been widely investigated, predictions of seagrass performance under future climates need to consider multiple environmental factors. Here, we performed a mesocosm study to assess the effects of OA on shallow and deep Posidonia oceanica plants. The experiment was run in 2021 and repeated in 2022, a year characterized by a prolonged warm water event, to test how the effects of OA on plants are modulated by thermal stress. The response of P. oceanica to experimental conditions was investigated at different levels of biological organization. Under average seawater temperature, there were no effects of OA in both shallow and deep plants, indicating that P. oceanica is not limited by current inorganic carbon concentration, regardless of light availability. In contrast, under thermal stress, exposure of plants to OA increased lipid peroxidation and decreased photosynthetic performance, with deep plants displaying higher levels of heat stress, as indicated by the over-expression of stress-related genes and the activation of antioxidant systems. In addition, warming reduced plant growth, regardless of seawater CO 2 and light levels, suggesting that thermal stress may play a fundamental role in the future development of seagrass meadows. Our results suggest that OA may exacerbate the negative effects of future warming on seagrasses. • Shallow and deep plants were exposed to ocean acidification and thermal stress; • Plants were unaffected by ocean acidification when not exposed to thermal stress; • Ocean acidification reduced plant performance under thermal stress; • Deep plants showed higher levels of heat stress at genetic and physiological levels; • Warming may play a key role in structuring future seagrass meadows. [ABSTRACT FROM AUTHOR]

Abstract Marine coastal ecosystems are facing structural and functional changes due to the increasing human footprint worldwide, and the assessment of their long-term changes becomes particularly challenging. Measures of change can be done by comparing the observed ecosystem status to a purposely defined reference condition. In this paper, a geospatial modelling approach based on 2D mapping and morphodynamic data was used to predict the natural position of the upper limit (i.e., the landward continuous front) of Posidonia oceanica seagrass meadows settled on soft bottom. This predictive model, formerly developed at the regional spatial scale, was here applied for the first time at the Mediterranean spatial scale in eight coastal areas of Spain, France, Italy, and Greece showing different coastal morphologies and hydrodynamic characteristics, and affected by a number of natural and/or human local disturbances. The model was effective in measuring the regression (i.e., seaward withdrawal) of the meadow upper limit. In all the meadows investigated the upper limit was regressed, laying deeper than the reference condition, with the proportion of regression ranging from 17.7% to 98.9%. The highest values of regression were found in Spain and in France, and were consistent with the highest levels of fragmentation detected with map analysis and of coastal pressures. This geospatial modelling approach represents an effective tool to define the reference conditions when proper pristine areas or historical data are not available, thus allowing the assessment of long-time changes experienced by seagrass ecosystems due to human impacts. Highlights • Reference conditions are needed to measure long-term change in marine ecosystems. • We modelled the upper (landward) limit of seagrass meadows at Mediterranean scale. • All meadow upper limits appeared regressed and fragmented due to local disturbances. • Modelling represents an effective tool for evaluating seagrass meadow conditions. [ABSTRACT FROM AUTHOR]

Abstract This paper investigates the combined role of hydrodynamic forcing, sediment transport processes and sea bottom features in determining the location of the meadow upper limit of the endemic Mediterranean seagrass Posidonia oceanica. For this purpose, an approach including extreme wave analysis and numerical modelling is applied to two sandy beach systems located in southern Sardinia (Italy). Information about bathymetry, P. oceanica meadow extension and bottom features has been made available by previous published studies. Based on the 30-year long NOAA hindcast dataset, a wave climate analysis is carried out to identify the incoming storm wave conditions (1 year return period) to be simulated with the Delft3D model package. The simulation results extend the current knowledge highlighting the importance of wave-induced hydrodynamic parameters as limiting factors for the survival of P. oceanica meadows. In particular, the results suggest that on sandy substrata the meadow upper limit lies well offshore of the surf zone, in areas with little morphological activity, where the wave orbital velocities associated to storms are on the order of 0.8 m/s and the mean current magnitude does not exceed 0.5 m/s. On rocky bottoms, the P. oceanica meadow can extend up to the outer surf zone of storms and is thus able to colonize shallow areas subject to stronger hydrodynamic forcings than those observed on sand. This difference in the plant tolerance to wave forcing depending on the substratum type shows how both hydrodynamic and geological factors play a key role in defining the environmental conditions for the development of seagrass meadows. [ABSTRACT FROM AUTHOR]

Abstract Seagrass ecosystems are net autotrophic systems which contribute to organic carbon burial in marine sediment. Dead seagrass leaves are often exported outside the seagrass beds and may form accumulations (exported macrophytodetritus accumulations, hereafter EMAs) from littoral zones to deepest canyons. Understanding how seagrass organic matter is channeled in its associated trophic web is necessary to assess the role of the seagrass ecosystem as blue carbon service providers. We used gut content and stable isotope analyses to delineate the Posidonia oceanica EMA food web structure and to determine the importance of detrital material in the diets of macrofauna. Evidence from gut contents and stable isotopes showed that this food web is fuelled mainly by two food sources found in the detritus accumulations: 1) P. oceanica detritus itself and 2) epiphytes and drift macroalgae. Dead leaves of P. oceanica enter the diet of dominant species, representing more than 60% of animal abundance. The food web is structured in five trophic levels with a numerical dominance of detritivore/herbivore species at the first consumer level. Animals act as a vector for seagrass organic matter transfer to upper trophic levels and this "dead seagrass signal" is followed through the entire food web. Seagrass primary production and seagrass organic matter processing by animals are spatially decoupled and this should be taken into account in assessments of seagrass ecosystems as key actors in C cycles in coastal areas. [ABSTRACT FROM AUTHOR]

The annual export of the Mediterranean seagrass ( Posidonia oceanica ) litter to adjacent beaches and coastal dunes was quantified by examining the fortnight evolution of seagrass beach-cast volume on two beaches in the NW Mediterranean (Son Real and Es Trenc, Mallorca Island, Spain) for two years and analyzing the wind speed and direction obtained from the closest Meteorological Spanish Agency surface weather stations. The decomposition stage of the deposits was examined by analyzing the total hydrolysable amino acids, its percentage distribution and derived degradation indexes. Prevalent winds exceeding 6 m s −1 , the coastline morphology and type of terrestrial vegetation determine the annual dynamics of the seagrass beach-cast. In the most protected beach (Son Real) the seagrass beach-cast remained nearly stationary during the two studied years while it exhibited wide annual fluctuations in the less protected one (Es Trenc). The amounts of P. oceanica wrack washed on Son Real and Es Trenc beaches, respectively, were estimated at 309 kg DW m coastline −1 yr −1 and 1359 kg DW m coastline −1 yr −1 . They supplied between 20 kg CaCO 3 m coastline −1 yr −1 and 47 kg CaCO 3 m coastline −1 yr −1 . Between 54% (Son Real) and 70% (Es Trenc) of seagrass beach-cast, respectively accounting for 1.5 kg N m coastline −1 yr −1 and 8.6 kg N m coastline −1 yr −1 , were annually exported from the beaches to adjacent dune systems. Our results reveal that Mediterranean seagrass meadows might be an important source of materials, including sand and nutrients, for adjacent terrestrial systems, able to support their functioning. [ABSTRACT FROM AUTHOR]

This paper presents a methodology for observing and analyzing marine ecosystems using images gathered from autonomous marine vehicles. Visual data is composed in photo-mosaics and classified using machine learning algorithms. The approach expands existing solutions, enabling extended monitoring in time, space, and depth. Imagery was collected during a field campaign in the Spanish marine and terrestrial protected area of Cabrera, Balearic Islands, colonized by the endemic seagrass species Posidonia oceanica (Po). The operations were performed using three distinct platforms, an Autonomous Underwater Vehicle (AUV), an Autonomous Surface Vehicle (ASV) and a Lagrangian Drifter (LD). Results are compared to prior habitat maps to assess seagrass meadow distribution. The proposed solution can be scaled and adapted to other locations and species, considering limitations in data storage and battery endurance. [Display omitted] • Seafloor habitat mapping with autonomous robots, filling gaps missed by divers. • Assessment of marine habitats using convolutional neural networks. • Methods apply to georeferenced imaging (e.g. drones, satellites) beyond subsea mapping. • Field campaign data from a marine area compared to previous habitat map data. [ABSTRACT FROM AUTHOR]

Seagrass meadows are 'blue carbon' ecosystems widely recognised for their potential role in climate change mitigation. Previous studies have focused mainly on carbon storage within meadows and sediments. However, little is known about contribution of outwelling (i.e., lateral transport) to seagrass carbon budgets. Here, radium isotopes (223Ra and 224Ra) were used to assess dissolved inorganic carbon (DIC) and total alkalinity (TA) outwelling from a Mediterranean Posidonia oceanica meadow during early autumn. DIC outwelling was 114 ± 61 mmol m−2 day−1 and exceeded above-meadow CO 2 outgassing (3 ± 1 mmol m−2 day−1). Production of DIC was uncoupled from TA and fuelled by net heterotrophy and aerobic processes within the meadow. The small export of TA (5 ± 6 mmol m−2 day−1) implied that ∼90% of outwelled DIC may return to the atmosphere as CO 2 in offshore waters. Combining these fluxes with above-meadow outgassing suggested a total carbon loss that exceeded long term burial in sediments. Overall, the meadow acted as a carbon source to the atmosphere during the early autumn season. Further studies quantifying outwelling at multiple spatial and temporal scales are required to better resolve seagrass carbon budgets and their contribution to carbon sequestration. • Radium isotopes enabled calculation of mixing rates and outwelling fluxes. • Dissolved inorganic carbon outwelling exceeded above-meadow CO 2 outgassing. • Production of dissolved inorganic carbon and alkalinity was uncoupled. • Most of the outwelled dissolved inorganic carbon may return to the atmosphere offshore. • Dissolved inorganic carbon may be a significant component of seagrass carbon budgets. [ABSTRACT FROM AUTHOR]

Climate change has increased interest in seagrass systems as natural carbon sinks and recent studies have estimated the carbon stocks associated with seagrass meadows. However, the factors that affect their variability remain poorly understood. This paper assesses how landscape-level attributes (patch size and matrix composition) influence carbon storage in seagrass sediments. We quantified the organic carbon (C org ) content and other geochemical properties (δ 13 C and particle size) in surface sediments of continuous Posidonia oceanica meadows, patchy meadows interspersed with rocky-algal reefs and patchy meadows on sedimentary bottoms. We also took samples of potential carbon sources for isotopic composition determination. Our results indicate that the continuous meadows accumulated larger amounts of C org than patchy meadows, whether embedded in a rock or sand matrix. The C org from continuous meadows was also more 13 C enriched, which suggests that a high proportion of the carbon was derived from plant material (autochthonous sources); in contrast in patchy meadows (especially in a sand matrix), lower δ 13 C values indicated a higher contribution from allochthonous sources (mainly suspended particulate organic matter, SPOM). These findings suggest that the sediment of continuous meadows stores more C org in than that of patchy meadows. This is probably due to the increased contribution from seagrass leaves, which are much more refractory than SPOM. In general, certain landscape configurations, and especially patchiness, appear to reduce the carbon storage capacity of seagrasses. Since the current decline of seagrass is leading to habitat fragmentation, our results increase the argument for the promotion of effective measures to preserve the integrity of these natural carbon sinks. [ABSTRACT FROM AUTHOR]

Although terrestrial and marine ecosystems are often perceived as clearly distinct, in coastal areas biological communities on land and at sea are in fact intimately linked. One way in which terrestrial and marine systems interact is through the accumulation of seagrass wrack on beaches, which plays an important role as a nutrient input in coastal dune food webs. Here we test whether accumulated beach-cast wrack also influences the structure and diversity of coastal dune plant communities. Relying on a database of 572 vegetation surveys distributed across the island of Sardinia, we used mixed-effects models to compare the vegetation cover and species richness of plant communities exposed to different amounts of Posidonia oceanica beach-cast wrack. We found that beaches which receive high amounts of P. oceanica wrack have considerably greater vegetation cover (10% on average) than those with fewer deposits. The positive relationship between beach-cast wrack and vegetation cover was especially strong in nearshore plant communities, becoming progressively weaker along the habitat zonation. A similar pattern was found for species richness: beaches with high levels of accumulated wrack had more diverse drift line and foredune plant communities, while habitats further away from the shoreline were unaffected. Our study is the first to present evidence suggesting that activities which reduce wrack accumulation on beaches – either through direct removal of deposits or by causing P. oceanica seabeds to decline – can have effects on both the structure and diversity of coastal dune plant communities. Effective management of Mediterranean coastal dune ecosystems will require developing clear strategies for the removal and relocation of accumulated beach-cast wrack. [ABSTRACT FROM AUTHOR]

Mediterranean seagrass ecosystems are endangered by increased colonization of Caulerpa species, which may replace them, affecting key ecosystem processes. The fish Sarpa salpa (L.) is one of the main macroherbivores in the western Mediterranean seagrass meadows which is known to feed on a wide range of macroalgae such as Caulerpa species. In order to elucidate if this consumption could minimize the spread of invasive species, during summer-autumn 2012, we investigate the importance of S. salpa herbivory pressure on C. prolifera and C. cylindracea compared to Posidonia oceanica and Cymodocea nodosa in a mixed meadow. A combination of field experiments and dietary analyses were used to investigate consumption rates, dietary contributions, and feeding preferences for the different macrophytes, including the role of epiphytes and nutrient contents in mediating fish herbivory. In summer, C. nodosa was the most consumed macrophyte (12.75 ± 3.43 mg WW·d −1 ), probably influenced by higher fish densities, higher nutritional quality of leaves and epiphytes, and by differences in epiphyte composition. Feeding observations suggest that fish may have a variable diet, although with a consistent selection of mixed patches with C. nodosa and C. prolifera . Indeed, food choice experiments suggest that when seagrass leaves are not epiphytized, fish prefer feeding on C. prolifera. Gut content and stable isotopic analyses supported the dietary importance of epiphytes and C. prolifera but also suggested that C. cylindracea could occasionally be an important food item for S. salpa . Our results highlight the role of epiphytes in S. salpa feeding decisions but also suggest that C. nodosa and C. prolifera may have an important contribution to fish diet. The variability in S. salpa diet confirm the need to carry out multiple approach studies for a better understanding of its potential influence over different macrophytes species. [ABSTRACT FROM AUTHOR]

A multidisciplinary approach was applied to investigate the role of abiotic constraints in the settlement of Posidonia oceanica on shallow rocks in two coastal areas of the Ligurian Sea (Italy, NW Mediterranean). Meadows developed very shallow upper limits, at 1.5 m depth in both areas, and with a distinctive morphology of stripes growing on rocky outcrops orthogonal to the coastline. Application of a predictive model to indicate the reference condition zone for the meadow upper limit, already validated on meadows developing on soft-bottoms, was not adequate for these rocky substrata as the meadow upper limits were found shallower than model predictions (>5 m depth). Geological and geomorphological characteristics of the rocky shores were analysed through geomechanic and petrographic analyses (i.e. thin sections, SEM analyses, rock hardness tests) whilst the shape and the features of the meadows (i.e. shoot density and maximum leaf length) were assessed through scuba diving surveys. Among the different lithotypes occurring at the sites in the alternating and interbedded outcrops, P. oceanica was passively selected (i.e. due to the seedlings survival and settlement there) on the strongest (i.e. less erodible) lithotypes, whilst the comparatively weaker and more erodible rocks remained unvegetated and covered by a layer of soft-sediments. P. oceanica , settling on specific rocky substrata with favourable lithological and geomechanical characteristics, is able to establish outside the theoretical reference zone predicted by the model for soft sediments due to greater attachment strength and possible resistance to hydrodynamic forces. Combining biological, ecological, mineralogical, geological and geomorphological approaches was effective for explaining the primary role of substratum nature in the spatial variability of seagrass meadows, with geomechanical and lithological characteristics of the rocks being equally important abiotic factors than sedimentological features. [ABSTRACT FROM AUTHOR]

Posidonia oceanica meadows are ecosystem engineers which, despite their ecological relevance, are experiencing habitat fragmentation and area decrease. Cartography and information on the ecological status of these habitats is key to an effective maritime spatial planning and management for habitat conservation. In this work we apply an acoustic methodology to map benthic habitats (substrate and vegetation) in an archipelago of the Natura 2000 Network close to the coast of Murcia (SE Spain) where dense and sparse areas of P. oceanica , and patches of Cymodocea nodosa appear over a sandy and had bottom. The methodology uses dual frequency information (200 kHz and 38 kHz) acquired with a single-beam echosounder to develop a bathymetry, and performs sea bottom and vegetation supervised classifications, using video and scuba diver observations as groundtruthing data. Sea bottom was classified from acoustic features of the first and second 200 kHz echoes into 5 substrate classes using a random forest classifier: sand, fine sand, coarse sand, hard bottoms and hard bottoms with sandy patches. The vegetation was classified from features extracted from the "above-bottom" part of the echo (height and backscattering intensity) in both frequencies, resulting also in a 5 class classification: C. nodosa meadows, dense P. oceanica meadows, dispersed P. oceanica meadows, dense P. oceanica with sand patches, and no-vegetation; according to the random-forest Gini index, 38 kHz features were the most informational variables for this classification. The validation accuracies of both classifications were 85% (substrates) and 70% (vegetation), close to accuracies reported in the literature when using a similar number of classes. The results of this article (including bathymetric, and substrate and vegetation thematic maps), together with the acoustic methodology described and used, are contributions that can improve the continuous monitoring of Mediterranean seagrasses. [ABSTRACT FROM AUTHOR]

A mass balance ecosystemic approach, based on bacterial carbon demands and primary production data, was used to investigate if the bacterial community (freewater bacterioplankton and benthic bacteria of the oxygenated sediment layer) could be sustained by the main primary producers ( Posidonia oceanica and its epiphytes, adjacent macroalgae and phytoplankton communities; hereafter called the P. oceanica system) of a non-eutrophic Mediterranean bay. Unexpectedly, the findings of this study differed from previous works that used benthic incubation chamber and O 2 optode methods. In this study, data were grouped in two categories, corresponding to two time periods, according to the seawater temperature regime (<18 °C or >18 °C): from May to October and from November to April. Between May and October, the produced benthic macrophyte tissues could not provide the carbon required by the bacteria of the oxygenated sediment layer, showing that the balance production of the investigated bay was clearly heterotrophic ( i.e . negative) during this time period. In contrast, between November and April, benthic bacteria respiration nearly equated to carbon production. When integrating the open water carbon dynamics above the meadow in the model, a negative carbon balance was still observed between May and October, while a slight carbon excess was noticed between November and April. In the light of these findings, the carbon balance being negative on an annual basis, alternative carbon sources are required for the maintenance of the bacterial carbon production. [ABSTRACT FROM AUTHOR]

Seagrass meadows are complex ecosystems representing an important source of biodiversity for coastal marine systems, but are subjected to numerous threats from natural and human-based influences. Due to their susceptibility to changing environmental conditions, seagrasses are habitually used in monitoring programmes as biological indicators to assess the ecological status of coastal environments. In this paper we used a non-destructive photo mosaicing technology to quantify seagrass distribution and abundance, and explore benefits of micro-cartographic analysis. Furthermore, the use of photogrammetric tools enhanced the method, which proved to be efficient due to its use of low-cost instruments and its simplicity of implementation. This paper describes the steps required to use this method in meadows of Posidonia oceanica, including: i) camera calibration procedures, ii) programming of video survey, iii) criteria to perform sampling activities, iv) data processing and micro-georeferenced maps restitution, and v) possible study applications. [ABSTRACT FROM AUTHOR]

Sustained functioning of ecosystems is predicted to depend upon the maintenance of their biodiversity, structure and integrity. The large consensus achieved in this regard, however, faces to the objective difficulty of finding appropriate metrics to measure ecosystem functioning. Here, we aim at evaluating functional consequence of the phase shift occurring in meadows of the Mediterranean seagrass Posidonia oceanica , a priority habitat that is undergoing regression in many coastal areas due to multiple human pressures. Structural degradation of the P. oceanica ecosystem, consequent to increasing coastal exploitation and climate change, may result in the progressive replacement of this seagrass by opportunistic macrophytes, either native or alien. Reviewing published information and our personal records, we measured changes in biological habitat provisioning, species richness and biomass associated to each of the alternative states characterizing the phase shift. Then, ecosystem functioning was assessed by computing the exergy associated to each state, exergy being a state variable that measures the ecosystem capacity to produce work. Phase shift was consistently shown to imply loss in habitat provision, species richness, and biomass; structural and compositional loss was parallelled by a reduction of exergy content, thus providing for the first time an objective and integrative measure of the loss of ecosystem functioning following the degradation of healthy seagrass meadows. [ABSTRACT FROM AUTHOR]

The Alboran Sea is the westernmost ecoregion of the Mediterranean Sea. It is located in the vicinity of Strait of Gibraltar, the only natural connection of the Mediterranean Sea with global circulation. This ecoregion presents steep and highly variable environmental gradients, thus acting as a natural filter for Mediterranean and Atlantic species. This study aimed to analyse spatial patterns of littoral and upper sublittoral communities and their relationship with oceanographic conditions and coastal geomorphology, and to quantify littoral and upper sublittoral rocky shore communities at landscape scale. The results suggest that oceanographic conditions are the main factor to explain landscape patterns along the studied area, while geomorphological features should be related with local-scale variability. In this sense, three biogeographic subregions, matching with oceanographic patterns, were identified: western, central and eastern. These subregions showed significant differences in the structure and the composition of the littoral and upper sublittoral community, which can be explained by regional oceanographic dynamics. Posidonia oceanica , Cystoseira ericaefolia group and Mytilus spp. were the species that most contributed to landscape dissimilarity between the three subregions identified. The central oceanographic region, where the environmental conditions were more variable, showed the poorer and less differentiated flora, suggesting the existence of a divergent boundary between the Atlantic and the Mediterranean Sea. [ABSTRACT FROM AUTHOR]

A large scale survey of the trace element (TE) contamination of Mediterranean coastal waters was performed from the analysis of Ag, As, Cd, Cu, Hg, Ni and Pb in the bioindicator Posidonia oceanica , sampled at 110 sites differing by their levels of exposure to contaminants. The holistic approach developed in this study, based on the combined utilization of several complementary monitoring tools, i.e. water quality scale, pollution index and spatial analysis, accurately assessed the TE contamination rate of Mediterranean coastal waters. In particular, the mapping of the TE contamination according to a new proposed 5-level water quality scale precisely outlined the contamination severity along Mediterranean coasts and facilitated interregional comparisons. Finally, the reliability of the use of P. oceanica as bioindicator species was again demonstrated through several global, regional and local detailed case studies. NB: The designations employed and the presentation of the information in this document do not imply the expression of any opinion whatsoever on the part of the authors concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. [ABSTRACT FROM AUTHOR]

Herbivory on Mediterranean seagrass species is generally low compared to consumption of some other temperate and tropical species of seagrasses. In this study we: (1) investigate the feeding preference of the two dominant Mediterranean seagrass herbivores, the sea urchin Paracentrotus lividus and the fish Sarpa salpa, on Posidonia oceanica and Cymodocea nodosa and (2) elucidate the role of epiphytes in herbivore choices. We assessed consumption rates by tethering seagrass shoots, and preferences by food choice experiments with the following paired combinations: 1) Epiphytized leaves of both C. nodosa vs. P. oceanica (CE vs PE); 2) Non-epiphytized leaves of C. nodosa vs. P. oceanica (CNE vs. PNE); 3) Epiphytized vs non-epiphytized leaves of C. nodosa (CE vs. CNE) and 4) Epiphytized vs non-epiphytized leaves of P. oceanica (PE vs PNE). We found that preference for C. nodosa was weak for S. salpa , but strong for P. lividus , the species responsible for most consumption at our study. Overall both herbivores showed preference for epiphytized leaves. The higher nutritional quality of C. nodosa leaves and epiphytes together with the high coverage and diversity of the epiphyte community found on its leaves help explain the higher levels of herbivory recorded on epiphyted leaves of C. nodosa. Other factors such as seagrass accessibility, herbivore mobility and size, and behavioral responses to predation risks, may also affect the intensity of seagrass herbivory, and studies addressing the interactions with these factors are needed to improve our understanding of the nature, extent and implications of herbivory in coastal ecosystems. [ABSTRACT FROM AUTHOR]

Ecological analyses are aimed at characterizing the complexity of the structure of natural objects, yet their heterogeneity is hardly described by the Euclidean concepts. For such purpose, the fractal geometry can be best suited due to its ability in describing, with mathematical rigor, the inherent irregularity of nature. Fractal dimension provides indeed a measurement of the complexity of the analyzed object in terms of space occupation. In this study, we applied the fractal geometry to Posidonia oceanica in order to characterize the structural complexity of its meadows, which are widely recognized as one of the most important coastal ecosystems in the Mediterranean basin. For achieving our aim, we developed an ad hoc implementation of the Box-Counting algorithm based on the Moore neighborhood analysis. Our approach allowed to render the structural complexity of P. oceanica meadows spatially explicit, thus expressing an intrinsic ecological property. The fractal analysis suggested that the complexity of meadows structure is intimately connected with the ecological conditions of P. oceanica. In fact, meadows in living and mixed conditions showed a higher fractal dimension, suggesting a largely uniform and smooth structure. While the fractal dimension associated to the regressed ecological condition of P. oceanica meadows exhibited lower values, highlighting a more jagged and rough structure. Therefore, the fractal theory may prove useful to both fundamental and applied ecological research focusing on P. oceanica and its interactions with Mediterranean coastal ecosystems. In fact, the fractal analysis we performed could result in an effective and straightforward approach for assessing the condition of P. oceanica at large spatial scale, enhancing an integrated maritime spatial planning over the whole Mediterranean basin. • Posidonia oceanica meadows are paramount ecosystems in the Mediterranean basin. • The complexity of meadows structure is a crucial property of Posidonia oceanica. • Fractal geometry is capable of describing the inherent complexity of nature. • Fractal dimension resulted strictly related to the ecological condition of meadows. • Fractal analysis proved to be effective for the assessment of meadows condition. [ABSTRACT FROM AUTHOR]

Seagrass meadows support complex species assemblages and provide ecosystem services with a multitude of socio-economic benefits. However, they are sensitive to anthropogenic pressures such as coastal development, agricultural run-off, and overfishing. The increasing prevalence of marine heatwaves (MHWs) due to climate change poses an additional and growing threat. In this study, we apply the environmental sensitivity mapping approach MESA (Mapping Environmentally Sensitive Assets) to explore the potential consequences of MHWs on the ecosystem services that Posidonia oceanica provides to coastal communities. Under the intermediate climate change scenario Representative Concentration Pathway 4.5, Mediterranean marine heatwaves will be severe by 2050, and will very likely increase mortality of P. oceanica. However, the societal risk of seagrass loss is not evenly distributed across the Mediterranean. The spatial distribution of socio-economic implications of seagrass loss is highlighted through two case studies on seagrass-dependent fisheries and coastal hazards. Coastal communities in Tunisia and Libya show very high sensitivity to a loss of fisheries due to a combination of increasingly intense and frequent MHWs, coupled with high proportions of regional seagrass-dependent fisheries catch. The coastlines of Italy, Tunisia, and Cyprus are shown to potentially be highly sensitive to loss of seagrass due to high levels of coastal hazards, and seagrass meadows susceptible to MHW-induced degradation. These coastlines are likely to suffer from reduced coastal protection services provided by intact seagrass meadows. We demonstrate the implications of MHWs for ecosystem service provision to coastal communities in the Mediterranean and the need for policy instruments to help mitigate and adapt to its effect. We also highlight the potential for environmental sensitivity mapping to help support policymakers with rapid screening tools to prioritize resources more effectively to areas where in-depth local planning is needed. [Display omitted] • Posidonia oceanica provides important ecosystem services for coastal communities. • P. oceanica is affected by magnifying marine heatwaves due to climate change. • MHWs will affect the provision of fishing and coastal protection services. • Areas that heavily rely on these services should prioritize adaptation planning. • Environmental Sensitivity Mapping is used here to prioritize adaptation responses. [ABSTRACT FROM AUTHOR]

Abstract: [1.] Posidonia oceanica (L.) Delile meadows are experiencing severe decline, with significant effects on P. oceanica meadow biodiversity, food webs and associated ecological processes. Despite the importance of this habitat, very little information with which to comprehend and predict the effects of habitat loss on the trophic traits of meadow-dwelling species is available. [2.] The study addressed the effect of changes in P. oceanica cover on the trophic niche of macroinvertebrate litter-associated species and on their consumption of various basal resources along the upper limit of a P. oceanica meadow in the central Tyrrhenian Sea. [3.] Census data and C and N stable isotope analysis were used to characterize changes in the trophic niche of species and the contribution of basal resources to food webs along a meadow coverage gradient. [4.] Falling P. oceanica cover was associated with lower species richness and animal density. In the low-coverage area of the meadow, the organic matter content of basal resources decreased, and the trophic generalism of species increased while their trophic niche overlap declined. In addition, consumption of living P. oceanica plants and epiphytes decreased, and consumption of sediment-derived organic matter increased. [5.] The results suggest that the community associated with P. oceanica litter is particularly sensitive to meadow degradation, which affects trophic structuring and nutrient pathways within the food web. [Copyright &y& Elsevier]

Abstract: The European Water Framework Directive (WFD) 2000/60/EC requests the achievement of the Good Status for all surface waters, including the coastal waters, by 2015. In order to check compliance with the needs of Directive, Italian national monitoring data on Posidonia oceanica meadows have been explored and the relationships among the Posidonia Rapid and Easy Index (PREI), and human-induced pressures have been analyzed along the Italian coasts. The aim of this work is to establish functional relationships between a response variable (i.e. the PREI) and a set of potential pressure (i.e. land use, potential organic and nutrient loading, pesticides) and status (i.e. transparency, trophic level and stability of the water column) indicators in a quantitative way. The ecological responses of coastal marine environment have been evaluated using appropriate statistical tools, such as the multiple linear regression analyses and “linear programming” techniques. Results show that more than 70% of the variability of the P. oceanica meadows status, expressed as PREI value, is significantly explained only by a few pressure/status indicators (namely: potential organic load, specific nitrogen load, natural areas extent, water column transparency), among all those initially considered in the model. The application of the proposed model could allow decision makers to better address remedial actions and to achieve the environmental targets proposed by the EU Directives. [Copyright &y& Elsevier]

Abstract: We implement a Bayesian spatial approach to predict and map the probability of occurrence of seagrass Posidonia oceanica at high spatial resolution based environmental variables. We found that depth, near-bottom orbital velocities and a spectral pattern of Landsat imagery were relevant environmental variables, although there was no effect of slope or water residence time. We generated a data inventory of P. oceanica samples at Palma Bay, NW Mediterranean, from three main sources: side scan sonar, aerial imagery and a customized drop-camera system. A hierarchical Bayesian spatial model for non-Gaussian data was used to relate presence-absence data of P. oceanica with environmental variables in the presence of spatial autocorrelation (SA). A spatial dimension reduction method, the predictive process approach, was implemented to overcome computational constraints for moderately large datasets. Our results suggest that incorporating spatial random effects removes SA from the residuals and improves model fit compared to non-spatial regression models. The main products of this work were probability and uncertainty model maps, which could benefit seagrass management and the assessment of the ecological status of seagrass meadows. [Copyright &y& Elsevier]

Abstract: Temporal trends in abundance, demographic structure of the population and secondary production of Siriella clausii were studied at three different sectors of Posidonia oceanica meadows in the SE Iberian Peninsula, with different levels of habitat complexity. Secondary production was calculated through three methods widely employed in marine invertebrates: the Hynes method (based on demographic structure data) and models by Morin-Bourassa and Brey (based on biomass data). This filter/grazer and detritus feeder is the most abundant mysid in P. oceanica seagrass meadows in the Mediterranean. The population structure was similar in the three sectors, but the temporal pattern of abundance was different, at times even opposite. This pattern may be related to the aggregated spatial distribution, metapopulation dynamics or stochastic factors, but other potential factors could be involved, such as habitat features or predation rates. The values of production oscillated between 25 and 60 mgAFDW/m2/year, depending on the calculation method that was applied, and showed differences between sites and seagrass complexity. The present values are intermediate between those characterising highly productive systems, such as estuaries, and systems of limited productivity (bathyal habitats). Compared to other peracarids, mysid production is greater than that of isopods and less than amphipod production. This is possibly related to the life cycle and reproductive strategies. Detritus feeders such as S. clausii may play an important trophic role due their capacity to use as food the biomass associated to detritus, which constitutes larges reservoirs of carbon derived from seagrass primary production. [Copyright &y& Elsevier]

Abstract: The increasing rate of human-induced environmental changes on coastal marine ecosystems has created a demand for effective descriptors, in particular for those suitable for monitoring the status of seagrass meadows. Growing evidence has supported the useful application of biochemical and genetic descriptors such as secondary metabolite synthesis, photosynthetic activity and genetic diversity. In the present study, we have investigated the effectiveness of different descriptors (traditional, biochemical and genetic) in monitoring seagrass meadow conservation status. The Posidonia oceanica meadow of Monterosso al Mare (Ligurian sea, NW Mediterranean) was subjected to the measurement of bed density, leaf biometry, total phenols, soluble protein and photosynthetic pigment content as well as to RAPD marker analysis. This suite of descriptors provided evidence of their effectiveness and convenient application as markers of the conservation status of P. oceanica and/or other seagrasses. Biochemical/genetic descriptors and those obtained by traditional methods depicted a well conserved meadow with seasonal variability and, particularly in summer, indicated a healthier condition in a portion of the bed (station C), which was in agreement with the physical and sedimentological features of the station. Our results support the usefulness of introducing biochemical and genetic approaches to seagrass monitoring programs since they are effective indicators of plant physiological stress and environmental disturbance. [Copyright &y& Elsevier]

Abstract: Early life history phases are crucial stages limiting species distribution and abundance, thus influencing assemblage composition in marine benthic environments. In seagrass systems the period between seed germination and establishment is one of the most vulnerable phases for plant development. This study analyzes the influence of microhabitat structure, in terms of substrate nature and algal canopy, on the persistence and growth over two years of seedlings of Posidonia oceanica, the dominant Mediterranean seagrass. Long time persistence of seedlings only occurred on microhabitats providing vegetated rocky substrates, with a maximum value of 81% on rock covered by Cystoseira spp. No seedling was found on unvegetated sand and gravel after the first year. Seedling growth resulted increased on rock covered by Halopteris spp. and Dilophus spp. than on rock covered by Cystoseira spp. Results suggest that high canopy onto a stable substrate enhances seedling persistence, probably because these allow the best anchorage by roots while hampering water flow. In contrast, turf algal cover promotes better seedling growth, possibly through higher light irradiance and nutrient availability. Our findings support the view that the understanding of the factors controlling early life processes is a necessary prerequisite for the comprehension of seagrass species distribution patterns, colonization and recovery potentials, which, in turn, can guide sound strategies for seagrass management and restoration. [Copyright &y& Elsevier]

Abstract: Previous research has demonstrated the low tolerance of Posidonia oceanica mature shoots to salinity variability but there is no information about the response of its seeds and seedlings to this impact. In the present study, two independent experiments were conducted to investigate the effects of salinity variation on germination, development and survival of P. oceanica seeds and seedlings under laboratory-controlled conditions. Maximum P. oceanica seed germination occurred at the control salinity (37), while mortality was lowest for this treatment. However, no significant differences were detected with other salinity levels (39–49). In contrast, salinity appears to play an important role in seedling development. The number of blades, the length of the primary root and the maximum length of leaves of seedlings were significantly reduced with increased salinities. The results observed in the present study are consistent with those obtained for mature P. oceanica shoots, and suggest that hypersalinities, such as those associated with brine discharge of a desalination plant, may limit P. oceanica seedling recruitment, affecting the expansion and recovery of Posidonia meadows. [Copyright &y& Elsevier]

Abstract: The seasonal dynamics of the molluscan fauna associated with the westernmost populations of the Mediterranean seagrass Posidonia oceanica, has been studied throughout an annual cycle in the northwestern coasts of the Alboran Sea. Samples were collected seasonally (5 replicated per season) using a non-destructive sampling technique (airlift sampler) on quadrats of 50 × 50 cm at 2 sites located 7 km apart. Several environmental variables from the water column (temperature, chlorophyll a), the sediment (percentage of organic matter) and the seagrass meadows (shoot density, leaf height and width, number of leaves per shoot) were also measured in order to elucidate their relationships with the dynamics of the molluscan assemblages. In these meadows, a total of 17,416 individuals of molluscs were collected, belonging to 71 families and 171 species, being Rissoidae, Pyramidellidae and Trochidae the best-represented families, and Mytilidae, Nassaridae and Trochidae the dominant ones in terms of abundance. The assemblages were dominated by micro-algal grazers, filter feeders and ectoparasites (including those feeding on sessile preys). The species richness and the abundance displayed significant maximum values in summer, whereas evenness and diversity displayed maximum values in spring, being significant for the evenness. Both abundance and species richness values were positively correlated to seawater temperature and percentage organic matter, only for the latter, and negatively to leaf width. Significant seasonal groupings were obtained with multivariate analyses (MDS, Cluster, ANOSIM) using qualitative and quantitative data that could be mainly related to biological aspects (i.e. recruitment) of single species. The molluscan assemblages are influenced by the biogeographical location of the area (Alboran Sea), reflected in the absence or scarcity of most Mediterranean species strictly associated with P. oceanica (e.g. Tricolia speciosa, Rissoa ventricosa) and by the occurrence of subtropical fauna (e.g. Ungulina cuneata). The patchiness of the meadows, intercalated in other habitats like photophilous algae, and the strong hydrodynamism resulting from the unusually shallow setting, may promote the movement of species between these habitats and the seagrass and therefore enhance the species richness. This large faunistic list for the molluscan fauna associated with P. oceanica is therefore different than those observed in other meadows, and highlights the conservation value of these fragmented meadows located in their western distributional limit. [Copyright &y& Elsevier]

Abstract: We experimentally investigated the effect of Mediterranean seawater summer warming projected for the 21st century under Intergovernmental Panel on Climate Change (IPCC) scenarios of greenhouse gas emissions on Posidonia oceanica sulphide stress. The results reveal that projected warming would enhance sulphide toxicity in P. oceanica and, hence, the risk of seagrass loss. We assessed sulphide stress of seagrasses by estimating the sulphide intrusion (F sulphide) into leaf tissue from the sulphur isotopic signature in sediments (δ 34 S sulphide), in the water column (δ 34 S sulphate) and in P. oceanica leaves (δ 34 S tissue). Seagrasses were grown at 6 seawater temperature treatments, ranging from 26 °C to 32 °C, in 3 replicated experimental mesocosms for 50 days. At the end of the experiment, the seagrass shoots in all of the treatments exhibited sulphide intrusion (F sulphide > 0%), which had been negligible when the experiment started. Similarly, the leaf δ 34 S tissue was lighter at the end of the experiment than at the beginning, when seagrasses in the field were experiencing a seawater temperature of 23 °C. F sulphide linearly increased with experimental warming, while leaf δ 34 S tissue declined with increasing temperature at a rate of 0.3‰ for each centigrade degree of warming. The daily production of new leaves per shoot decreased as the sediment became more reduced. Leaf δ 34 S tissue below 20.5‰, indicative of 1% sulphide intrusion, is expected to compromise P. oceanica performance. [Copyright &y& Elsevier]

Abstract: Nutrient increases in coastal systems are becoming a world-wide concern since they promote strong structural and functional changes in shallow ecosystems. Increased nutrient availability in the water column may strongly enhance the leaf epiphytic communities of key habitat-forming species such as seagrasses through a bottom-up mechanism, competing for light and nutrients with the leaves. Epiphytes support an abundant and diverse community of resident invertebrates which fuel higher trophic levels in Posidonia oceanica food webs. We evaluated the response of seagrass, epiphytes and the invertebrate community to an experimental increase of water column nutrient availability. Nutrient increase was followed by a rise of epiphyte biomass. The increase in epiphytic biomass promoted invertebrate abundance, but appeared to have negative effects on P. oceanica shoot size. On the other hand, the increase in invertebrate abundance did not seem to control epiphytic biomass, which was not reversed to pre-nutrient enrichment levels. This work suggests that the abundance of invertebrate populations is limited by epiphyte biomass, through food or habitat provision, in P. oceanica systems and points to nutrients as the main driver of epiphyte biomass during summer. The results illustrate the control mechanisms at community level in P. oceanica meadows and the possible responses of a threatened ecosystem to human impact such as eutrophication. [Copyright &y& Elsevier]

Abstract: Rapid, easy and low cost semi-quantitative methods were tested to study the heterotrophic bacterial community of Posidonia oceanica meadows and were compared to techniques commonly used in microbial ecology. Free and pore-water bacterial densities were estimated by luminescence, and principal enzymatic activities, metabolic capabilities and benthic mineralisation processes were studied with microtitration methods: ApiZym galleries, Biolog microplates and BART™ tests. Bacterial densities varied little throughout the year and were around 5.0·105 and 6.0·106 cells ml−1 of free and pore-water, respectively. The combined use of the ApiZym gallery and the Biolog microtitration plate permitted highlighting bacterial enzymatic activities susceptible to degrade principal organic polymers present in the Posidonia meadow, and to correlate these enzymatic activities to the subsequent potential utilization of resulting monomeric products. Levels of enzymatic activities (1.80–8.36 nmolessubstrates h−1 ml−1) and energetic bacterial metabolism (1.80–6.42 nmolessubstrates h−1 ml−1) presented seasonality relying on the temperature regime and on the primary production (Posidonia and phytoplankton). Main mineralization processes of buried organic matter through sulfate and iron reduction activities were successfully detected. Despite the complexity of the studied ecosystem, results obtained by this semi-quantitative approach, compared to studies applying commonly used methods in microbial ecology, highlighted the same bacterial dominant key processes. Their low cost, rapid and easy use, and the low level of expertise and sophistication they require means that these techniques are of use to many employed in environmental surveys. [Copyright &y& Elsevier]

Abstract: Posidonia oceanica, the most important and abundant seagrass in the Mediterranean Sea, forms large meadows from the sea surface down to 40 m. The depth of the lower limit of the meadows marks the boundary between the infralittoral and the circalittoral zone, and is said to be normally set by light attenuation underwater, while the role of water movement has been little explored. In this paper, the position (i.e. distance from the shoreline and depth) of P. oceanica meadow lower limits along the whole Ligurian coastline (about 350 km) was related to the annual storm wave base. This depth represents the limit of interaction between waves and seafloor and corresponds to L 0/2, where L 0 is the annual offshore wave length. In all meadows, the lower limit has never been found deeper than the annual storm wave base, and its depth (Zc ) showed related to L0 according to the equation Zc  = 0.32∙L 0 + 5.62. In the coastal tracts affected by the least intense waves, the reduction of water movement with depth represents the most important constraint to the seaward development of the meadow, whereas light availability plays a major role in meadows affected by the most intense waves. The present study represents the first attempt at understanding the role of hydrodynamic factors in setting the depth limit of seagrass meadows. If corroborated by future research at other sites, this will have important implications for both basic and applied science, as it would imply rethinking about the relative importance of water movement and light in seagrass depth distribution, and could allow for a better estimate of the extent of meadow regression in anthropized areas. [Copyright &y& Elsevier]

Abstract: Quantitative sampling in combination with classification of fish species into six major feeding guilds revealed the position and contribution of non-indigenous species (NIS) in the food web of Posidonia oceanica and sandy habitats in an area of the eastern Mediterranean. In P. oceanica beds and on sandy bottoms 10 and five species, respectively, were non-indigenous fish of Indo-Pacific origin. The proportional contribution of NIS individuals on P. oceanica beds was lower than that of sandy bottoms (12.7 vs. 20.4%) a pattern that also followed for biomass (13.6 vs. 23.4%), indicating that low diverse systems may be more liable to introductions than species-rich communities. The two habitats had similar fish feeding guilds, but the biomass contribution from NIS varied within each guild, indicating different degrees of impact on the available resources. This study showed that only few non-indigenous fish species contributed to the differences in biomass between habitats. No support could be found in postulating that taxonomic affiliation could predict invasion success. Size was considered highly important due to habitat shift of species with increased size. Two of the aspects considered in this study, the chance of establishing vs. the chance of being very dominant will depend upon competitive abilities strongly coupled to size and grounds for habitat shift. However, success of establishment will also depend on appropriate food resources in the recipient community as well as competitive abilities and level of competition in the food web within habitats. [Copyright &y& Elsevier]

Abstract: While coastal management activities have long been known to exert a strong influence on the health of marine ecosystems, neither scientists nor administrators have realized that small interventions may lead to disproportionately larger impacts. This study investigated the broad and long-lasting environmental consequences of the construction of an ill-planned, although small (only 12 m long) jetty for pleasure crafts on the hydrodynamic conditions and on the meadow of the seagrass Posidonia oceanica of an embayed cove in the Ligurian Sea (NW Mediterranean). There, P. oceanica used to develop on a high (>1.5 m) matte (a lignified terrace causing seafloor elevation) in which the leaves reach the surface and form a compact natural barrier to waves in front of the beach. Such a so-called ‘fringing reef’ of P. oceanica is today recognized of high ecological value and specific conservation efforts are required. The construction of the jetty implied the cutting of the matte, which directly destroyed part of the fringing reef. In addition, meadow mapping and sedimentological analyses coupled with morphodynamic modelling showed that the ecosystem of the whole cove had been greatly altered by the jetty. We used the geometric planform approach, a proper tool in the study of headland-controlled embayment, both to characterise the present situation of Prelo cove and to simulate the original one, before the jetty was built. In the long term, such a small jetty completely altered the configuration and the hydrodynamic conditions of the whole cove, splitting the original pocket beach into two smaller ones and creating strong rip-currents flowing seaward along the jetty. These rip-currents enhanced erosion of residual shallow portions of the meadow and further modified the sedimentary fluxes in shallow waters. A century after the construction of the jetty, an irreversible environmental damage has occurred, as the slow growing rate of P. oceanica implies that the high matte terrace and the fringing reef will hardly form again, even after the removal of the jetty. The lesson learnt from this study is that even such small, and therefore reputed intrinsically ‘innocent’, interventions on the coastal zone require accurate planning based on interdisciplinary studies to understand and respect the delicate interplay among morphological, hydrodynamic and ecological components. [Copyright &y& Elsevier]

Abstract: Multiple stable isotope analyses were used to examine the trophic shifts at faunal assemblages within the invading macroalga Caulerpa racemosa in comparison to established communities of Posidonia oceanica seagrass meadows. Sampling of macrobenthic invertebrates and their potential food sources of algal mats and seagrass meadows in Mallorca (NW Mediterranean) showed differences in species composition of faunal and primary producers among seagrass and C. racemosa. Accordingly, changes in food web structure and trophic guilds were observed, not only at species level but also at community level. The carbon and nitrogen isotope signatures of herbivores, detritivores and deposit feeders confirmed that the seagrass provided a small contribution to the macrofaunal organisms. δ13C at the P. oceanica seagrass and at the C. racemosa assemblages differed, ranging from −6.19 to −21.20‰ and −2.67 to −31.41‰, respectively. δ15N at the Caulerpa mats was lower (ranging from 2.64 to 10.45‰) than that at the seagrass meadows (3.51–12. 94‰). Significant differences in isotopic signatures and trophic level among trophic guilds at P. oceanica and C. racemosa were found. N fractionation at trophic guild level considerable differed between seagrass and macroalgae mats, especially for detritivores, deposit feeders, and herbivores. Filter feeders slightly differed with a relatively lower N signal at the seagrass and CR values at community level and at trophic guild level were higher in the C. racemosa invaded habitats indicating an increase in diversity of basal resource pools. C. racemosa did seem to broaden the niche diversity of the P. oceanica meadows it colonised at the base of the food web, may be due to the establishment of a new basal resource. The extent of the effects of invasive species on ecosystem functioning is a fundamental issue in conservation ecology. The observed changes in invertebrate and macrophytic composition, stable isotope signatures of concomitant species and consequent trophic guild and niche breadth shifts at invaded Caulerpa beds increase our understanding of the seagrass systems. [Copyright &y& Elsevier]

Abstract: Seagrass meadows are ecosystems of great ecological and economical value and their monitoring is an important task within coastal environmental management. In this paper, an acoustic mapping technique is presented using a profiling sonar. The method has been applied to three different sites with meadows of Zostera marina, Zostera noltii and Posidonia oceanica respectively, with the aim to test the method’s applicability. From the backscatter data the seabed could be identified as the strongest scatterer along an acoustic beam. The presented algorithm was used to compute water depth, seagrass canopy height and seagrass coverage and to produce maps of the survey areas. Canopy height was estimated as the distance between the bed and the point where backscatter values decrease to water column values. The algorithm was extended from previous methods to account for a variety of meadow types. Seagrass coverage was defined as the percentage of beams in a sweep where the backscatter 5–10 cm above the bed was higher than a threshold value. This threshold value is dynamic and depends on the average backscatter value throughout the water column. The method is therefore applicable in a range of turbidity conditions. Comparison with results from other survey techniques (i.e. dive surveys, underwater videos) yielded a high correlation which indicates that the method is suited to measure seagrass coverage and height. Analysis of the data showed that each seagrass species has a characteristic canopy height and spatial coverage distribution. These differences were used to undertake a preliminary species identification, as each species has a typical canopy height and preferred depth range. Furthermore, the results show that these differences can be used to track boundaries between species remotely. Finally, the application of the meadow distribution pattern to the health of a meadow is discussed. [Copyright &y& Elsevier]

Abstract: The sedimentary features of the inner-middle shelf of the strait of Bonifacio (western Mediterranean) were analyzed to evaluate the relationship between the production and transport of biogenic carbonate sediments and the basin morphology and hydrodynamics. A three-dimensional hydrodynamic modeling was performed in order to simulate the influence of waves and currents at seabed level. Superficial sediments were collected at depths ranging from 5 to 80 m and were analyzed for grain size, mineralogical composition and skeletal carbonate composition. Posidonia oceanica seagrass meadows border the coasts in a narrow strip on both sides of the strait down to a depth of 40 m. At greater depths, the seabed is characterized by the presence of plateaus and ridges which are controlled by outcropping bedrock morphology. Waves and seabed currents are driven by the prevailing northwest and northeast winds. For both wind directions, higher values for the seabed current velocity, associated with wind-storm events, were detected in shallower sectors and along an east–west-oriented belt that connects the western Mediterranean and the Tyrrhenian Seas. The sediments range from sand to gravel and show a mixed biogenic carbonate/siliciclastic composition. This is due to the carbonate production associated with benthic ecosystems and the mixing of modern carbonate with relict sediments. Biogenic gravelly sands were found in association with Posidonia oceanica seagrass meadows and extended down to the circalittoral zone outside the deeper boundary of the meadows. This sedimentary facies was derived from the modern carbonate production associated with the P. oceanica ecosystem. Sediments collected outside the deeper limit of the meadows were identified as sediments deposited during the stand of the meadows at a deeper level during lower sea level conditions. Maërl (free-living calcareous red algae) beds are the main carbonate factory and are mainly located at the top of the rocky plateaus (at ∼50–55m) formed by the outcropping of the bedrock. Downward, the increased currents at the seabed level in the east–west-oriented belt, which connects the western Mediterranean and the Tyrrhenian Seas, limit the extension of this carbonate factory. This results in a mixed sedimentary facies composed of biogenic carbonate and relict siliciclastic sandy gravel. Compared to other Mediterranean shelves, the strait of Bonifacio is characterized by a distinct oceanographic setting. This is due to the connection between two basins. The currents at the seabed play a crucial role in controlling the distribution of the carbonate factories. [Copyright &y& Elsevier]

Abstract: This study aims to examine the effect of increased salinity on the photosynthetic activity of the Mediterranean seagrass Posidonia oceanica in a laboratory mesocosm system. To do this, large rhizome fragments were transplanted in a mesocosm laboratory system and maintained at 37 (ambient salinity, control treatment), 39, 41 and 43 (hypersaline treatments) for 47 days. Pigment content, light absorption, photosynthetic characteristics (derived from P vs. E curves and fluorescence parameters), and shoot size, growth rates and net shoot change were determined at the end of the experimental period. Both net and gross photosynthetic rates of plants under hypersaline conditions were significantly reduced, with rates some 25–33% and 13–20% lower than in control plants. The pigment content (Chla, Chlb, Chlb:Chla molar ratio, total carotenoids and carotenoids:Chla ratio), leaf absorptance and maximum quantum yield of PSII (Fv /Fm ) of control plants showed little or no changes under hypersaline conditions, which suggests that alterations to the capacity of the photosynthetic apparatus to capture and process light were not responsible for the reduced photosynthetic rates. In contrast, dark respiration rates increased substantially, with mean values up to 98% higher than in control leaves. These results suggest that the respiratory demands of the osmoregulatory process are likely to be responsible for the observed decrease in photosynthetic rates, although alterations to photosynthetic carbon assimilation and reduction could also be involved. As a consequence, leaf carbon balance was considerably impaired and leaf growth rates decreased as salinity increased above the ambient (control) salinity. No significant differences were found in the percentage of net shoot change, but mean values were clearly negative at salinity levels of 41 and 43. Results presented here indicate that photosynthesis of P. oceanica is highly sensitive to hypersaline stress and that it likely account for the decline in leaf growth and shoot survival reported in this and previous studies in response to even small increments of the ambient salinity. [Copyright &y& Elsevier]

Abstract: 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. [ABSTRACT FROM AUTHOR]

Abstract: The demand for sensitive biological tools to assess the environmental quality of coastal waters at broad spatial scales is increasing. Many of the tools used are based on the taxonomic composition of biotic assemblages. They usually require a valuable taxonomic expertise while are unique reflecting the overall ecosystem integrity. Here, we evaluate the potential indicator value of several features of the epiphytic community (overall assemblage composition, species richness, and proportion of the main taxonomic groups) growing on the seagrass Posidonia oceanica leaves. We do so by empirically examining their changes along a disturbance gradient where multiple human activities have interactive and cumulative impacts, sampling at different spatial scales and at two different depths (5 and 15 m). Our results show that the specific composition of the epiphytic assemblages (i.e. species composition) closely reflects, in the deep meadows, the combined effects of different anthropogenic stressors along the gradient, showing an integrative and non-specific response. Similarly, an increase in the proportion of hydrozoans, and a decrease in the proportion of rhodophytes and chlorophytes are observed in deep meadows along the gradient. In shallow meadows, grazing and biotic features of the seagrass seem the main forcing factors determining species composition, and therefore masking the response of epiphytes to the deterioration gradient. After address the effect of natural sources of variability (water depth, within- and between-meadow heterogeneity), changes in epiphyte assemblages and in the proportion of hydrozoans, rhodophytes and chlorophytes in relatively deep meadows seem promising monitoring tools for detecting coastal environmental deterioration. [Copyright &y& Elsevier]

Abstract: Habitat fragmentation in meadows of Posidonia oceanica, the most important and abundant seagrass in the Mediterranean Sea, was investigated at a region-wide spatial scale using a synthetic ecological index, the Patchiness Index (PI). We tested the hypothesis that human impacts are the major factor responsible for habitat fragmentation in P. oceanica meadows contrasting fragmentation of meadows located in “anthropized” areas with that of meadows located in areas with low anthropization and considered as virtually “natural”. We also related fragmentation of meadow with the morphodynamic state of the submerged beach (i.e. distinctive types of beach produced by the topography, the wave climate and the sediment composition) in order to investigate the influence of one natural component on the seagrass meadow seascape. Results demonstrated that fragmentation in the P. oceanica meadows is strongly influenced by the human component, being lower in natural meadows than in anthropized ones, and that it is little influenced by the morphodynamic state of the coast. The use of landscape approaches to discriminate natural disturbance from human impacts that affect seagrass meadows is thus recommended for the proper management of coastal zones. [Copyright &y& Elsevier]

Abstract: Processes acting on the early-life histories of marine organisms can have important consequences for the structuring of benthic communities. In particular, the degree of coupling between larval supply and adult abundances can wield considerable influence on the strength of trophic interactions in the ecosystem. These processes have been relatively well described in rocky systems and soft-sediment communities, and it is clear that they are governed by very different bottlenecks. Seagrass meadows make interesting study systems because they bear structural affinities to both soft sediments as well as rocky substrates. We examined the early-life history of Paracentrotus lividus, one of the dominant herbivores in Mediterranean seagrass meadows, to identify the drivers of population dynamics in this species. We measured spatial and temporal variability in sea urchin post-settlement in 10 Posidonia oceanica meadows in the North-Western Mediterranean over a period of two years, and compared the numbers with the one-year old cohort a year later (i.e. the new population recruitment) as well as between successive size–age groups. Urchin post-settlers differed substantially between meadows but were present in both years in all meadows surveyed, suggesting that larval supply was not limiting for any of the studied sites. However, in six of the studied meadows, the one-year cohort of urchins was absent in both years, indicating that post-settlement processes strongly affected urchins in these meadows. In contrast, in four of the studied meadows, there was a strong coupling between post-settlers and one-year cohort individuals. These meadows were structurally different from the others in that they were characterised by an exposed matrix of rhizomes forming a dense seagrass mat. This mat apparently strongly mediates post-settlement mortality, and its presence or absence dictates the successful establishment of urchin populations in seagrass meadows. As the population aged, the relationship between size–age groups decreased evidencing the action of other processes. Yet, these results indicate that differences in physical structure are a vital bottleneck for sea urchin populations in seagrass meadows. Exploring the interaction between ecosystem structure and early-life history may provide a broader and more unified framework to understand the dynamics of a range of benthic habitats, including rocky substrates, soft sediments and seagrass meadows. [Copyright &y& Elsevier]