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

Does transplanted Posidonia oceanica act as a sink or source of trace elements? Ecological implications for restoring polluted coastal areas

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.

Dead foundation species drive ecosystem dynamics

Foundation species facilitate communities, modulate energy flow, and define ecosystems, but their ecological roles after death are frequently overlooked. Here, we reveal the widespread importance of their dead structures as unique, interacting components of ecosystems that are vulnerable to global change. Key metabolic activity, mobility, and morphology traits of foundation species either change or persist after death with important consequences for ecosystem functions, biodiversity, and subsidy dynamics. Dead foundation species frequently mediate ecosystem stability, resilience, and transitions, often through feedbacks, and harnessing their structural and trophic roles can improve restoration outcomes. Enhanced recognition of dead foundation species and their incorporation into habitat monitoring, ecological theory, and ecosystem forecasting can help solve the escalating conservation challenges of the Anthropocene.

Anchoring pressure and the effectiveness of new management measures quantified using AIS data and a mobile application

Large boats can have a major impact on sensitive marine habitats like seagrass meadows when anchoring. The anchoring preference of large boats and their impacts can be mapped using Automatic Identification System (AIS). We found a constant increase in the number of anchoring events with, until recently, a large part of them within the protected Posidonia oceanica seagrass meadows. French authorities adopted a new regulation in 2019 forbidding any anchoring within P. oceanica seagrass meadows for boats larger than 24 m. The number of large ships (>24 m) anchoring in P. oceanica meadows significantly decreased after the enforcement of the regulation. The surface of avoided impact thanks to the new regulation corresponds to 134 to 217 tons of carbon sequestered by the preserved meadow in 2022. This work illustrates that a strict regulation of anchoring, based on accurate habitat maps, is effective in protecting seagrass meadows.

Signs of local adaptation by genetic selection and isolation promoted by extreme temperature and salinity in the Mediterranean seagrass Posidonia oceanica

Adaptation to local conditions is known to occur in seagrasses; however, knowledge of the genetic basis underlying this phenomenon remains scarce. Here, we analysed Posidonia oceanica from six sites within and around the Stagnone di Marsala, a semi-enclosed coastal lagoon where salinity and temperature exceed the generally described tolerance thresholds of the species. Sea surface temperatures (SSTs) were measured and plant samples were collected for the assessment of morphology, flowering rate and for screening genome-wide polymorphisms using double digest restriction-site-associated DNA sequencing. Results demonstrated more extreme SSTs and salinity levels inside the lagoon than the outer lagoon regions. Morphological results showed significantly fewer and shorter leaves and reduced rhizome growth of P. oceanica from the inner lagoon and past flowering events were recorded only for a meadow farthest away from the lagoon. Using an array of 51,329 single nucleotide polymorphisms, we revealed a clear genetic structure among the study sites and confirmed the genetic isolation and high clonality of the innermost site. In all, 14 outlier loci were identified and annotated with several proteins including those relate to plant stress response, protein transport and regulators of plant-specific developmental events. Especially, five outlier loci showed maximum allele frequency at the innermost site, likely reflecting adaptation to the extreme temperature and salinity regimes, possibly due to the selection of more resistant genotypes and the progressive restriction of gene flow. Overall, this study helps us to disentangle the genetic basis of seagrass adaptation to local environmental conditions and may support future works on assisted evolution in seagrasses.

An Enhanced Photogrammetric Approach for the Underwater Surveying of the Posidonia Meadow Structure in the Spiaggia Nera Area of Maratea

The Posidonia oceanica meadows represent a fundamental biological indicator for the assessment of the marine ecosystem's state of health. They also play an essential role in the conservation of coastal morphology. The composition, extent, and structure of the meadows are conditioned by the biological characteristics of the plant itself and by the environmental setting, considering the type and nature of the substrate, the geomorphology of the seabed, the hydrodynamics, the depth, the light availability, the sedimentation speed, etc. In this work, we present a methodology for the effective monitoring and mapping of the Posidonia oceanica meadows by means of underwater photogrammetry. To reduce the effect of environmental factors on the underwater images (e.g., the bluish or greenish effects), the workflow is enhanced through the application of two different algorithms. The 3D point cloud obtained using the restored images allowed for a better categorization of a wider area than the one made using the original image elaboration. Therefore, this work aims at presenting a photogrammetric approach for the rapid and reliable characterization of the seabed, with particular reference to the Posidonia coverage.

Transplantation on a Posidonia oceanica meadow to facilitate its recovery after the Concordia shipwrecking

Ecological restoration is an important tool to reverse habitat loss and recover ecosystem services. Here, for two years, we examine the dynamic of Posidonia oceanica following the restoration of a 1149 m² meadow damaged by the Concordia shipwreck. To evaluate the suitability of a recently employed seagrass restoration protocol, we assessed the patches' survival and development by high-spatial resolution photomosaics over the whole transplanted surface. To estimate recovery trajectories, we quantified the cuttings' survival, shoot density, and Daily Leaf Production within fixed monitoring squares. The outcomes confirmed that our protocol could be efficiently applied at larger scales, showing diminutions in cuttings' survival and shoot density over the first year (up to -20%), followed by stability in the number of living cuttings and increases of leaf bundles (up to +5%/year). Our insights demonstrate that the recovery of P. oceanica can be speeded up and underline the need for case-specific transplantation strategies.

Assessing Seagrass Restoration Actions through a Micro-Bathymetry Survey Approach (Italy, Mediterranean Sea)

Underwater photogrammetry provides a means of generating high-resolution products such as dense point clouds, 3D models, and orthomosaics with centimetric scale resolutions. Underwater photogrammetric models can be used to monitor the growth and expansion of benthic communities, including the assessment of the conservation status of seagrass beds and their change over time (time lapse micro-bathymetry) with OBIA classifications (Object-Based Image Analysis). However, one of the most complex aspects of underwater photogrammetry is the accuracy of the 3D models for both the horizontal and vertical components used to estimate the surfaces and volumes of biomass. In this study, a photogrammetry-based micro-bathymetry approach was applied to monitor Posidonia oceanica restoration actions. A procedure for rectifying both the horizontal and vertical elevation data was developed using soundings from high-resolution multibeam bathymetry. Furthermore, a 3D trilateration technique was also tested to collect Ground Control Points (GCPs) together with reference scale bars, both used to estimate the accuracy of the models and orthomosaics. The root mean square error (RMSE) value obtained for the horizontal planimetric measurements was 0.05 m, while the RMSE value for the depth was 0.11 m. Underwater photogrammetry, if properly applied, can provide very high-resolution and accurate models for monitoring seagrass restoration actions for ecological recovery and can be useful for other research purposes in geological and environmental monitoring.