Seagrasses rank among the most productive yet highly threatened ecosystems on Earth. Loss of seagrass habitat because of anthropogenic disturbances and evidence of their limited resilience have provided the impetus for investigating and monitoring habitat restoration through transplantation programmes. Although Structure from Motion (SfM) photogrammetry is becoming a more and more relevant technique for mapping underwater environments, no standardised methods currently exist to provide 3-dimensional high spatial resolution and accuracy cartographic products for monitoring seagrass transplantation areas. By synthesizing various remote sensing applications, we provide an underwater SfM-based protocol for monitoring large seagrass restoration areas. The data obtained from consumer-grade red-green-blue (RGB) imagery allowed the fine characterization of the seabed by using 3D dense point clouds and raster layers, including orthophoto mosaics and Digital Surface Models (DSM). The integration of high spatial resolution underwater imagery with object-based image classification (OBIA) technique provided a new tool to count transplanted Posidonia oceanica fragments and estimate the bottom coverage expressed as a percentage of seabed covered by such fragments. Finally, the resulting digital maps were integrated into Geographic Information Systems (GIS) to run topographic change detection analysis and evaluate the mean height of transplanted fragments and detect fine-scale changes in seabed vector ruggedness measure (VRM). Our study provides a guide for creating large-scale, replicable and ready-to-use products for a broad range of applications aimed at standardizing monitoring protocols in future seagrass restoration actions.

The role of habitat degradation on the spread of the alien green alga Caulerpa cylindracea is reported here by comparing observations achieved through a multi-year assessment on three Mediterraneans habitats, namely Posidonia oceanica meadows, Phyllophora crispa turf, and coralligenous reefs. Due to the peculiarity of the study site, both natural-reference and impacted conditions were investigated. C. cylindracea occurred in all the studied habitats under impacted conditions. High susceptibility to the invasion characterized impacted P. oceanica, where Caulerpa cover reached 70.0% in summer months. C. cylindracea cover did not differ significantly among conditions in P. crispa turf, where values never exceeded 5.0%. Conversely, the invasive green algae was low in abundance and patchily distributed in coralligenous reefs. Our results confirmed that habitat loss enhances the spread of C. cylindracea, although with different magnitudes among habitats. Dead matte areas of P. oceanica represented the most vulnerable habitat among those analyzed, whereas coralligenous reefs were less susceptible to the invasion under both the studied conditions.

Sea cucumbers constitute common benthic organisms in the subtidal zones capable of providing key ecosystem services. Due to the recent harvesting and increased commercial interest in Mediterranean species, fundamental ecological knowledge is required to promote adequate management measures. In this regard, a remotely sensed mapping method is proposed for deriving length-frequency distribution and defining habitat preferences of a common sea cucumber species. Image analyses and classification carried out on high spatial resolution cartographic products, including orthophoto mosaics and digital surface models derived by 'Structure-from-Motion' photogrammetric processing, allowed accurate estimation of sea cucumber densities over an area of 5500 m². The mean length of mapped specimens ranged from 8 to 50 cm, with a peak in abundance recorded in April. A distinct preference for sandy substrates was observed, characterized by low vector ruggedness measure values ranging from 0 to 0.15. A negative correlation was also identified in areas with slopes exceeding 15°, especially among larger size classes. Spatial patterns investigated with a Log-Gaussian Cox Process showed significant temporal trends with well-defined aggregation patterns during the summer months in specific areas covered by transplanted P. oceanica fragments with higher (up to 0.25) vector ruggedness measure values. This finding confirmed the essential role of this restored habitat in retaining fine organic particle used as a source of food for deposit-feeding holothurians. This approach is expected to enhance the creation of tailored management plans for sea cucumber populations in coastal regions, facilitating accurate assessments of the impacts of fishing activities on these species.