Ventura et al. 2022. Seagrass restoration monitoring and shallow-water benthic habitat mapping through a photogrammetry-based protocol

Ventura, D., Mancini, G., Casoli, E., Pace, D. S., Lasinio, G. J., Belluscio, A., & Ardizzone, G. (2022). Seagrass restoration monitoring and shallow-water benthic habitat mapping through a photogrammetry-based protocol. Journal of Environmental Management , 304 , 114262. Redirecting

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.

Fig. 2. Graphical layout of the equipment used for high-accuracy SfM-based mapping of seagrass transplantation (a) Permanent visible markers or ground control points (GCPs) and 1 × 1 m quadrats (in green) were placed on the seafloor inside the transplanting area, previously divided into 20 sub-plots by a 5 × 5 m grid (black dotted line). From the surface, XY and (-Z = depth) coordinates perpendicular to the vertical plane of each GCP were determined by a snorkelling operator using a low-cost RTK GNSS system and a plumb line equipped with a digital depth gauge. (b) The image acquisition was carried out by a SCUBA diver operator aided by a diver propulsion vehicle (DPV) equipped with a GoPro Hero 5 Black action camera. The inset in figure b shows the survey grid followed by the diver for a large-scale survey. The camera’s position (green dots), images in-track and cross-track overlaps (red frames) are also shown, as well as the action camera and the adjustable adhesive mount used. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)