Space-time evolution of a lagoon seagrass meadow colonized by Pinna nobilis using UAV and satellite data

Abstract:

The presence and density of seagrass meadows is a key element in the stability of the bottom sediments of lagoons, deeply influencing the water turbidity and the sediment resuspension, transport and deposition. Seagrass density is also one of the driving factors in the colonization of Pinna nobilis within the meadows, the largest bivalve mollusk endemic of the Mediterranean Sea, reaching a size of up to 120 cm and strictly protected by the European Habitats Directive (92/43/EEC). After several decades, the presence of Pinna nobilis has been recorded in some lagoons along the Adriatic Sea, where the fan mussel disappeared at the beginning of the last century and started to re-colonize the seagrass meadows 10-15 years ago. In the Venice lagoon (Italy), Pinna nobilis has colonized several areas in the central and southern basins, and there are now hundreds of individuals that grow and live on vegetated tidal flats.

In this study we map the abundance of Pinna nobilis using UAV’s images and link it to the seagrass density and to the main geomorphological features of the tidal flat. UAV’s data are also used to calibrate Sentinel-2, ASTER and Landsat data in order to explore the evolution of the meadow density in the last two decades, when the mollusk re-colonized the Venice lagoon reaching the current high spatial coverage. Field surveys and lab analyses as well as 18 years of data from a multi-parametric probe allowed us to explore several environmental factors (as for example suspended sediment concentration) that likely affected the Pinna nobilis population growth and health conditions.

It is important to underline that Pinna nobilis has been recently listed Critically Endangered on the IUCN Red List of Threatened Species due to a pathogen that is causing mass mortality events across the entire Mediterranean region. Surprisingly, the population currently present in the Venice lagoon appear to be still in good health conditions, even though the situation might change soon. One of the goals of this research is to detect the main geomorphological and environmental factors that drove the population growth in the last 15 years in order to provide management indications for protection and/or restoration methods in case the population will be attacked by the pathogen.