Posidonia oceanica restoration, a relevant strategy after boat anchoring degradation?

Trabajo presentado al 7th Mediterranean Symposium on Marine Vegetation, celebrado en Génova (Italia), los días 19 y 20 de septiembre de 2022.The anchoring of large pleasure boats constitutes one of the main threats in shallow marine habitats and particularly for seagrass beds. In the Mediterranean, this activity has seen constant development during the last decades, causing major physical disturbances in Posidonia oceanica meadows and associated ecosystem services, notably in terms of climate change mitigation (i.e. carbon fixation and sequestration capacities). In this context, the aims of the present study are to estimate the impact of these anchoring activities on P. oceanica meadows in a particularly highlyfrequented area (Sant’Amanza gulf, SE Corsica Island) and to set up a strategy to restore this major carbon sink. Since the last decade, time-series of marine habitat maps revealed an important regression of P. oceanica meadows, with a loss of 72.9 ha, corresponding to 11% of the meadow surface and 9% decline in the total carbon fixation and sequestration performed each year. Moreover, in the most impacted part of the bay (Balistra bay), a loss of 16.6 ha (28%) has been recorded between 2011 and 2022. Following recent enforcement of anchoring regulation, prohibiting the anchoring of large units (greater than 24 m), and the lockdown linked to the COVID-19 pandemic, anchorages in the seagrass reduced by 92%, between 2018 and 2022. Natural recovery was observed at the edge of the meadow (plagiotropic rhizomes) but this growth is slow and the areas to recover are large. As a result, a transplant experiment, from cuttings harvested from the adjacent meadows, was initiated in the spring 2021. Four restoration techniques are being tested in the some pilot sites and the development of these transplants will be monitored and compared to the natural recovery.This research has been co-financed by INTERREG program (GIREPAM), Collectivity of Corsica Corse (PADDUC-CHANGE), SETEC Foundation, Environment Office of Corsica and the French Office of Biodiversity (RenforC program)

Mapping of Posidonia oceanica beds around Elba Island (western Mediterranean) with integration of direct and indirect methods

Direct and indirect methods were used for mapping Posidonia oceanica beds around Elba Island. Side scan sonar and a visual infra red scanner were used to detect the extension of beds. Also, direct observations were adopted to assess the bed density and to calibrate the indirect methods. Samples of P. oceanica were collected and phenological variables and macroalgal epiphytes were studied to assess the health of the beds. P. oceanica around Elba Island covered an area of 3 680 ha. The largest beds occupied the four main gulfs of the island, characterized by gentle slopes and sandy bottoms. There, the beds showed uniform cover while a patchy structure was common on the rocky and steep bottoms of the western and south-eastern coasts of the island. P. oceanica beds at Elba Island demonstrated a good state of health. The epiphytic algal assemblage of the leaves was dominated by species generally found on this habitat, while, on the rhizomes, we observed a high covering of turf-forming species. The methods adopted allowed accurate maps at a scale of 1:10000. The integration of data obtained both by direct and indirect methods and the precision of the positioning system were particularly important.Le sonar latéral et la télédétection aérienne ont été utilisés pour la localisation des herbiers, tandis que des observations directes en plongée autonome et au moyen de véhicules sous-marins l'ont été pour identifier certaines images ou pour mesurer la densité des herbiers. Des faisceaux de P. oceanica ont été prélevés pour l'étude de la phénologie de la plante et de la communauté macroalgale épiphyte, de manière à évaluer la condition écologique des herbiers. P. oceanica autour de l'ı̂le d'Elbe couvre une surface de 3 680 ha. La pente limite l'expansion des herbiers, d'où une étendue très basse par rapport à la longeur de la côte (25,3 ha km−1). Les herbiers les plus étendus apparaissent dans les baies principales de l'ı̂le : Campo, Lacona, Stella et Procchio. Les herbiers y présentent une couverture uniforme, tandis qu'une structure en taches est commune sur les fonds rocheux et à pente forte, le long des côtes occidentales et sud-orientales. Les paramètres structuraux et phénologiques traduisent une bonne condition écologique des herbiers à P. oceanica. De petites aires de régression sont présentes autour des ports principaux et le long de la côte nord-orientale, où les effets d'anciennes mines sont encore visibles. Les méthodes utilisées ont permis la réalisation de cartes très précises à une échelle de 1:10 000. L'intégration des données directes et indirectes et la précison du système de positionnement (DGPS) ont été particulièrement importantes pour la connaissance de l'étendue des herbiers et de leurs situation écologique. Ces résultats constituent un document pour la gestion du système littoral de l'ı̂le d'Elbe