Assessing edge-effects in Posidonia oceanica seagrass meadows: A multidisciplinary approach in Calvi Bay (Corsica)

Supervisor: Sylvie Gobert (Univ. Liege)
Structural boundaries in ecosystems play an important role both in the context of seascape architecture and ecological processes. The Mediterranean Seagrass Posidonia oceanica (L.) Delile is considered an ecosystem engineer species, forming habitats of great ecological value and providing many ecosystem services. However so far, only few studies have addressed the importance of edges on this habitat. Therefore, this study aims to (1) determine whether there are differences between seagrass-sand corridors edges and the adjacent continuous meadow, and (2) investigate whether anthropogenic pressures can cause disturbances in the measured parameters along the edges and the meadow. To reach those objectives, we have developed a multidisciplinary approach combining plant physiology, canopy community studies and seagrass structural parameters characterization. Two sites located ca. 15 m depth with contrasted natural and anthropogenic influences were selected in Calvi Bay (Corsica, France). At each site two natural occurring edges-sand corridors (SC) were examined. We performed in situ chlorophyll fluorescence measurements using a Pulse Amplitude Modulated (Diving-PAM) fluorometry in order to assess the photosynthetic rate of the shoots. Vagile-macrofauna of the leaf stratum was sampled by a hand-towed net, and the major taxonomic groups were sorted, counted, and identified. Meadow’s biometric measurements and the epiphytic biomass were also determined. Regarding edge-meadow matrix, results have shown highest differences on matrix structural parameters such as shoot density and shoot type proportions. Shoot density was found to decrease in edges considerably. We found c.a to 60 % plagiotropic shoots on edges while in continuous meadow orthotropics were predominant (up to 90 %). Vagile-fauna population densities and diversity did not differ significantly among stations studied, neither by sites. Photosynthetic rate and leaf surface values also did not show changes between edges and continuous meadow. However, results did show that plagiotropic shoots had higher photosynthetic rate than orthotropics, and also that epiphyte abundance sorted out to be much higher (up to 54 %) on edges. The overall results showed that edges differ from continuous meadow especially referring to habitat architectural parameters; nevertheless ecological effects regarding anthropogenic impacts are far from being well understood. We concluded that particular attention must be given to these edge areas for the purpose of developing and enhancing seagrass management and conservation strategies.