Swinging boat moorings: Spatial heterogeneous damage to eelgrass beds in a tidal ecosystem

Seagrass meadows are currently known to be subjected to huge physical disturbances including boat moorings in shallow bays. We aimed to identify the impact of permanent swing mooring on the fast-growing seagrass Zostera marina in a mega-tidal area. Coupling the hydrodynamic MARS3D model to simulate mooring chain movements and in situ measurements of plant traits, we analyzed the structural responses of the eelgrass bed to scraping disturbance in the western English Channel (France). A comparison of the results with a reference site without any permanent swing boat mooring showed a significant impact on eelgrass structure (shoot density, leaf size, leaf dry weight), depending on the direction and distance from the mooring. Zostera marina was absent close to the mooring fixation point in three out of the four directions we evaluated. Beyond 5 m, the canopy height remained lower than in the reference site, most likely due to regular disturbances by mooring chains. Conversely, shoot density beyond 5 m was higher than in the reference site. This adaptive response counter-balanced the decrease in canopy height at these distances. The fluctuations of the structure of the eelgrass cover (number of shoots, leaf length) at a small spatial scale was clearly in accordance with the scraping intensity simulated by the MARS3D model. The tidal currents coupled to tidal amplitude variability imply a small-scale heterogeneous effect of permanent mooring on the benthic compartment, previously undetected by an aerial survey. The present results highlight the interest of coupling approaches so as to understand how physical pressure influences fast-growing species traits. The resulting important modifications could imply a more functional impact such as biodiversity loss and carbon sequestration, which is beyond the scope of the present paper.