Protected by international, European and local acts, Zostera marina L. meadows, a marine flowing plant (angiosperm), are subject to considerable human pressure related to their geographical distribution: abrasion of anchor chains, trawling due to professional or recreational boat fishing, trampling due to foot fishing at low tide. This study takes part of the European Life Impact project to assess the disturbance/stress of human activity on the subtidal seagrass meadow at La Varde (48°40’59 N ; 1°59’13 W) in the commune of Saint-Malo in Brittany.
The aim of this study is to monitor the surface evolution and fragmentation of the seagrass meadow on a fine spatial and temporal scale using a drone. three drone campaigns per year were carried out on 2021 and 2022. Cross-analysis of underwater truths by snorkeling and drone imagery in Red-Green-Blue natural colors were used to extract the overall envelope of the seagrass meadow. Five classes were identified (immersed seagrass, emerged/immersed sediment, emerged rock, and immersed macroalgae) and a machine learning algorithm, namely the maximum likelihood, was processed.
Preliminary results have revealed a loss of 465.18 m2 at annual scale, between 2021 and 2022 of the seagrass meadow. Analysis of the results on a seasonal scale highlights a shrinkage of the meadow during the winter period and an expansion during the summer period, i.e. a differential of 382.4 m2 between March and September 2021 and 370.64 m2 between February 2022 and October 2022. Seagrass meadow fragmentation has measured (fragmented envelope area) and highlight a difference of -12.15 m2 between 2021 and 2022. These temporal variations are attributable by the sensitivity of the Zostera marina L. species to variations in sedimentation and turbidity, which are more pronounced in winter due to the higher level of hydrodynamism at this time of year. As regards fragmentation, the meadow recovery can be explained by the protection measures adopted, such as restricting fishing on foot and setting up ecological moorings.
