Numerical investigation of seasonal wind-wave responses to severe seagrass decline in a coastal lagoon

The intensification of environmental stressors and anthropogenic pressures has resulted in significant seagrass loss worldwide, reducing the capacity of vegetation to damp hydrodynamic energy. While the broad-scale impact of seagrass decline on wind waves has been partially investigated in coastal lagoons, the role of seasonal variations in seagrass characteristics in influencing these responses remains not fully understood. We propose to explore the seagrass-wave interaction through the implementation of a novel coupled wave-flow-vegetation model, which considers deflected canopy height in WAVEWATCH-III, and applied to the Arcachon lagoon. We perform a scenario analysis using different wind conditions and various maps of seagrass coverage (from 1989 to 2016) to provide new insights into how seasonality affects wind wave responses to Zostera noltei decline. Our results reveal that during the summer, when seagrasses are fully developed, seagrass decline significantly reduces the ability of Z. noltei to dissipate wave energy, leading to a wave height increase (reaching 30 %, for H m0 between 0.05 m and 0.35 m) in the areas where seagrass coverage is most reduced. However, during the winter, when canopy height and density are low, no significant changes in wave dissipation are observed due to Z. noltei decline. This study also shows that modifications of the ambient current within the canopy further decreases the capacity of Z. noltei to attenuate waves, as the intensification of flow conditions due to seagrass decline reduces canopy height. • Implementation of a coupled wave-flow model accounting for flexible vegetation. • Definition of an adaptative cutoff frequency for wind wave reconstruction. • Zostera noltei decline significantly modified wave height in summer. • Zostera noltei decline had no significant influence on wave conditions in winter. • Flow intensification due to Z. noltei loss reduced their ability to dissipate waves.