Résumé
Currently used indicators such as shoot density and biomass are being questioned for their efficacy in reflecting seagrass ecological status. In the current study, meadows along the English Channel, Atlantic coast and Mediterranean coast of France were investigated. Multiple seagrass growth traits were tested for their efficacy in reflecting meadow carrying capacity. The distance of each meadow to the interspecific boundary line (dgrass), defined by the seagrass biomass-density relationship, was measured, and all meadows were classified using ordinal scaling into different levels of ecological status. Both inter- and intra-site differences, as well as temporal variations, were analysed. Correlation analyses were performed to determine patterns among dgrass and other biological variables. Sites varied in their carrying capacities based on seagrass species. Among Zostera marina (ZM) meadows, the Dinard site had the highest efficiency of space occupancy, and the Thau site had the least. Among the Nanozostera noltei (NZN) meadows, the Thau site had the highest efficiency of space occupancy, and the Arcachon site had the least. Among the two species, the NZN meadows had better efficiencies of space occupancy compared to the ZM meadows. Overall, both species had lower efficiencies compared to the most efficient plants studied worldwide. The dgrass was moderately inversely correlated with aboveground and belowground biomass and density and strongly inversely correlated with relative cover and leaf area index (LAI). The distribution of sites based on the biotic community followed a similar pattern to the distribution of sediment grain size. Higher relative algal cover was observed in sites with lower proportions of fine sand particles. Epifauna biomass or richness was more often associated with algal biomass and cover and sometimes with seagrass traits such as aboveground biomass or LAI, depending on the seagrass species. Endofauna was associated to seagrass growth traits or bare sediments, depending on the seagrass species. The current study differentiates between suitable and unsuitable plant morphometric traits for use as indicators. While traits such as shoot density are used extensively, the current study shows that relative cover and LAI may be much more reflective of ecological status. Temperature was the most influential factor on LAI, and sediment structure modified competition between seagrasses with algae. The current study highlights the importance of identifying site- and habitat-specific indicators. Future work from our study will further evaluate the environmental pressures faced by these four sites to characterize them based on conservation status.