Ecological indicators for seagrass meadow status: Adopting efficiency of space occupancy as a reference

• Seagrass efficiencies of space occupancy differed based on site and species. • NZN had better overall efficiencies compared with ZM. • Relative cover and leaf area index were efficient indicators of ecological status. • Relative algal cover was higher when proportions of fine sand particles were lower. • Associations of epi- and endofauna with habitat type varied with seagrass species. Currently used indicators, such as shoot density and biomass, are increasingly questioned for their effictiveness in reflecting seagrass ecological status. In the present study, meadows along the English Channel, Atlantic, and Mediterranean coasts of France were investigated. Multiple seagrass growth traits were tested for their ability to reflect meadow carrying capacity. The distance of each meadow to the interspecific boundary line (d grass ), as defined by the seagrass biomass-density relationship, was measured, and all meadows were classified into different levels of ecological status using ordinal scaling. Both inter- and intra-site differences, as well as temporal variations, were analyzed. Correlation analyses were performed to identify patterns between d grass and other biological variables. Sites varied in their carrying capacities depending on the seagrass species present. Among Zostera marina (ZM) meadows, the Dinard site had the highest efficiency of space occupancy, while the Thau site had the lowest. For Nanozostera noltei (NZN) meadows, the Thau site had the highest efficiency, and the Arcachon site had the lowest. Overall, NZN meadows demonstrated better efficiencies of space occupancy compared to ZM meadows, but both species showed lower efficiencies than the most efficient plants studied worldwide. The d grass 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, with higher relative algal cover observed in sites with lower proportions of fine sand particles. Epifaunal biomass or richness was most often associated with algal biomass and cover, and occasionally with seagrass traits like aboveground biomass or LAI, depending on the species. Endofauna was linked to either seagrass growth traits or bare sediments, depending on the species. This study distinguishes between suitable and unsuitable plant morphometric traits for use as indicators. While traits like shoot density are widely used, our findings suggest that relative cover and LAI may be by far more reflective of ecological status. Temperature was the most influential factor on LAI, while sediment structure played a role in modulating competition between seagrasses and algae. Overall, the study emphasizes the need to identify site- and habitat-specific indicators. Future work will further assess the environmental pressures on these four sites to classify them according to their conservation status.