Ecological consequences of seagrass decline on macrozoobenthic communities: a case study of a Zostera noltii meadow in Arcachon Bay, France

Seagrasses form critical coastal habitats that are experiencing worldwide decline, often resulting in reduced abundance and diversity of associated macrofauna. This study focuses on Arcachon Bay (SW France), a coastal lagoon characterised by an extensive Zostera noltii meadow (40 km 2 in 2019) that has declined by 44% since 1989. This work aimed to characterise benthic community responses to environmental changes, including seagrass loss, using uni and multivariate analyses of two broad-scale surveys (49 stations) conducted in 2002 and 2023, supplemented by a biennial survey at a single station over 22 years. Results reveal significantly different benthic communities in 2002 and 2023 associated with a significant loss in macrofauna densities, while diversity remained similar. Temporal β-diversity (β SOR = 0.65) was primarily due to turnover (species replacement) rather than nestedness (species loss/gain). Five distinct benthic communities were identified, mainly influenced by seagrass biomass, elevation, distance to the ocean and sediment characteristics. The biennial surveys offered additional insights into the temporality of these changes at one station. The combined effect of these modifications and habitat fragmentation on the ecosystem functioning remains to be addressed more specifically.