Morphosedimentary evolution and benthic recovery dynamics 45 years after cessation of intensive marine aggregate...

Dredging activities can alter seabed morphology and benthic communities, with long-lasting ecological consequences. This study investigates the long-term morphosedimentary and biological evolution of the CNEXO dredging pit, located in the eastern Bay of Seine (English Channel, France), more than four decades after extraction ceased. A multidisciplinary approach combining bathymetric data (1981, 2002, 2025), sediment analyses and benthic community surveys (1995, 2010, 2025) was applied to evaluate physical infilling processes and ecological recovery. Bathymetric data indicates that the pit remains partially infilled in 2025, despite substantial sediment accumulation since 1981, particularly between 2002 and 2025. Pit sediments remain heterogeneous, comprising sand, mud, and gravel derived from slope erosion and post-extraction deposition. This heterogeneity, coupled with bathymetric gradients, exerts a strong structuring effect on benthic assemblages, with redundancy analysis (RDA) showing that sediment composition and depth explain approximately 32% of community variability. Biotic indices (taxonomic richness, abundance, Shannon diversity and Pielou's evenness) revealed higher diversity and abundance within the pit than at peripheral stations. Non-metric multidimensional scaling (nMDS) highlights clear spatial and temporal gradients, reflecting a gradual convergence of pit and peripheral communities over time, consistent with the ongoing siltation of the eastern Bay of Seine. Overall, the CNEXO pit remains morphologically and ecologically distinct from surrounding habitats, a pattern supported by the Ecological Quality Status indices (MUMARINEX, GPBI, M-AMBI, and EQA), underscoring the long-term legacy of intensive, fixed-point dredging. These findings reinforce ICES recommendations advocating extensive or semi-extensive extraction practices to facilitate faster physical and biological recovery of impacted marine ecosystems.