Understanding morphological evolution and sediment dynamics at multi-time scales helps balance human activities and protect coastal ecosystems: An example with the Gironde and Pertuis Marine Park.

Finding a balance between the preservation of habitat, species and the sustainable development of human activities in Marine Protected Areas (MPAs) is made even more challenging in coastal areas where sediment dynamics entails naturally changing habitats. To achieve this goal, a solid knowledge base is needed, and reviews are essential. Starting from an extensive review of sediment dynamics and coastal evolution at three-time scales (from millenaries to events), in the Gironde and Pertuis Marine Park (GPMP, French Atlantic coast), we investigated the interactions between human activities, sediment dynamics and morphological evolution in the GPMP. Five activities were identified as having a maximum interaction with coastal dynamics: Land reclamation, shellfish farming, coastal defences, dredging and sand mining. In sheltered areas, where natural sediment fill occurs, land reclamation and shellfish farming increase sedimentation through a positive feedback mechanism, leading to instability. Natural coastal erosion and sediment fill in harbours and tidal channels are fought by coastal defences and dredging, respectively, creating negative feedback and stability. However, these activities also generate negative side effects such as upper beach erosion, pollution, and increased turbidity. Sand mining, mainly developed in submarine incised valleys, results in a deepening of the sea floor, which is naturally filled by sediments from surrounding areas, tending towards shoreface profile restoration. However, sand extraction exceeds natural renewal rates, and may impact the stability of coastal ecosystems in the long term. These activities are at the heart of environmental management and preservation issues. This review and a discussion of the interactions between human activities and coastal behaviour enabled us to make recommendations that could counteract instabilities and negative side effects. They mainly include depolderization, strategic retreat, optimization and sufficiency. Given the diversity of the coastal environments and human activities found in the GPMP, this work is transferable to many MPAs and coastal areas whose objective is to foster sustainable human activities compatible with habitat preservation.