Disentangling effects of protection on seabed organic carbon and biodiversity in a rare highly protected mud-dominated MPA

Soft sediment habitats on the European shelf have endured centuries of bottom trawling, causing major biodiversity loss and long-term degradation. Due to sustained value to commercial fisheries and perceptions of low biodiversity value, they have been overlooked in the designation of MPAs. Recently, the role these habitats and their ecological communities may play in carbon storage has emerged as a compelling argument for strict protection. We investigated the effects of seabed protection on biodiversity and sediment organic carbon (OC) in a rare, highly protected, mud dominated MPA, the South Arran Marine Protected Area. Despite the challenge of confounding environmental variables inherent to coastal MPAs, we showed strong positive responses of infaunal biodiversity metrics to protection. Organic carbon concentration was driven primarily by sediment mud content; however, protection increased the total OC–to–mud ratio and stabilized the positive mud–OC relationship which was significantly less reliable in the unprotected zone. Our results demonstrate that protecting soft sediments can enhance biodiversity and may contribute to climate mitigation. We call for their consideration in the establishment of strict, well-enforced MPAs as an opportunity to re-calibrate chronically shifted ecological baselines, and to understand their potential in carbon storage. • Bottom trawling has degraded soft sediment habitats on the European shelf, causing biodiversity loss and long-term ecological shifts. • Rare, mud-dominated, highly protected MPAs provide valuable opportunities to assess biodiversity recovery and carbon storage potential. • Seabed protection had a significant positive effect on multiple epifaunal and infaunal biodiversity metrics and altered fish community structure. • Protection increased the organic carbon to mud ratio and stabilised the mud–organic carbon relationship. • Soft sediment protection can enhance biodiversity and support early carbon recovery processes, with implications for climate mitigation and MPA policy design.