Addition of mussel shells enhance marine benthic biodiversity in two degraded coastal soft sediment ecosystems

The decline in coastal shellfish populations, attributed to anthropogenic stressors such as harvesting via dredging, can deplete the seabed of vital biogenic habitat. This removal of shellfish significantly reduces habitat complexity, biodiversity, and the hard substrate required for the natural recovery of some key species. A common method for oyster restoration involves deploying oyster shell material to provide habitat and substrate for settling larval oysters. In contrast, hard substrate is not required for the mussel lifecycle and as a result the deployment of mussel shell for seabed restoration has received minimal attention, with the effects of reintroducing this material poorly understood. Deploying mussel shell material has the potential to aid in ecosystem recovery, including reviving seabed biodiversity, especially in areas that have been depleted of shellfish via damaging methods such as dredging. This study aimed to investigate the ecosystem effects of returning mussel shells onto two locations with differing soft sediment (sand and mud) in areas historically subjected to excessive dredging of mussel populations and sedimentation. Within 2 years 4 months the infaunal taxa richness was higher in the shell areas compared to the reference areas in both locations, while abundance of infauna was higher in only the sand location and infaunal diversity did not differ at either location. Epifaunal diversity was also higher in the shell area compared to the reference areas at both locations, while the abundance and richness did not differ. Few significant differences were seen in benthic biogeochemical variables between the reference and shell areas and those observed were likely the result of the addition of the shell material changing the sediment composition. These findings demonstrate that mussel shells are a restoration tool that can enhance epifaunal biodiversity and infaunal taxa richness in soft sediment ecosystems historically impacted by shellfish overharvesting and sedimentation.