Restoring mussel beds in highly dynamic environments by lowering environmental stressors

Restoration of coastal ecosystem engineers that trap sediment and dampen waves has proven to be difficult, especially in the wave-exposed and eroding areas where they are needed the most. Environmental stressors, such as hydrodynamic stress and predation, can only be overcome if transplanted organisms are able to establish self-facilitating feedbacks. We investigate if the artificial lowering of multiple environmental stressors can be used to give transplanted juveniles the opportunity to form a self-sustainable system and thereby increase their long-term survival on wave-exposed and eroding shores. We designed a large field experiment using juvenile mussels (Mytilus edulis) as model species on a wave-exposed tidal flat in the Oosterschelde estuary (the Netherlands). We tested if the environmental stress caused by a high predation pressure and wave-driven dislodgement could be reduced by a combination of artificial structures such as fences (to exclude predatory crabs), attachment substrates (such as coir-net or oyster shells), and breakwaters. Despite a low overall mussel survival (29%), we found that under strong hydrodynamic conditions, experimental fences and attachment substrates increased the retention of transplanted mussel seed. However, modification of local hydrodynamic conditions using breakwaters did not improve mussel coverage preservation. Overall, this study highlights the potential of using techniques that lower multiple environmental stressors to create a window of opportunity for establishment in highly dynamic ecosystems.