Limitations of using repurposed dredged sediments and seabed capping for Posidonia australis restoration

Seagrass meadows are among the most degraded coastal habitats worldwide, with limited natural recovery. Interest in using dredged sediments for restoration is increasing, but further study is needed to ensure sediment-seagrass compatibility. We tested a novel restoration approach in Cockburn Sound, southwestern Australia, to evaluate: (a) the feasibility of sediment capping using containment structures in a subtidal environment, (b) the potential benefits to seagrass of dredged products as a cap over port sediment, (c) the influence of adding natural organic matter (seagrass wrack) and (d) benefits of seagrass root microbiomes to restoration outcomes. We established four sediment treatments: control plots (undisturbed), plots with containment structures (experimental controls) and plots with containment structures filled with dredged products, half of which with seagrass wrack. We transplanted 1120 sprigs of the seagrass Posidonia australis, half with experimentally disrupted root-associated microbiomes. While containment structures remained intact, their addition, regardless of sediment treatment, had negative impacts on seagrass survival. By 12 months, average survival dropped to 54% (±3.6SE) for all containment structures compared to 82% (±5.6SE) in control plots. Net shoot numbers declined in treatments with containment structures (approx. 40% shoot loss). Neither wrack addition nor intact root microbiomes enhanced plant performance. This study suggests that while seabed manipulation techniques such as sediment capping with dredged products using containment structures are technically feasible, they may not support seagrass restoration. With careful matching of the structures and sediments to ecological requirements, such approaches may still be valuable for restoration or other coastal management objectives in sensitive marine systems.