Lessons learnt from revisiting decades of seagrass restoration projects in Cockburn Sound, southwestern Australia

Seagrass loss is impacting coastal communities globally, and significant efforts are being spent to address this loss through restoration. Yet, the success of restoration projects and methodologies has rarely been assessed over decades. In this case study, we reviewed past and continuing seagrass restoration projects (66 restoration sites from 1990s to 2020s) in Cockburn Sound and Owen Anchorage, temperate Western Australia, to (1) address whether they were successful in rehabilitating seagrasses, (2) whether seagrasses could be restored at appropriate scales, and (3) what the requirements for successful seagrass restoration were. In 2022, 28 individual restoration sites were revisited to establish long-term restoration success. Methods of seagrass restoration included shoots (as sprigs, plugs, cores, and sods), seedlings, and seeds. Approximately 70% of sites revisited in 2022 showed demonstrable success in restoring seagrasses. Project extent ranged from meters to hectare scales, including a study that restored 3 ha using sprigs. In the 2010s, seed-based restoration research became a major success at hectare scales. Pre-existing environmental conditions and processes were extremely important in determining restoration success, which was both site-and time-specific and influenced the choice of restoration methods. Restoration required the environment to be suitable for natural seagrass revegetation, or it needed modification. Researchers' focus on small-scale experiments testing methods across a range of environments has prepared us for scaling up to hectares. In long-lived seagrasses, decades of hysteresis were overcome with restoration, as it assisted natural recovery.