Summary1. In coastal and estuarine systems, foundation species like seagrasses, mangroves, saltmarshes or corals provide important ecosystem services. Seagrasses are globally declining and their reintroduction has been shown to restore ecosystem functions. However, seagrass restoration is often challenging, given the dynamic and stressful environment that seagrasses often grow in. 2. From our world-wide meta-analysis of seagrass restoration trials (1786 trials), we describe general features and best practice for seagrass restoration. We confirm that removal of threats is important prior to replanting. Reduced water quality (mainly eutrophication), and construction activities led to poorer restoration success than, for instance, dredging, local direct impact and natural causes. Proximity to and recovery of donor beds were positively corre- The meta-analysis shows that both trial survival and seagrass population growth rate in trials that survived are positively affected by the number of plants or seeds initially transplanted. This relationship between restoration scale and restoration success was not related to trial characteristics of the initial restoration. The majority of the seagrass restoration trials have been very small, which may explain the low overall trial survival rate (i.e. estimated 37%). 4. Successful regrowth of the foundation seagrass species appears to require crossing a minimum threshold of reintroduced individuals. Our study provides the first global field evidence for the requirement of a critical mass for recovery, which may also hold for other foundation species showing strong positive feedback to a dynamic environment. 5. Synthesis and applications. For effective restoration of seagrass foundation species in its typically dynamic, stressful environment, introduction of large numbers is seen to be beneficial and probably serves two purposes. First, a large-scale planting increases trial survival -large numbers ensure the spread of risks, which is needed to overcome high natural variability. Secondly, a large-scale trial increases population growth rate by enhancing selfsustaining feedback, which is generally found in foundation species in stressful environments such as seagrass beds. Thus, by careful site selection and applying appropriate techniques, spreading of risks and enhancing self-sustaining feedback in concert increase success of seagrass restoration.

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Sharing experiences and results among scientists and managers working on seagrass restoration was the main objective of the first European Seagrass Restoration Workshop that gathered researchers from around Europe. The meeting was the first forum in Europe that allowed for scientists, NGOs, and managers to interact and share their experiences relating to seagrass restoration and management. The results show that none of the seagrass restoration programs developed in Europe by the participants during the last 10 years was successful. Furthermore, an informal review of data published in seagrass restoration success, showed that the results reported were biased because they were mostly based on a very short monitoring period (i.e. <1 year). Numerous decision trees, guidelines, and restoration models have been developed to aid seagrass restoration management, but the results of this workshop point toward a new paradigm in seagrass restoration were efforts should shift to give priority to natural restoration potential, with an emphasis on the fact that restoration should never be considered the first alternative when planning for the mitigation of coastal development projects or to justify mitigation as a compensation measure for economic activities.

Some of the major challenges in seagrass restoration on exposed open coasts are the choice of transplant design that is optimal for coastlines periodically exposed to high water motion, and understanding the survival and dynamics of the transplanted areas on a long timescale over many years. To contribute to a better understanding of these challenges, we describe here part of a large-scale seagrass restoration program conducted in a Marine Park in Portugal. The goal of this study was to infer if it was possible to recover seagrass habitat in this region, in order to restore its ecosystem functions. To infer which methods would produce better long term persistence to recover seagrass habitat, three factors were assessed: donor seagrass species, transplant season, source location. Monitoring was done three times a year for 8 years, in which areas and densities of the planted units were measured, to assess survival and growth. The best results were obtained with the species Zostera marina transplanted during spring and summer as compared to Zostera noltii and Cymodocea nodosa. Long-term persistence of established (well rooted) transplants was mainly affected by extreme winter storms but there was evidence of fish grazing effects also. Our results indicate that persistence assessments should be done in the long term, as all transplants were successful (survived and grew initially) in the short term, but were not resistant in the long term after a winter with exceptionally strong storms. The interesting observation that only the largest (11 m 2) transplanted plot of Z. marina persisted over a long time, increasing to 103 m 2 in 8 years, overcoming storms and grazing, raised the hypothesis that for a successful shift to a vegetated state it might be necessary to overpass a minimum critical size or tipping point. This hypothesis was therefore tested with replicates from two donor populations and results showed effects of size and donor population, as only the larger planting units (PUs) from one donor population persisted and expanded. It is recommended that in future habitat restoration efforts large PUs are considered.