Deliverable 3.1 Report on best practices for marine restoration protocols

1 Deliverable 3.1 Report on best practices for marine restoration protocols Ref. Ares(2023)5925541 - 31/08/2023 2 Project number: 101093865 Project duration: 1 Dec 2022 – 30 Nov 2025 Project coordinator: Ida Beathe Øverjordet, SINTEF Ocean Web site: www.climarest.eu Deliverable ID: D3.1 Due month: M9 Preparation date: 2023-08-30 Title: D3.1 Report on best practices for marine restoration protocols Lead beneficiary: UNIVPM Prepared by: Cristina Gambi, Cinzia Corinaldesi, Antonio Dell’Anno, Alice Premici & Roberto Danovaro - UNIVPM Simonetta Fraschetti - UNINA Abstract A literature review has been conducted to assess which best practices are adopted for restoration across marine ecosystems along the EU Seas and behind. More than 50 documents (i.e., scientific papers and technical reports) have been analysed to identify methodologies and protocols of ecological restoration for different marine habitats. Overall ca 200 references are included in the document to support the description of protocols and best practices. The output of this literature review is included in the Deliverable 3.1: Report on best practices for marine restoration protocols. Dissemination level PU Public X CO Confidential, only for members of the consortium (including the Commission services) CI Classified information as referred to in Commission Decision 2001/844/EC) Deliverable type R Document, report X DEM Demonstrator, pilot, prototype DEC Websites, patent filings, videos, etc. OTHER Software, technical diagram, etc. Authorship information Editor Cristina Gambi, UNIVPM Contributing partners UNIVPM & UNINA Version history Version number Date Description of changes 1 2023-08-30 Report submitted to the project officer 3 Table of contents 1. Ecological Restoration in marine coastal ecosystems 4 2. Restoration projects: defining Target, Goals and Objectives 5 3. Literature review 6 4. Restoration protocols and best practices 7 5. Coastal ecosystem restoration 8 5.1 Restoration of seagrass meadows 10 5.1.1 Traditional transplanting approaches and methodologies for adult plants 11 Plug Method 12 Staple Method 12 Popsicle-stick technique 13 Peat-Pot Method 13 Other transplanting methods 14 Cotton mesh bag method 14 5.1.2 Planting seeds 15 Seed-based methods 15 5.1.3 New restoration tools and techniques 16 Buoy-Deployed Seeding (BuDS) 16 Dispenser injection seeding 17 Nurseries 19 Anchoring of shoots with iron nails 19 Biodegradable bags and containers 20 Artificial in-situ structures 21 Alternative sources of transplant units and use of seagrass wrack 21 Promoting positive biological interactions 22 5.2 Restoration of oyster reefs 22 5.2.1. Oyster restoration methodologies 23 Oyster’s reefs clutch 23 Factors influencing the oyster reef restoration 24 Oyster Seeding 26 Species Selection 26 5.2.2 Native Oyster Restoration Alliance (NORA) 28 5.3 Kelp and algal forests restoration 30 5.3.1 Kelp forest 30 Transplanting 32 Removing competitors 33 Grazing avoidance 33 Artificial reefs 34 5.3.2 Algal forests 35 Selection of restoration techniques 36 In situ approach 37 Ex situ approach 37 5.4 Saltmarshes 41 5.4.1 Saltmarshes restoration methods and approaches 42 Vegetation development on restored saltmarshes 43 Wave protection and sediment retention features 44 Sedimentation fields (potential hold the line option) 46 Realigning defences 46 5.4.2 Summary on the abiotic and biotic active approaches 48 6. Final considerations 49 7. References 49 4 1. Ecological Restoration in marine coastal ecosystems Despite the goods and services that marine coastal ecosystems provide to humans (UNEP, 2006), several habitats such as coral reefs, seagrass, mangroves, saltmarsh, and oyster reefs are being lost at alarming rates worldwide mainly due to unsustainable land use, coastal development, and climate change (Duke et al., 2007). Many areas have little natural habitat left to conserve (Aronson and Precht, 2001, Beck et al., 2011) and require restoration interventions. The UN Decade of Ecosystem Restoration (2021 - 2030) is a reaction to the urgent need to massively accelerate global efforts to reverse centuries of ecosystem damage, and to address our current climate and biodiversity crisis (UN General Assembly 2019). While much of this effort is focused on increasing forests on land (e.g., Bonn Challenge, IPCC-IPBES 2019), restoring marine forests and other costal habitats presents a unique - but underappreciated and underutilized - way to protect biodiversity, enhance CO2 sequestration, and provide other relevant benefits to Nature and People (Ortega et al., 2019, Feehan et al., 2021). Growing awareness of marine forests as a source of climate, environmental, and socio-political solutions come from their capacity as carbon sinks, important nutrient filters, and focal points for high biodiversity, as well as an untapped source of sustainable materials and source of opportunities to redress gender and societal inequality (Duarte et al., 2017, Mouritsen et al., 2021). Ecological restoration or “the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed” (Society for Ecological Restoration) is urgently needed to assist ecosystems where natural recovery is hindered or impeded (Perrow and Davy, 2002). Ecological restoration principally seeks to recover the biodiversity and functioning of degraded ecosystems, while providing a range of ecological and socio-economic benefits, such as coastal protection from flooding and erosion, fisheries habitat, water quality improvements, and carbon sequestration and storage (Abrantes et al., 2019, Gilby et al., 2020). The Society for Ecological Restoration (SER) was founded in 1988 to “advance the science, practice and policy of ecological restoration to sustain biodiversity, improve resilience in a changing climate, and reestablish an ecologically healthy relationship between nature and culture”. The International Standards for the Practice of Ecological Restoration (second edition released by SER, 2019) contain several best practice guidelines developed over decades of research and practice from wellestablished restoration of terrestrial habitats, however, is aimed to be transferrable to marine and freshwater ecosystems (McDonald et al., 2019). SER has developed many tools to help restoration practitioners track their progress toward a full ecosystem restoration, such as the “recovery wheel” used to assess advancement based on metrics categorized under the attributes: absence of threats; ecosystem function; external exchanges; physical conditions; species composition; and structural 5 diversity (McDonald et al., 2016). In addition to best practices and metrics, the outcomes from a restoration project can be categorized into ecological, social, and economic or a combination thereof following the framework by Wortley et al. (2013). The increasing experience acquired in the last decades on marine ecosystems at global scale provides evidence on the feasibility of restoration activities also in marine habitats. The success of restoration activities is highly dependent upon different factors: local environmental conditions, the presence of anthropogenic impacts and the protocols applied for restoration actions that are specific for each marine habitat and, can be partly compromised by the occurrence of extreme/episodic events (e.g., storms, heat waves). Independently by the methodologies utilized for marine ecological restoration, the presence of extreme climate-driven events immediately after the beginning of restoration actions can compromise the survival of the target species and the associated biodiversity. The success of most of the pilot restoration actions and the failure, are now lessons learn which allow the identification of the best solutions to make successful future restoration actions and to upscale interventions from pilot small-scale action to larger spatial scale. 2. Restoration projects: defining Target, Goals and Objectives International Standards of Ecological Restoration recognise the need for setting clear targets, goals, and objectives for each restoration project by which its success can be measured and encouraged the development of both social and ecological goals (McDonald et al., 2019). These elements should be agreed on early, through both stakeholder and community engagement as well as expert consultation. This approach also recognises that individual projects and practitioners will have different motivations to support marine ecosystems restoration. The project target is likely to be the recovery of healthy and resilient populations of target species (e.g., seagrasses, algal and kelp forests, coral reefs, mangroves, saltmarsh, and oyster reefs) in each area, but there are multiple reasons why this might want to be achieved, including: • ecological outcomes; • meeting legislative commitments or outcomes; • stakeholders’ and the community’s expectations. In details, Project target: describes the site and native ecosystem to be restored. It should be broad, general, and inspiring but also related to the local conditions of the intervention. Goals: are normally several and open. They describe the level of recovery and outcomes desired, both in social and ecological terms. 6 Objectives: translate goals into clear, distinct, and measurable outcomes or expected changes. They can be helpful in determining the restoration strategies to implement, assessing progress, and managing the project. Often relating to a site’s distinct aspects or a project’s timeframe, they are critical to allowing the project to meet expectations, operate within budget and deliver outcomes against performance criteria. Before starting a restoration project, independently from the target species/habitat, a comprehensive feasibility study and site selection process is recommended. Setting clear goals and targets for restoration projects enables success and progress to be measured, and the purpose of the project to be clearly communicated. Communicating the benefits of the marine ecosystem restoration can help to justify the initiative across stakeholders and society. 3. Literature review Here the aim is to document specific guides to marine habitat restoration showing pros and cons, practical recommendations to support future restoration interventions. The literature review was conducted to identify the best and successful restoration protocols adopted in different marine ecosystems worldwide with special reference to the habitats identified in the CLIMAREST demonstration sites (seagrasses, oyster reefs, soft bottom, algal and kelp forests). We included in the search both scientific (through WoS and Scopus) and grey literature, through an in-depth search on Google Scholar. To comprise in the research as much as possible information, we selected key terms as “marine”, “active”, “restoration”, “transplant”/“outplant”, “seeding” and “restoration protocols or restoration best practices”. We considered both scientific (i.e., papers on international journals, contributions to scientific congresses) literature and technical reports (e.g., projects reports, deliverables and guidelines) on restoration interventions carried out at the international, national or regional level by the scientific communities or public administrations for territory management purposes. We examined methodological approach and protocols to identify the best practices in each different habitat type. Our survey allowed to select more than 50 main contributions, specifically for each habitat as following: • Seagrass: 18 • Saltmarshes: 9 • Oyster reefs: 3 • Algal and Kelp forests: 8 7 • Coastal areas and Nature based solutions: 6. Moreover, our survey included other 6 documents on marine ecosystem restoration without focusing a specific habitat type and recent reports on restoration guidelines tested in specific oceanic regions in the framework of international initiatives and alliances promoted within the UN Decade on Ecosystem Restoration. Overall ca 200 references are included in this document to support the description of protocols and best practices described below. 4. Restoration protocols and best practices Good planning is a critical, but often overlooked, stage of the restoration process. Inadequate planning is often listed as a reason for project failure on different attributes of ecosystems, not only species target but also ecosystem functions. Planning a restoration project includes several steps and requires: • to collect information about the local area, potential restoration, creation, or enhancement sites, historical trends, and other topics that will help you to better frame the project you are initiating. • to select the best site to achieve your goals, or, if you already have a site in mind, planning will help you determine the most reasonable goals for your site. • to establish clear and feasible objectives given the factors that may constrain the project. • to identify the materials, expertise, best practices/protocols, and activities that will be needed to achieve the project’s goals. • to establish clear objectives and target criteria that can be monitored in the short and long time. • to inform people, including potential funders/partners, and the local community with clear goals and objectives that can be easily explained to other. The experience acquired worldwide in restoration projects, independently in terrestrial and marine ecosystems, suggests that key steps in designing and implementing a monitoring plan include: 1. Identify restoration and monitoring objectives: Link the monitoring plan directly to the underlying objectives of the restoration to ensure that the information collected during monitoring is usable for decision making. 2. Create a sampling plan: Ensure that the information collected during monitoring can be easily analysed, synthesized, and evaluated. 8 3. Select monitoring strategy: Acknowledge available time and resources and clarify strategy to ensure that monitoring is rigorous, feasible and can be implemented. 4. Select response variables, metrics, and methods: identify how to measure restoration success in the monitoring plan according to the specific characteristics of restoration site or project so that information collected during monitoring is usable for decision making. 5. Create a data analysis, management and information sharing plan: Ensure that the information collected during monitoring is analysed, synthesized, evaluated, and disseminated. 6. Create an inspection and maintenance plan: Help adjust inspection and site maintenance practices across projects and within a project or site. Not every project will require all the planning steps described above, nor will everything in each step be needed. The extent of the planning required will depend on the condition of the project site and expected goals. Experiences gathered until now both in terrestrial and marine ecosystems revealed that more complex projects require more planning. 5. Coastal ecosystem restoration Coastal zones, concentrating high population densities, economic assets, and cultural heritage, are urbanizing more rapidly than inland regions (Nicholls et al., 2019), while coastal ecosystems provide highly productive and biodiverse environments, with an important and often underappreciated carbon storage potential (Mcleod et al., 2011, Owers et al., 2020). The narrow coastal fringe, a dynamic ecotone comprising water and land zones with a fuzzy boundary (Sanchez-Arcilla et al., 2016), is experiencing progressive degradation and escalating risks (Reguero et al., 2020), with deep uncertainties affecting how to restore sustainably (Brugnach et al., 2008, UNEP 2021). Coastal ecosystem restoration is increasingly recognized as a scientifically credible Nature-based Solution approach capable of supporting the delivery of multiple benefits. A growing body of evidence demonstrates that restoration interventions in coastal areas can be effective over large spatial scales (1,000s-100,000s ha), persist for decades, rapidly expand in size, be cost-effective, and generate social and economic benefits (Abelson et al., 2020, Bayraktarov et al., 2020, Saunders et al., 2020). Models indicate that restoration should produce greater outcomes when complemented by protected area management but can also be a cost-effective investment as an alternative to expanding protection (Possingham et al., 2015). It is becoming clear, however, that the success of restoration actions is context-dependent and, therefore, seascape context (e.g., surrounding sociopolitical and ecological variables) must be considered to avoid inappropriate site selection and for 9 effective scaling up of Nature-Based Solutions (Bradley et al., 2020, Sheaves et al., 2021). Examples of Nature-Based Solutions are summarized from Eggermont et al. (2015) and Riisager-Simonsen (2022). A. Sustainable use and protection of natural ecosystems - Large marine protected areas; - Rebuilding marine life stocks: algae, plants, and animals; B. Improved multifunctionality of managed marine ecosystems - Seagrass and algal forests restoration; - Shoreline protection using boulders, shellfish reefs and seagrass; C. Novel, restored or deliberately designed artificial marine ecosystems - Nature-inspired surface on built marine infrastructures; - Low trophic aquaculture; D. Nature inspired designs which reduce environmental pressures - Wind powered shipping; - Nature-based antifouling agents on ships. All these Nature Based Solutions can address the social challenges identified by United Nations on 1. Climate change mitigation and adaptation 2. Disaster risk reduction 3. Economic and social development 4. Human health 5. Food security 6. Water security 7. Environmental degradation and biodiversity loss Experience acquired from costal restoration projects reveals that the success is driven by several factors including barriers (technical, financial, management or commitment limits) and supporters (biophysical knowledge, economic advances, favourable governance, or social engagement) that are deeply interconnected. Coastal restoration barriers includes technical limitations (engineering expertise; data and metrics for biodiversity and ecosystem services; monitoring and maintenance plans; delayed performance and room for adaptation), financial limitations (benefit-cost ratios; returns from investments; business plans suited to local constraints; short term and small-scale bias; long term support) and governance limitations (integrated approach; coordinated decision making; social perception and pervasive inertia; short term policies; convergence of stakeholder interests). 16 An alternative approach in species that produce seeds contained within spathes on flowering shoots (e.g. Zosteraceae), is to 1. Collect large quantities of fertile shoots prior to seed release. 2. Place these shoots in mesh nets (suspended from buoys) anchored at the restoration site, allowing for natural seed release with time as the seeds ripen, falling out of the nets onto the seafloor and germinating. While this method may be suitable for community-based restoration projects in areas without access to facilities required to separate seeds from other plant material, the method is more costly and time consuming because of the large number of buoys, nets and anchoring devices required. Practical recommendations Irrespective of the methodology used for seed-collection and broadcasting, the percentage of broadcasted seeds that survive and become established as seedlings is generally low (<10%) and sometimes very low (1-2%). However, in areas where seeds collection is easy during the reproductive season (for seagrass species in which mass flowering and fruiting is common), it is quite easy to broadcast very large numbers of seeds to compensate for their low survival. For the smaller seagrass species, to obtain a few hundred seedlings per m2, it is generally required to broadcast several thousands of seeds per m2. Impacts on the donor meadows from harvesting such large quantities of seeds, however, has rarely shown to be significant. 5.1.3 New restoration tools and techniques New seagrasses restoration tools and techniques have been recently set-up revealing relatively high degrees of success. The recent experience accumulated worldwide shows that there is no “one solution fits all” approach to suit the life history traits of all species across all environmental and local conditions. The emerging tools now make seagrass restoration feasible for many species and at the large spatial scales and the success of these techniques and approaches are underpinned from previous lessons learned, including many restoration failures. Buoy-Deployed Seeding (BuDS) The BuDS technique involves the collection of mature reproductive shoots which are placed in mesh nets attached to buoys, suspended above plots to be restored with the aim that negatively buoyant seeds when released, will settle over the desired restoration plot (Pickerell et al., 2005). The collection of reproductive shoots can be relatively easy and rapid (depending on the target species), 17 and BuDS can be deployed over relatively large spatial scales. Recruitment based on this technique is currently low, at approximately 1 (Marion and Orth, 2010) to 7% of seeds deployed (Pickerell et al., 2005). This approach ensures high genetic diversity, which is positively correlated to rates of sexual reproduction, vegetative propagation, and overall shoot density (Williams, 2001; Reynolds et al., 2012). Potential limits BuDS approach has been tested only for Z. marina and appears not successful in all environmental conditions. Trails conducted in areas with strong hydrodynamic conditions revealed that BuDS was less effective as seeds can be washed away at high rates. Moreover, many of the buoys deployed can be coated in drifting macroalgae and potentially grazed by amphipods and juvenile crabs, reducing the number of seeds available. Dispenser injection seeding This recent developed technique is based on the mixing of seeds with local sediment to create a sediment-seed mixture that is then injected into the substrate using modified sealant guns. The procedure is described below: 1. A predetermined number of seeds is mixed with sieved fine-grained sediment (median grain size <100 mm) and loaded into sealant tubes. 2. The mixture is injected into the sediment using calibrated sealant guns up to a depth of 1-4 cm, depending on the depth that the seeds of the target species naturally recruit from. Sediment is collected close to the restoration sites to avoid any foreign substrate and allows a cohesive substrate to keep the seeds together for a standardised injection into the sediment. Inorganic clay can also be added to the natural sediment to improve cohesiveness of the seedsediment mixture. This method was trailed in the intertidal Dutch Wadden Sea, using Z. marina seeds in 2017 and 2018 (Govers et al., 2022, Figure 1). Figure 1. Dispenser Injection Seeding (credits Laura Govers) from Tan et al. 2020. 18 Resulting plant densities exceeded target densities of 10 plants m-2. This method is promising, especially for sites with strong tidal currents, such as the intertidal zone, where hand-casting and BuDS have not been very successful. However, direct injecting of seeds has yet to be trailed for other seagrass species and is likely more labour intensive compared to other seeding techniques such as hand-casting. The technique is also currently suitable for seeds between 0.5 and 4 mm in size, however, the equipment needed can be adjusted accordingly for different seed sizes. It is a valuable and promising technique and is still less labour-intensive than attempting restoration via planting vegetative fragments. An adapted version of this method is currently being trailed for underwater seeding. Potential limits In case of coarse-grained sediment, the use of mud (fine-grained sediment) required by this technique might not be ideal. Recently, Glover et al. (2022) applied various seeding methods from Buoy Deployed Seeding (BuDS) and ‘BuDS-in-frame’ in fall to a newly developed ‘Dispenser Injection Seeding’ (DIS) method (Figure 2). This adaptive experimental approach revealed high seed losses between seeding and seedling establishment of the BuDS methods (>99.9%), which were mitigated by controlled harvest and storage of seeds throughout fall and winter, followed by DIS-seeding in spring. These iterative innovations resulted in 83 times higher plant densities in the field and a small reduction in seed loss between the three experimental years (2015-2017). These outcomes suggest that an iterative, research-based restoration approach that focuses on technological advancement of precisionseeding may result in advancing knowledge and improved seed-based seagrass restoration successes and up-scaling. Figure 2. Overview of the different approaches used for an adaptative seed-based seagrass restoration in the Dutch Wadden Sea (credits Laura Govers) from Govers et al. 2022. 19 Nurseries The use of aquaculture systems in seagrass restoration is relatively new, and the few published studies to date have shown promising results. Under controlled conditions were able to germinate and grow Z. marina plants to a size that was large enough for transplanting within 70-100 days. Furthermore, the nursery reared plants had a higher survival rate and better growth than plants transplanted from donor meadows (Tanner et al., 2010, Tanner and Parham, 2010). The survival rates of cultured plants are highly variable when different species are considered (Prunus angustifolia, Irving et al., 2010; P. australis, Statton et al., 2013). However, the aquaculture of seagrass seedlings is a viable source of planting units in restoration and can be effective in seagrass restoration projects, especially for areas or species where seed production is high. Anchoring of shoots with iron nails The use of shoots has been widely used in restoration since they are often planted directly into the substrate (e.g., Matheson et al., 2017), however, several anchoring techniques have been used to varying degrees of success. One of the most successful examples of the use of anchored shoots for seagrass restoration has been in Denmark (Lange et al., 2022). Danish waters are typically characterized with periods of severe wave action, and it has not been possible to transplant Z. marina as unanchored shoots which tend to be uprooted within short periods. Instead, the transplanted shoots which had 5-10 cm rhizome were attached to iron nails of 0.3 cm by winding a thin iron wire of 0.5 mm thickness around the rhizome and the nail. The nails were uncoated pure iron (not corrosion treated) and corroded within the first year without leaving heavy metals in the sediment. During transplanting, the rhizome and nail are gently pushed about 1 cm down into the sediment, ensuring that the base of the shoots is sitting at the sediment-water interface. This technique provides sufficient weight to keep the transplanted shoots in place and has led to the successful restoration of about 1.5 ha of seagrass in three estuaries (Lange et al., 2022). The success of this technique could be due to the addition of iron into the sediment when the nail corrodes. Iron addition into a well-oxidized seagrass rhizosphere increases the absorption capacity for phosphorus and reduces sulphide toxicity, in turn increasing seagrass productivity (Ruiz-Halpern et al., 2008). As such, the benefits of iron addition in combination with this anchoring technique should also be considered as a mechanism for increasing restoration success. 20 Biodegradable bags and containers An additional seagrass transplanting methodology used biodegradable bags made of corn starch inserted in biodegradable containers (made with rice husks), which were anchored with U-shaped stainless-steel rods (Da Ros et al., 2020; Figure 3). The procedure is described below: 1. A manual stainless-steel corer was used to dig a clod from the donor seagrass meadow, avoiding any damage to the roots and leaves. 2. This clod was immediately inserted in a biodegradable bag, and in turn, the bag was inserted in the biodegradable container to maintain the consistency of the clod. 3. The container was then inserted in the receiving sediments and anchored with a Ushaped stainless-steel rod. Figure 3. Schematic representation of the Biodegradable bags and containers approach as described in Da Ros et al. (2020). Practical recommendation Preliminary survey is important to assess the shoot density of the donor seagrass meadows to select a conservative shoot density to transplant limiting the potential impact of shoots removal from the donor site. The life cycle of the selected species (fast vs slow growth) could favour the success of the seagrass transplanting that should be carried out immediately after the removal from the donor seagrass to limit the operative stress to the plants. The selection of the best suitable site for transplanting is a priority: an area close to a natural park, as far as possible from the crowded beaches (especially in Summer), limits the anthropogenic impact which could potentially compromise the conservation of the transplanted seagrasses. Spring has been identified as the best suitable period to successfully conduct seagrass transplantation in temperate ecosystems. This period favours the settlement, maintenance, and the vegetative growth of the underground rhizome of the transplanted seagrass. Good environmental conditions immediately after the 21 transplanting favour the settlement and conservation of the transplanting seagrass. The use of biodegradable bag and jar maintains and stabilizes the consistency of the clod with rhizomes and leaves. Besides, the lack of high-energy events along the shoreline immediately after the transplanting favours the expansion of the roots and the settlement of the transplanted plants in the bare sediments. Artificial in-situ structures The use of artificial in-situ structures to protect restoration trials is not new, and some have been shown to improve survival of both transplanted shoots and seedlings (Tuya et al., 2017). Researchers are developing artificial seagrass made entirely out of fully biodegradable materials to help facilitate restoration without the use of plastic (The SeaArt Project, 2020). Artificial in-water structures can also be used as anchoring devices to increase the chance of transplant unit survival. These include tying seagrass shoots to metal frames which are lowered to the seafloor (e.g., Transplanting Eelgrass Remotely with Frame Systems -TERFS, Wendländer et al., 2020), or to oyster shells (Lee and Park, 2008). These methods increase restoration success through ensuring adequate anchoring and tend to be more cost-effective as they do not require the planting of individual shoots one at a time. Biodegradable materials, such as hessian and jute, have also been trailed with great success since these materials can i) promote the establishment of naturally dispersing seedlings (Tanner, 2015), ii) protect seeds from predation (Orth et al., 2006), iii) enhance survival of restored shoots (Ferretto et al., 2019), and iv) exclude bioturbating animals, thus increasing survival rates (Wendländer et al., 2020). Alternative sources of transplanting units and use of seagrass wrack Seagrass propagules are often limited and highly seasonal, and the collection of transplantation material could potentially put greater risks on donor meadows. Thus, alternative sources of transplant units are required to minimize the overall negative impact of sourcing restoration material. A potentially viable source of transplant units is seagrass wrack, detached biomass transported by wind and tides and accumulated on beaches globally (Macreadie et al., 2017). Seagrass wrack has many important ecological functions but can also pose problems for coastal managers as its over-accumulation is often viewed as a nuisance by the public and high costs are incurred in their removal (Macreadie et al., 2017, Del Vecchio et al., 2017). Terrados et al. (2013) used Posidonia oceanica seedlings from beach-cast fruits for seagrass plantings and obtained relatively high success (44% survival for 3 years). Wrack-collected P. angustifolia seedlings showed 22 survival of 6-9% after 11 months in aquaculture, and survival upon out planting was also low (Irving et al., 2010). Storm-generated rhizome fragments of Posidonia found within the wrack have also been used successfully for restoration in the Mediterranean (Balestri et al., 2011) and are currently being used successfully to restore P. australis in Australia (Ferretto et al., 2019). Promoting positive biological interactions Harnessing positive biological interactions can increase restoration success (Silliman et al., 2015, Gagnon et al., 2020). Biological interactions in seagrasses include plant-substrate, plant-microbial communities, plant-plant (both intraand interspecific), and between seagrass and other marine organisms/species such as shellfish, mangroves, and coral reefs. Plant-bivalve interactions have been shown to be largely positive, with a review which included all marine angiosperms (i.e., seagrasses, salt marshes, mangroves, and freshwater submerged aquatic vegetation) showing that 70% of studies with a restoration focus showed positive interactions compared to 5% for negative interactions (Gagnon et al., 2020). Oyster reefs have been shown to facilitate seagrass productivity through a variety of mechanisms since oysters can i) provide physical protection from wave action (Piazza et al., 2005), ii) improve water clarity through filtering particulate organic matter (Plutchak et al., 2010), and iii) increase sedimentation and nutrient inputs through addition of faeces (Newell and Koch, 2004). Facilitation can also occur between seagrass species as documented for Halodule wrightii, a fast-growing, opportunistic species used to facilitate the recovery of Thalassia testudinum, a slow-growing, climax species by promoting more suitable conditions and reducing additional erosion (Fonseca et al., 2000; Kenworthy et al., 2018). Practical recommendation These positive interactions could be especially important for species that are not abundant seed producers and should be carefully considered to not only increase restoration success, but also potentially reduce donor seagrass meadow impact through making the best use of the plants harvested. 5.2 Restoration of oyster reefs Oyster reefs, complex three-dimensional structures created from aggregations of oysters, are among the coastal habitats that are increasingly the focus of restoration efforts. Oyster reefs support diverse and abundant ecological communities and underpin highly valued ecosystem services such as coastal protection, water filtration, fisheries productivity, and carbon sequestration 23 (Alleway et al., 2015). Though once broadly distributed globally across temperate and tropical coastlines, oyster reefs experienced an 85% decline during the 1700s to early 1900s largely due to overharvest using destructive fishing practices (Ermgassen et al., 2012, Gillies et al., 2018). Despite the reduction in fishing pressure, oyster reefs did not recover, perhaps because dredge harvest removed not only live oysters but also the dead shell base on which oyster reefs accrete, or perhaps due to the emergence of new threats such as disease, declining water quality, warming temperatures and ocean acidification (Beck et al., 2011). Until recently oyster reef restoration projects were largely confined to the United States and focused on the eastern oyster Crassostera virginica (Luckenbach et al., 1999) or the Olympia oyster, Ostrea lurida (White et al., 2009). However, recently efforts have expanded to additional species including Saccostrea glomerata, Ostrea edulis, Ostrea angasi, Magallana (Crassostrea) sikamea, (Crassostrea) hongkongensis, and new geographic regions such as Australia, New Zealand, Europe, and Asia (Fitzsimons et al., 2019, 2020). Key goals of the oyster reefs restoration projects may include: • biodiversity enhancement, • nature-based coastal defence, • fisheries productivity as well as improvement of coastal water quality (Gilby et al., 2018, Morris et al., 2019). Additionally, because intertidal oysters can, through the effects of moisture retention and shading, mitigate heat stress to associated organisms, oyster reef restoration may contribute to management strategies aimed at climate change adaptation of biodiversity (McAfee et al., 2017, 2018). 5.2.1. Oyster restoration methodologies The success of oyster reefs restoration is influenced and driven by several aspects that should be considered before starting restoration projects to make the interventions successful. Oyster’s reefs cultch Once the location is determined for new oyster reefs to form cultch (fossilized shell, coral or other similar materials produced by living organisms designed to provide points of attachment for oysters) is often obtained from sustainable recycling programs: 24 1. Oyster shells and clam shells are collected from farmers and restaurants and get disinfected by volunteers to then be used in oyster restoration. 2. Once the used clam and oyster shells are returned to the water, these recycled shells provide substrate for oyster larval eggs to begin populating oyster beds that were laid out by volunteers. Clutching consists of collecting materials that would be suitable for spat to attach too and would be considered as a "natural oyster reef" and it is considered the most successful approach when it comes to oyster beds getting reformed. There are many other ways oyster reefs can get reformed. NOAA team has restored the oyster beds by distributing large quantities of shells in the ocean as a strong foundation for other oysters to attach themselves. Additionally, the NOAA team has reenhanced shoreline stability by building a linear reef to provide protection for not only other marine life but the oyster beds as well. Oyster reefs restoration can be also addressed by investing in more hatchery facilities where oysters’ volunteers and scientists would have the ability to artificially control and breed viable oyster eggs. Main limits In the ocean when eggs are released onto the oyster beds not all the eggs make it to full term, thus making the population of oyster mats at a risk for extinction. The hatchery facilities would guarantee for oyster eggs to be bred appropriately and until they can survive on their own. Factors influencing the oyster reef restoration Several aspects as documented also for other target species, should be considered in case of oyster reefs restoration projects: substrate, material type, seeding, species selection and populations sources, including genetic considerations and implications. Historic dredge harvest of oysters, and subsequent estuarine habitat modification have at many sites removed or buried the shell base required for oyster settlement and reef accretion. At these sites, substrate addition is required for oyster reef restoration. Restoration projects have historically used recycled, fossilized, or dredged native oyster shell as substrate, because it is the natural material on which oyster reefs accrete (Schulte et al., 2009). The increased demand for oyster shells in many systems and the decreasing overall amount of shell have, however, limited the availability and affordability of natural oyster shells for restoration projects (Bersoza Hernández et al., 2018). Shell may be infeasible or undesirable to use due to biosecurity regulations (Bushek et al., 2015), increased rates of dissolution of calcium carbonate shell bases in acidifying oceans (Waldbusser et al., 2011), or introduction of 25 microplastics to the environment if shell is deployed in plastic mesh bags (Hunsucker et al., 2021). Given limitations surrounding use of shell substrate, a diversity of natural and artificial substrates is now being applied to oyster reef restoration (Goelz et al., 2020). These substrates include other bivalve shells (e.g., scallop shells, surf clams), crushed limestone or rock, standard concrete (e.g., oyster castles), concrete with various additives often aimed at lowering pH or resource consumption during manufacture (e.g., Econcrete; Sella et al., 2018, Reefcrete; Dennis et al., 2018) and biodegradable products such as BESE-elements (a zigzag mesh constructed of a potato waste polymer) that can be layered to produce a high surface-area structure with protective microhabitat (Herbert et al., 2018; Temmink et al., 2020). Habitat complexity can provide protective microhabitats to species from stressors and is generally positively associated with biodiversity (Coen et al., 2007). Substrates that are either complex by nature (e.g., oyster shell, limestone rock) or are fabricated to incorporate complexity (e.g., concrete blocks with cracks, surface texture, pools, and holes; BESE units) can protect oyster recruits from predators as well as environmental stressors such as high temperatures that are being exacerbated by climate change (McAfee et al., 2018, Strain et al., 2018). Complex substrates can facilitate biodiversity while oyster reefs are developing, and still acquiring their own biogenic complexity (Strain et al., 2020). Consequently, utilization of complex substrates will be particularly important in environments where high post-settlement mortality limits oyster reefs establishment, or where enhancement of native biodiversity is a key ecosystem service goal. Substrates utilized for oyster reefs restoration vary in chemistry, color, surface roughness, porosity, density, and longevity. These factors interact with local environmental conditions to determine oyster settlement and post-settlement survival-key factors underpinning oyster reef development. Substrate chemistry, brightness and micro-texture are key determinants of invertebrate settlement (Coombes et al., 2015, Ells et al., 2016). Oysters are gregarious settlers, responding positively to the chemical cues of conspecifics (Zimme-Faust and Tamburri, 1994) and this behaviour appears to be driven by peptides that may also be associated with biofilms growing on the alkaline surface of some other bivalve shells (e.g., surf clams, scallops) and concrete (Anderson, 1996). Nevertheless, concrete surface texture rather than chemistry appears to be a more important determinant of oyster settlement (Potet et al., 2021). The density and longevity of materials can also be a key determinant of their suitability for restoration, especially in dynamic environments. Without stabilization, relatively low-density shell materials placed in wave-swept environments are rapidly 32 method has not substantially changed but the acquired experience reveals different lessons learned from each method. Transplanting Transplanting kelp typically involves adhering the holdfast to some artificial material and then adding that to the sea floor with the intention that the holdfast migrates to the benthos or the plant acts as a seed source and provides a suitable environment for new plants. Different methods, including gluing holdfasts to the rock (Susini et al., 2007), attaching them to small concrete blocks or stones (Fredriksen et al., 2020), tying them to ropes (North, 1976), attaching them to existing holdfasts (Hernandez-Carmona et al., 2000), and attaching them to mesh mats, themselves anchored to the seafloor (Campbell et al., 2014) or to artificial substrata (Marzinelli et al., 2009) have been tested. Main limits The key limitation with each of these techniques is scalability and how well the plant can attach to the seafloor. Physical transplantation of kelp is a laborious process and manual installation will likely prove cost prohibitive for large-scale restoration projects. Practical recommendations The benefit of transplanting is that it immediately introduces plants into the environment and these plants can create conditions more suitable for new recruits (Layton et al., 2019; Japanese Fisheries Agency, 2021). Transplanting may therefore be a necessary first step that can establish source populations that then self-propagate. However, based on the acquired experience, it is recommended that these transplanted patches need to be close to the source of other existing kelps to survive (Eger et al., 2020, Layton et al., 2020). Green gravel is a new kelp restoration approach that allows to reduce deployment time avoiding the support of divers and increases restoration scalability by using laboratory-cultured gametophytes that are attached to small stones (i.e., gravel), grown in the laboratory and then transferred into the ocean (Fredriksen et al., 2020). This method provides some successful results and is supported by a working group (greengravel.org) testing the feasibility of this approach also in high wave-exposed sites. 33 An alternative to the transplanting is seeding that involves dispersing and/or growing the juvenile life stage (i.e., seeds, gametophytes, propagules, zoospores) of the kelp in the ocean. Seeding kelp populations has received much less attention than transplantation due to the extremely high mortality of kelp propagules (Schiel and Foster, 2006) and the perceived advantage of focusing on sporophytes where survival is many orders of magnitude higher. The seeding occurs using weighted mesh bags filled with fertile kelp blades to the bottom to facilitate propagules settling on the sea floor (Westermeier et al., 2014). Main limits Seedling displays a limited success and requires the long-time efforts of divers to install and remove the bags from the bottom. Lesson learned from coral reefs restoration using ships to disperse coral propagules into the ocean (Doropoulos et al., 2019) could be also applied for kelp forests allowing the application at a much larger scale at relatively low cost (Saunders et al., 2020, Vanderklift et al., 2020). Removing competitors Removing kelp competitors from the sea floor has received very little attention outside of Japan, where they have developed a suite of techniques for clearing the rock bare (Japanese Fisheries Agency, 2021). Some of these methods can be maintained without continued input, for example, a chain spun by wave action, but others such as manual or mechanical removal are much more labour intensive. Regardless of the approach, large-scale scraping of the benthos is likely untenable in most countries and locations, thus this approach will likely be limited to small-scale transplanting sites where removing competitors may help to establish the desired kelp population. Grazing avoidance Controlling grazers relies on manual removal or exclusion of the animal from the targeted restoration area. For sea urchins this can entail crushing them (Leinaas and Christie, 1996), relocating them (Mead, 2021), harvesting them (Piazzi and Ceccherelli, 2019), or killing them with quicklime (Bernstein and Welsford, 1982). These methods are also restricted by their labour costs and feasibility varies by location considering urchin density, depth, water conditions, and topography. While urchin management is more scalable than transplanting, it still requires substantial resources. Urchins have been successfully baited to concentrate them in space and therefore make removal more efficient (James et al., 2017, Japanese Fisheries Agency, 2021, 34 https://www.urchinomics.com/). Another challenge associated with urchin removal is to maintain the sites where they have been removed because if the sites are not maintained, urchins often return and continue to graze transplanted kelp or recruits (Yoon et al., 2014). As an addition or an alternative to a continuous site maintenance, restoring healthy predator populations alongside kelp forests, that can keep sea urchin numbers low, may also help to create a self-sustaining ecosystem (Eger et al., 2020). Alternative solutions for managing grazer populations include the establishment of a fishery or ranching program which removes the animals from the ocean for food and/or profit (Lee et al., 2021, Verbeek et al., 2021). Destructive grazing of kelps by fishes is less common than by urchins but is a consistent issue in some areas such as southern California, southern Japan, and some regions of Australia because of a range-expanding herbivorous fishes due to the sea temperatures rise (Vergés et al., 2019). The same issues and potential solutions described above can be applied to control grazing fish populations on kelp forests. Artificial reefs This is another common approach although artificial reefs are used more often in afforestation when these are placed in habitats that did not contain kelp (e.g., sandy substratum). A key benefit of artificial reefs is that managers can place them where they are easily maintained, and kelp transplants can be attached more easily than to the natural sea floor. New materials for artificial reefs include those that structure the concrete to enhance rugosity and provide additional settlement area (Ishii et al., 2013, Bishop et al., 2017), as well as infusing the concrete with iron, nitrates, and other growth-enhancing materials that are slowly released over time (Oyamada et al., 2008). Main limits The materials required to build artificial reefs remain very expensive and require substantial investments. Practical recommendations Kelp restoration projects can use a combination of methodologies which may improve the chances of success and it is very important to consider the local conditions when applying any combination of different methods. The installation of reefs with transplants or seeding, or transplanting kelp or controlling grazer populations are not mutually exclusive and the use of multiple methods may enhance the success of the growth of emerging kelp populations in different ways. 35 5.3.2 Algal forests Macroalgal forests of canopy-forming fucoids are dominant on rocky reefs along all the coasts (Assis et al., 2020). They are recognized as hot spots of diversity and CO2 sink, providing food and shelter for diverse assemblages of understory species and enhancing coastal primary productivity (Sales et al., 2012, Cheminée et al., 2013, Thiriet et al., 2016). Macroalgal forests formed by fucalean brown algae (i.e., Cystoseira sensu latu, including the genera Cystoseira, Ericaria and Gongolaria; MolinariNovoa and Guiry, 2020) can potentially thrive from the intertidal to the circalittoral, with different species replacing one another along the bathymetric gradient. As a response to multiple stressors, including eutrophication, overgrazing, increasing coastal sediment loads, and impacts of urbanization, macroalgal forests are being lost at alarming rates with events of widespread canopyforming algae extension or reduction to remnant, fragmented, and isolated populations (Blanfuné et al., 2016, Mariani et al., 2019). Very little evidence of natural recovery has been reported in macroalgal forests (Iveša et al., 2016), even when the area switches back to conditions before Cystoseira forest decline (Pinedo et al., 2013). Relict populations will ultimately disappear if effective restoration strategies are not identified together with stressors identification, mitigation, or elimination, which should be an integral part of any restoration plan. Practical recommendations A priori knowledge of the phenology and recruitment periods of the species to be restored is needed because the proposed techniques are based on obtaining fertile branches and the survival of recruits (Cebrian et al., 2021). When Cystoseira s.l. fronds exhibit mature receptacles, fertile branches need to be collected for fertilization, therefore, it is crucial to act as fast as possible during the reproductive season of the selected species to collect an appropriate number of mature apexes. Their availability represents an intrinsic limit of any restoration technique since interventions cannot be repeated until the following reproductive period of the target species. Recently, Smith et al. (2023) published a decision-support framework for the restoration of Cystoseira sensu lato forests. Gaps were identified and recommendations were provided, dealing with stressors, coordinating, and networking stakeholders, integrating top-down policy and bottom-up initiatives, funding of restoration actions, establishing synergies between restoration, conservation, and marine spatial planning and finally communication and publicity. In the study, the critical steps of a restoration program have been also included (Figure 4). 36 Figure 4. Critical steps of a restoration program: key considerations (around the four pillars; society, competence, governance, and finance) that need to be addressed before implementing a restoration project (A), the key elements of restoration success evaluation (B), and long-term monitoring and adaptive management (C) (Smith et al., 2023). Selection of restoration techniques Several recruitment enhancement techniques (germlings out planting) and complementary actions have proved successful to restore Cystoseira populations (e.g., Verdura et al., 2018, De La Fuente et al., 2019, Tamburello et al., 2019, Medrano et al., 2020). They consist in obtaining recruits from fertile branches of the donor populations, which are 1) placed directly in the area to be restored (in situ technique) or 2) cultured on aquaria facilities and then transported to the field (ex situ technique). These techniques are especially suitable for Cystoseira s.l. species since they present a sexual reproduction from monoecious individuals, with male and female gametes housed within the same conceptacle, which are grouped in receptacles (Rodríguez-Prieto et al., 2013), thus ensuring male and female gametes for each fertile branch. Techniques can be selected according to different factors such as i) the availability of culture facilities, ii) dispersal capacity of the target species, iii) the hydrodynamic regime of the selected site to restore, and iv) the grazing pressure. For the exsitu technique, production is hampered by the availability of facilities (e.g., the dimension and number of environmentally controlled rooms, the number of aquaria and laboratories; Savonitto et al., 2021) and the proximity of the restoration site, since the transport to the field is a critical step 37 for the success of the intervention (Tamburello et al., 2019). The in-situ technique and the ex-situ technique appear to be especially suitable for species with a higher and a lower dispersal capacity, respectively (Verdura et al., 2018, De La Fuente et al., 2019). Under low hydrodynamic conditions the in-situ approach has proved to be the most effective while the ex-situ cultivation of zygotes should be prioritized in areas with high hydrodynamic conditions where zygotes can be swiped out to the water column (Verdura et al., 2018). Herbivory may lead to opting for the ex-situ technique, thus avoiding the grazing of the recruits (Savonitto et al., 2021). In situ approach The in-situ approach has been successfully tested with Ericaria barbata and Gongolaria elegans (Verdura et al., 2018; Medrano et al., 2020). The procedure is described below: 1. When the receptacles are mature, fertile apical branches are collected from donor populations using scissors and transported to the restoration site in plastic bags without any water and in cold and dark conditions. 2. Approximately ten fertile receptacles should be placed in each dispersal bag (8-10 cm) made of 36% fiberglass and 64% PVC, with a mesh size of 1.20-1.28 mm. 3. Dispersal bags are tied to a pick and fixed to the substratum using a hammer and/or epoxy putty at a vertical distance of 25 cm from the bottom. The distance between dispersal bags should be approximately 2-3 m. Practical recommendations Free substratum to promote the settlement of zygotes should be provided if the area to restore is dominated by turf algae that can outcompete the new recruits. This can be done by placing flat stones free from any organisms in the area, or by pre-empting the rocky substrate with a metal brush. The dispersal bags can be removed after 4 days. Ex situ approach The ex-situ technique has been tested for Ericaria amentacea, E. crinita, E. zosteroides, and Gongolaria barbata. Ex-situ seeding provides many healthy individuals to be reintroduced in the environment without impacting the natural populations (Verdura et al., 2018). The procedure is described as follow: 38 1. During harvesting, up to 3 fertile apexes should be collected from everyone to ensure a minimum degree of genetic variability and to avoid compromising the reproductive capability of exploited individuals. 2. During transportation to the laboratory, fertile branches should be preserved in plastic bags without water and in cold and dark conditions. Transport should be completed within 48 h of collection. Fertile branches should be preserved in plastic bags without water and in cold and dark conditions. Transport should be completed within 48 h of collection. 3. Once in the laboratory, fertile branches are stored in fridges (4°C) in dark conditions for 1224 h to promote zygote liberation. 4. Cultivation is carried out in aquaria with a close-water circuit of filtered natural seawater continuously aerated by air pumps. The photoperiod should be selected to reflect the seasonal conditions of the donor site. Temperature and irradiance conditions are set according to the specific requirements of the cultivated species (Verdura et al., 2015). The substratum used for cultivation can be natural stones previously cleaned with a hard brush. 5. Fertile branches should be placed uniformly in mesh bags, ensuring they float on the surface of the tanks while releasing the gametes and the zygotes. Water movement inside the tanks should be kept as low and stable as possible for the first 4 days to facilitate zygote settlement. Adult branches can then be removed to eliminate possible sources of contamination such as epiphytes and particulate organic matter. The water of the aquaria must be partially renewed once a week. Epiphytes on the stones can eventually be eliminated by gently cleaning their surface with a paint brush. 6. Cultured germlings should grow for at least 3 weeks (when they became visible with the naked eye) and up to 3 months, when early-stage individuals reach 1-1.5 mm and can be transported to the field. 7. The transport of germlings from the laboratory to the field should be carried out in dark and in situ temperature conditions. Once at the destination, the displacement or attachment of colonized stones with epoxy putty should take place rapidly (within a few hours) to ensure that the germlings do not undergo thermal stress. Practical recommendations Clay tiles can be used if stones are not available in the working areas, after carefully considering their rugosity (Tamburello et al., 2019). Tiles substrata show higher settlement of eggs when 39 compared with pebbles, which presented more roughness, although successive germling growth and survival are independent from the substrata. Main limits Multi factors can affect the success of algal forests restoration: competition with turf algae, grazing from fish and sea urchins’ population. Competition with turf algae Macroalgal forests face competition and overgrowth by turf, whose abundance may increase due to disturbance events such as chronic nutrient enrichment, sediment load and climate change (Filbee-Dexter and Wernberg, 2018). Turf can potentially compromise restoration success by quickly overgrowing and monopolizing primary substrate, limiting the availability of the suitable hard substratum required for canopy forming macroalgae settlement (Connell and Russell, 2010), and because of its ability to accumulate sediment (e.g., Isaeus et al., 2004, Filbee-Dexter et al., 2016), reducing the survival rates of canopy-forming macroalgae recruits (Gorman and Connell, 2009). Likewise, it has been suggested that turf-forming algae affect early life-history stages of Mediterranean canopy algae (Ballesteros et al., 2009). Therefore, active turf removal should be carried out as a management tool to reduce this competitive effect and provide space for new macroalgal recruitment. Providing available substrate by itself does not translate into a restored macroalgal forest if no neighbouring Cystoseira s.l. populations are available, but it does facilitate recruitment of new individuals when restored individuals become fertile. Scraping some plots to provide a newly available substrate for new recruits is preferably required in areas colonized by turf or other less complex macroalgal habitats (Verdura et al., 2018, Medrano et al., 2020), although it is probably not needed in barren areas. Cleaning periods are species-dependent, and they should match recruitment periods from the first year of fertility. They should be maintained annually or biannually. Grazing One of the main factors limiting the success of macroalgal restoration is the presence of herbivores at high abundance (mainly the sea urchins Paracentrotus lividus and Arbacia lixula and, especially in the Eastern Mediterranean, the fish Sarpa salpa and Siganus spp.). A preliminary assessment of the abundance of herbivores and grazing pressure at restoration sites should be carried out and, where possible, intensively grazed sites should be discarded. Alternatively, devices to prevent access by 40 grazers (fish and sea urchins) and grazers removal/culling should be considered (Guarnieri et al., 2020). Notably, knowledge on the critical threshold abundance of herbivores affecting recruitment success in macroalgae is still to be ascertained and will vary depending on environmental conditions, including nutrient regimes (Boada et al., 2017). Devices to prevent access by grazers can be used to protect the juveniles at least for the initial months of growth after settlement or transplantation from the aquaria. For instance, fish deterrent devices have shown to successfully limit the access of salema fish (Sarpa salpa) in intertidal forests of Ericaria amentacea (Gianni et al., 2020), while cages fixed to the substratum can protect Cystoseira s.l. juveniles (Tamburello et al., 2019). Cage structures made with double metal mesh and wire can be screwed to the substratum using an underwater drill and sealed with epoxy putty. If cages are provided with a top side, they can eventually also protect from fish grazing. Cages should be periodically cleaned with a metal brush to prevent shading due to epiphytes. Cage size depends on the hydrodynamic conditions, usually measuring 20 x 20 cm in shallow, high hydrodynamic areas and 50 x 50 cm in deeper areas or where hydrodynamism is negligible. However, setting up exclusion devices requires intense maintenance since epiphytism may strongly modify irradiance and water exchange conditions, and hydrodynamism can deteriorate fixed structures. Sea Urchin Population Recent studies have shown a positive effect of sea urchin removal (harvesting and culling) on the recovery of overexploited macroalgal beds in subtidal rocky habitats (Piazzi and Ceccherelli, 2019, Guarnieri et al., 2020). Removal actions can be opportunely carried out either to complement recruitment enhancement or to protect adult Cystoseira s.l. individuals seriously threatened by overgrazing. In the Mediterranean Sea it is widely accepted that P. lividus harvesting may be a potential effective method to mitigate overgrazing in areas of severe overfishing (Piazzi and Ceccherelli, 2019, Farina et al., 2020). Direct removal of P. lividus can result in community level effects triggering Cystoseira s.l. recruitment and recovery (Piazzi and Ceccherelli, 2019). Hence, an integrated management of P. lividus harvesting could be considered a useful complementary tool in areas where the density of sea urchins may compromise the viability of restoration actions. The different steps of the algal forest restoration are summarised in Cebrian et al. (2020) and reported in the Figure 5. 41 Figure 5. Image of the rocky reef before the restoration action (A) and after restoring the Cystoseira s.l. population (B). Restoration techniques: Cystoseira s.l. adult transplantation (C), ex situ approach (D) and in situ recruitment enhancement (E). Complementary actions: providing free substrate (F), herbivory exclusion (G), MPAs (H) and control of sea urchin populations (I). Photo credits: Xavi Calsina (@XCalsi; A, B, G), Margalida Monserrat (C), Jana Verdura (D), Simonetta Fraschetti (E), Alba Medrano (F), Josep Pascual (H) and Jordi Boada (I). From Cebrian et al. 2020. 5.4 Saltmarshes Coastal wetlands such as salt marshes are reported to be among the most abundant, accessible, and productive ecosystems on Earth (Deegan et al., 2012). Salt marshes are particularly efficient in the sequestration capacity of atmospheric CO2 because of their high rates of primary production (root, shoot, and leaf production) (Santini et al., 2019, Poppe and Rybczyk, 2021) and relatively low rates of microbial decomposition due to prevailing anaerobic conditions in the surface sediment. Besides, ecosystem functions of salt marshes include fishery support as leaves, roots, and stems provide vital shelter and nourishment (Billah et al., 2016, Taylor et al., 2018). In addition, several investigations reported their water purifications and nutrient retention capacities (Alvarez-Rogel et al., 2006), coastal protections through sediment stabilization (Taylor et al. 2019), and biodiversity provision (Costanza et al., 2008). Despite great ecological and economic importance, this valuable ecosystem is under threat due to the expansion of urban areas, land reclamation, and inputs of N from the runoff of upland agricultural sites due to fertilizer uses, freshwater influences, enriched groundwater, limited sediment supply, and sea level rise (Fagherazzi et al., 2019, Liu et al., 2021). 48 5.4.2. Summary on the abiotic and biotic approaches Billah et al. (2022) summarized the multiple aspects that need to be taken in account (biotic and abiotic) when restoration plans are setup for saltmarshes, including different restoration strategies and expected outcomes for assisted restoration: Abiotic approaches Recovery of tidal exchange: Tidal exchange/inundation has been recovered and restored through various means, including culvert constructions, canals and/or channels excavations. Recovery of sediment characteristics: To improve soil characteristics for revegetation effort, rubbles are removed and incorporated with organic vegetated soil. Managed realignment: Partial or whole scale breaching of coastal defence structures to ameliorate the salt marsh habitat for sustainable biodiversity response. Breaching of dikes is either intended or caused by natural processes, such as erosion, sea level rise, or wave actions/storms. Reconstruction of soil levels: Sediment slurry is added to the marsh bed to reduce the floodingrelated stress to the salt marsh plants. Created salt marsh through dredged sediment: In lagoons, salt marsh beds can be created through dredged sediment from the nearby navigation channels and excavation of the barrier. Construction of a new lagoon thereby creating new marshes: Construction of a new lagoon and removal of breakwaters, walking paths, connecting roads, and accumulated debris. Restriction of the vehicles and sediment level amendment: Exclusions (through fencing in the lagoon) of the recreational vehicles that are destructing the marsh beds through erosions and reconstructions of the soil level to accelerate the re-colonization of the salt marsh. Biotic approaches Control of invasive Phragmites spp.: Establishment and invasion of the Phragmites are eliminated by the large-scale application of herbicide and burning the marsh surface. Control of invasive Spartina alterniflora: In the salt marsh of the Yangtze Estuary, invasive Spartina alterniflora is removed mainly by herbicide application. Recovery of vegetation through revegetation: Usually carried out either transplantation of sediment cores with target salt marsh plant or transplantation of clumps of target salt marsh species from natural populations. 49 6. Final considerations As emerging from the previous analyses, habitat restoration is a complex issue, needing many elements to reach reasonable success. The habitats here included are relevant within the CLIMAREST project but are also the most widespread across the planet, showing important restoration interventions. Beside the differences among them, there are always common steps in best practices that should be considered. 1They are all habitat formers, often in a poor condition in many areas, due to loss and degradation from both anthropogenic and climate stressors. This means that active restoration should be always planned together with careful conservation interventions. 2Restoration of all these habitats can directly support several UN Sustainable Development Goals: SDG 2 (zero hunger), 8 (work and economic growth), 13 (climate action), and 14 (life under water) providing further arguments in favour of this activity. 3Key common considerations include restoration implementation (competence, society and support, finance, and governance), success evaluation (at the target species and the ecosystem level) and long-term adaptive management. 4Current and historical presence, site local condition assessment and choice of actions are central in all restoration interventions. 5Species selection and populations sources (including genetic considerations), substrate, material type, seeding, are also critical elements in restoration whose selection has to be addressed through a solid scientific knowledge. 6Decision-support framework should be developed for the restoration of all these habitats, comprising a stepwise decision tree with additional descriptions of key elements to be considered for a restoration action to facilitate the homogenization of restoration interventions leading to improved comparability of results. 7. 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