Year of publication: 2021
Authors: Kent, F., Lilley, R., Unsworth, R., Cunningham, S., Begg, T., Boulcott, P., Jeorrett, C., Horsburgh, R. and Michelotti, M.
Cite as: Kent, F., Lilley, R., Unsworth, R., Cunningham, S., Begg, T., Boulcott, P., Jeorrett, C., Horsburgh, R. and Michelotti, M. Seagrass restoration in Scotland - handbook and guidance. NatureScot Research Report 1286.
Global declines in seagrass beds along with a growing appreciation for the value of such habitats has stimulated an interest in seagrass restoration in a number of countries and regions. Seagrass beds occur along the British coastline and successful seagrass restoration projects in Wales along with a push towards net zero have stimulated enthusiasm for trials to begin in Scotland.
This handbook has been developed by NatureScot in collaboration with Scottish Government (Marine Scotland) and Project Seagrass to inform and guide potential seagrass restoration projects in Scotland and to ensure all appropriate policy, licensing and monitoring aspects have been considered. NatureScot is Scotland’s nature agency and have a role in assessing human activities in the Scottish marine environment. NatureScot also provide advice to ensure that natural features are maintained and enhanced. Marine Scotland is a directorate of the Scottish Government. Marine Scotland manages Scotland's seas and freshwater fisheries along with delivery partners NatureScot and the Scottish Environment Protection Agency (SEPA).
This handbook aims to provide an evidence-led approach to guide seagrass restoration proposals in Scotland. While guidance exists for other countries, for example the USA (Fonseca, 1998), Sweden (Moksnes et al., 2021), the Indian Ocean (UNEP, 2020), no equivalent is currently available for Scotland. A seagrass restoration handbook has been developed by the Environment Agency for the UK (Gamble et al., 2021) which provides further information.
Restoration may not be the most appropriate action for conservation management of seagrass at certain sites and priority should be to protect and promote recovery of existing seagrass beds. Where restoration is considered appropriate, this handbook can help guide projects and ensure all relevant aspects are considered.
Seagrass beds are a Priority Marine Feature (PMF) in Scotland and a protected feature in a number of marine protected areas (e.g. Special Areas of Conservation – SACs, Nature Conservation Marine Protected Areas and Sites of Special Scientific Interest - SSSIs). Restoration attempts that take place within a protected area should consider if and how the various activities (e.g. seed collection, planting, monitoring) could affect protected features of that site (and in some cases adjacent sites).
We highlight how seagrass restoration proposals should outline the potential risks and benefits of the project and clearly state the aims, objectives and timescales, including how success of the project will be measured. The handbook also provides background on policy, legislation and licensing considerations within Scotland and the roles that NatureScot and Marine Scotland can play in supporting projects with appropriate advice on a case-by-case basis.
It is important to fully assess project feasibility and site suitability (chapter 3) at an early stage. Chapters 4 provides background and guidance on seagrass restoration techniques as well as tips for seed collection and storage. A robust monitoring plan should be set out from the start, including data review, habitat suitability assessments and baseline monitoring (see chapter 6 for more information).
NatureScot have developed a Marine Enhancement Guidance Framework to assess enhancement and restoration proposals and provide guidance on aspects such as licensing, monitoring, biosecurity and planning. The level of assessment and licensing requirement will depend on the scale and location of the proposed project. Further information can be found in the Marine Enhancement Framework Guidance and report which should be used alongside this handbook.
This handbook highlights many of the knowledge gaps in our understanding of seagrass restoration, however, there are broad guiding principles to that have emerged from existing work globally. The guidance provided here therefore focusses around guiding practitioners to follow such guiding principles in order to shape projects appropriate to the local environmental, biological and social setting.
- There exists a critical need for projects to conduct feasibility studies and surveys in order to design restoration that can have a high chance of success. Studies have shown that in some areas, seagrass beds are often unviable at locations where they have existed in the past. It is thought that when a seagrass bed is lost, the entire ecosystem can shift to unsuitable conditions for the species to thrive. Seagrass beds stabilise the sediment, therefore when they disappear, resuspension can increase, and water quality degrades. Algal mats can then take over and reduce the light available, further hampering the potential for seagrass to grow. There are also other problematic processes driven by the underlying condition of coastal seas such as eutrophication and the overabundance of green shore crabs due to loss of predators. Seagrass restoration projects should assess site suitability before planting to ensure optimal ecosystem functioning and ensure local conditions will not inhibit recovery even if initial assessment indicates suitability. This should involve reviewing historic data, making use of habitat suitability models, and monitoring physical and biological parameters in the area that has been selected.
- It is important to understand the status of existing seagrass beds in a proposed area and the cause of any declines in bed condition or extent. If the pressure causing the decline persists, then it is likely that any restoration efforts will fail. Causes of seagrass decline can range from coastal development, eutrophication, disease, fishing, aquaculture, trampling or changes in hydrodynamics. Seagrass planting should not go ahead if known pressures continue to affect the proposed area of restoration.
- Seagrass beds naturally change over time - an individual bed will show considerable variation in extent and density throughout the year and between years. Events such as storms, algae blooms or unusually high summer temperatures can wipe out a seagrass bed. Therefore, even with thorough research and planning, restoration trials can fail. Scale is important, with large-scale seagrass restoration trials more likely to succeed. There may be a threshold size whereby a bed becomes self-sustaining. However, seagrass restoration research and trials are at a very early stage in Scotland and further research is required into the specific conditions required for seagrass on different parts of the coast. Small-scale test planting and trial plots should be used before large-scale seagrass restoration efforts are attempted.
- Biosecurity implications in terms of accidental transfer of diseases and invasive non-native species should be a consideration for any seagrass restoration proposal and discussed with NatureScot and Marine Scotland.
- A range of seagrass restoration techniques exist which largely fall into seed-based methods or adult transplants. The appropriateness of each technique will depend on the environmental conditions at a given site. Seed-based methods have been developed in recent years (e.g. the BoSSLine method – see Seed planting options section) to maximise success in some locations, however mixed methods of transplants and seeds have been found to be appropriate in many locations. Seeds should be sourced (where possible) from local beds to reduce biosecurity risks and may increase the chance of successful adaptations to the local environment.
- Restoration plans should consider connectivity of seagrass beds in the area and potential changes in connectivity over time. A changing climate may require future consideration of mixed populations or ‘pre-adapted’ donor beds to support increased ecosystem resilience. However, knowledge in this research field is weak.
- Monitoring is an important factor to include in any seagrass restoration proposal – this should include baseline measurements of physical and biological environment, test planting and long-term monitoring.
- Effective evaluation of a seagrass restoration project requires reference beds, i.e. natural unaffected seagrass beds that are as close to the restoration area as possible. This is very important for demonstrating whether changes observed at the restoration site are due to conditions at the planting site or methods used in the restoration, or to natural variations in seagrass between years or wider impacts (e.g. pollution or changes to hydrodynamics).
- Further research and site-specific studies on seagrass ecosystem functions and services are required in Scottish waters, particularly where there is an interest in restoring seagrass beds as a nature-based solution for carbon storage, coastal erosion and flooding protection, and biodiversity. Projects that aim to restore ecosystem functions and services related to seagrass beds will need to include appropriate controls in the monitoring design and consider the likely timescales for such processes to reflect natural seagrass habitats.
Scottish seagrass ecology and biology
Seagrasses (also known as eelgrass) are marine flowering plants, often found in shallow, sheltered areas along the Scottish coastline. The plants can be annual or perennial and can grow in dense beds or meadows. The term ‘beds’ is used in this handbook due to the Scottish policy context. Seagrass ecosystems play a multi-functional role in human well-being, e.g. food through fisheries, blue carbon, sediment stabilisation and coastal defence (Thorhaug, 1990; Bertelli & Unsworth, 2014; Röhr et al., 2018; Duarte et al., 2020; Orth et al., 2020). However, research on seagrass bed ecosystem function, processes and services largely comes from studies conducted outside of Scottish waters.
The seagrass species in Scotland are Zostera marina and Zostera noltii. Z.marina’s morphological characteristics vary across its range as a result of environmental conditions. It was previously thought there may be more than one species of Z. marina, giving rise to the name Z. angustifolia (Percival et al., 1996; Provan et al., 2008). However, this it is currently thought that Z.angustifolia is merely a phenotypic variant of Z. marina (Becheler et al., 2010) and thus an ‘ecotype’ which grows in different forms depending on the environmental conditions, rather than a distinct species (De Heij and Neinhuis, 1992). The focus of this handbook is on subtidal seagrass (Z. marina).
Seagrass bed biodiversity
Experiments to investigate the biodiversity of UK seagrass beds have largely focused on the mobile fauna component of the habitat (Jackson et al., 2006; McCloskey and Unsworth, 2015), although Frost et al. (1999) investigated infaunal diversity on intertidal seagrass beds. Attrill et al. (2000) found significant positive relationships between seagrass biomass and macrofauna abundance and number of species at sites in Devon, UK. Habitat disturbance and fragmentation can result in a reduction in epifaunal species richness, but the effect of patch size and scale are important (Reed and Hovel, 2006). Extensive seagrass beds in the Sound of Barra (Scotland) were found to harbour a diverse associated community in 2015, while low-density areas of seagrass were less diverse (Bunker et al., 2018).
Seagrass anatomy and reproduction
Understanding the biology and reproduction of seagrass is important for determining how and when to carry out different restoration or enhancement activities. In the seagrass, Z. marina, many populations are maintained by lateral (horizontal) shoot growth from roots or rhizomes (Figure 2) which persist for many years, whereas in other populations, annual plants grow each year from seeds (Figure 3) then die back. Many seagrass beds utilise both asexual and sexual reproductive strategies and pollination occurs in the water column. The presence of adult plants surrounding a seedling is likely to influence seedling survival.
Across its distribution, Z. marina populations invest and rely variably on sexual reproduction for the maintenance and resilience of existing populations, and for the dispersal to new areas (Phillips et al., 1983; Johnson et al., 2020). Sexual reproduction occurs through seedling recruitment; but population enhancement will only occur with the subsequent germination, growth and survival of seedlings into the reproductive population.
The seasonal pattern of seagrass reproduction varies between geographic areas. No comprehensive study of seagrass reproduction has been carried out in Scotland, although observations suggest that seeds are present from July to November with a peak during late August and early September. Along the Pacific coast of North America, Philips et al. (1983) found that Z. marina seeds germinate in October and November and flowering shoots appear from March to May.
The timing of growth and reproductive phases exhibited by Z.marina can vary significantly with the seasons and over relatively small spatial scales. Our understanding of the drivers of the reproductive effort is limited (Philips et al., 1983; Shields et al., 2018). However there is growing evidence that disturbance of seagrass beds due to digging by Otters may increase reproductive effort and lead to improved genetic mixing between sites (Foster et al., 2021).
Some of the most abundant seed densities have been recorded in the US in Chesapeake Bay, where flowering shoot densities can be up to 19% of the total shoots and produce up to 8,000 seeds/m2 (Silberhorn, et al. 1983). In the UK, densities are also highly variable, with densities ranging from from 200 to 1300 seeds per m2 from studies conducted around the Isle Of Wight and in North Wales (Furness et al., 2021) and potentially slightly less in some studies in the Isles of Scilly. Although seed production in this species is mostly vast, some sites are known to exhibit virtually no reproductive effort.
A recent study suggests that some more southerly seagrass populations (in South Carolina) have been recorded producing very small seeds compared to more northern populations. However, viability did not differ across seed sizes which suggests that seed size may not necessarily determine seed quality and therefore long term success (Combs et al., 2021).
Very little is known about the timing of germination in Scottish seagrass. Studies in the USA have shown that the majority of seeds germinate between October and December (Churchill, 1983) with the timing of germination linked to seasonal temperature changes and oxygen availability (Moore et al., 1993). Seedling growth is slow during the winter months and rapidly increases in the spring (temperatures greater than 10°C; Orth & Moore, 1983). In the UK, there appears to be a broadly latitudinal relationship with seed maturation, with sites in southern England reaching seed maturity by mid-July, and sites in North Wales by early August. In Scotland, experience from Argyll suggests spathe formation commences in early June, with seeds ripening through July ready for harvesting in early August. However, these development windows are likely to vary slightly from year to year and from site to site.
Connectivity and recruitment
Released mature seeds are negatively buoyant, and do not disperse more than a few meters (Hosokawa et al., 2015). It has been suggested that rafting of seeds on detached, floating reproductive shoots/spathes (see Figure 3) is as an important mode for long-distance dispersal of seagrass rather than movement of the individual seeds (Källström et al., 2008). Seagrass seeds are potentially an important recovery strategy for beds subjected to high disturbance (Unsworth et al., 2014).
Individual genetic analysis of floating, detached seed-bearing reproductive shoots indicates that seeds can disperse up to 50 km (Reusch, 2002). In the UK, genetic studies of Z. marina have shown no significant differences between populations in Wales and southern England (Nahaa and Bull, In Prep). No published data on the genetic make-up of Z. marina currently exists for Scottish beds, although research is underway to analyse samples from beds around Scotland. Early results indicate that all Z. marina sampled are closely related but some small scale geographic variation may exist.
Seagrass bed distribution and historic extent in Scotland
Seagrass beds are of regional importance in the NE Atlantic and Scotland holds 20% of the seagrass beds in north-west Europe. The evidence of anthropogenic factors leading to OSPAR ‘threatened and declining’ status indicates a vulnerability to physical disturbance, increases in turbidity (e.g. eutrophication, sand extraction and dredging activities) and susceptibility to disease.
Z. marina beds are widely recorded on the west coast of Scotland. For example, subtidal beds are found in Loch Gairloch, Gruinard Bay, Loch Ewe, Skye sea lochs, Linnhe Mhuirich, the Sound of Arisaig as well as the Outer Hebrides (e.g. the Sound of Harris and the Sound of Barra). Beds are found around the Orkney Isles (e.g. in Papa Sound, Stronsay, Shapinsay, Deer Sound, and Widewall Bay) and Shetland (e.g. Whiteness Voe). Subtidal seagrass beds are a protected feature within the South Arran MPA and the beds in Whiting Bay are believed to be the largest in the Firth of Clyde. Seagrass beds are also protected in a number of SACs as a subfeature of ‘Sandbanks which are slightly covered by sea water all the time’, for example in the Sound of Barra and the Sound of Arisaig. On the east coast of Scotland, intertidal seagrass beds (Z. marina and Z. noltii) occur in a number of firths and estuaries, e.g. the Firth of Forth, Dornoch Firth, the Tay and Eden estuaries.
Beds vary considerably in size, patchiness and plant density. The smallest of six beds surveyed in Gruinard Bay and Loch Gairloch in 2010 was ~0.03 ha in extent, essentially a discrete patch of seagrass of less than 20 m diameter (Moore et al., 2011). Extensive seagrass coverage has been reported in the Sound of Barra (360 ha - Harries et al., 2007) and Sound of Harris (280 ha - Malthus et al., 2006).
Seagrass beds are sensitive to smothering, organic enrichment, nutrient enrichment, physical disturbance, changes in water flow, and invasive non-native species (e.g. Spartina spp.) (FEAST, OSPAR, 2009; d’Avack et al., 2014). Although nutrient enrichment has found to be largely problematic for seagrass in many parts of the Europe including England and Wales (Jones et al., 2016), no such information is available for Scottish seagrass. Given the sensitivity of these systems this is a key information gap.
Anthropogenic activities that disturb the bed can cause fragmentation and mobilisation of sediment. Seagrass beds can be slow to recover from such impacts.
Determining the extent of loss of seagrass beds is difficult due to the uncertainty of techniques used to monitor the beds, especially in earlier in the last century. Attributing change to a cause or impact is often anecdotal rather than as a result of robust experimental sampling. This robust approach is required as seagrass extent and biomass varies naturally throughout the year with growth closely linked to seasonal changes in light conditions (Duarte 1989), confounding comparison between different times of year. In addition, whilst GPS accuracy has improved over time, surveyors may not be able to revisit the exact location or transects that were carried out in the past.
In a UK context, modelled estimates for seagrass show a general decline in seagrass beds (at least 44%), particularly in estuaries (Green et al., 2021). The authors estimate a total loss of 6697 ha since 1936 while noting that many beds have not been revisited in recent decades. This study also highlights the lack of recent survey work in Scotland, therefore trends for Scottish seagrass beds remain largely unknown. It is possible that industrialisation in some areas together with poor land management, unsustainable fishing practices and the expansion of the aquaculture in industry has resulted in ‘unseen’ seagrass loss. The absence of historic baselines limits our understanding of such changes.
However, there are examples where evidence exists for some form of decline or loss in Scotland, with evidence of loss in some east coast estuaries. On the other hand, ‘new’ seagrass beds continue to be discovered (although without previous absence data it is not possible to tell if these represent an increase) for example in sealochs on the west coast of Scotland.
Seagrass bed losses have been noted in Enard Bay, north-west Scotland (James, 2004) and in the Sound of Barra, beds are thought to have declined between 2005 and 2017 years since the construction of a causeway (see Figure 4). Extensive seagrass beds have recently (2021) been mapped in Westray, Orkney.
It is thought that loss of seagrass around Britain in the 1930s was in part due to a wasting disease (Labyrinthula zosterae). Although disease still exists across the distribution of seagrass species, it is at much lower levels than the 1930s and 1940s. Disease can be identified by the brown spots and streaks that spread down the seagrass leaves (Den Hartog 1989). A recent student project at Heriot Watt University found presence of Labyrinthula on seagrass along a transect in the Firth of Forth (Fraser et al., 2016). However, no comprehensive time series data exist to inform the prevalence of wasting disease in Scottish seagrass beds.
Policy and legislation
Seagrass conservation and protection
Seagrass beds (Zostera spp.) are an OSPAR threatened and / or declining habitat and a Priority Marine Feature (PMF) in Scotland. As a PMF, seagrass beds receive a degree of protection even if located outside MPAs through General Policy 9 of Scotland’s National Marine Plan, which states that the “Development and use of the marine environment must not result in significant impact on the national status of Priority Marine Features”. NatureScot have developed guidance on how to assess development proposals with potential to impact upon PMFs and a checklist to assist with such assessment.
Subtidal seagrass beds are a protected feature in a number of Marine Protected Areas (MPAs). The MPA network includes Nature Conservation MPAs, Special Areas of Conservation (SACs), Special Protection Areas (SPAs) and Sites of Special Scientific Interest (SSSI). Sites with seagrass beds include Sound of Arisaig; Sound of Barra; Loch nam Madadh; Dornoch Firth and South Arran. The feature can be part of the following broadscale habitats: Lagoons; Mudflats and sandflats; Sandbanks which are slightly covered by sea water all the time; and, Estuaries. European sites (SACs and SPAs) are internationally important for threatened habitats and species.
SSSIs are areas of land and water that best represent our natural heritage and can encompass intertidal areas. Many SSSIs are also designated as European sites, but the overlap is not exact and NatureScot has the role of the competent authority in dealing with applications for consent including for SSSIs. A Habitats Regulations Appraisal (HRA) may be required for seagrass restoration activities where a SSSI overlaps a European site.
Crown Estate Scotland owns most of the seabed from mean low water to the 12-nautical-mile limit. Therefore, permission should be sought where activities are planned in this area and a lease may be required.
Under Part 5 of the Marine (Scotland) Act 2010 (“Marine Protection and Enhancement: The Marine Protection Area”), conserving a feature in relation to a Marine Protected Area (MPA) may include “enabling or facilitating its recovery or increase”. Scotland’s National Marine Plan (Scottish Government, 2015) sets out wider policies and drivers for the conservation and enhancement of habitats. Enhancement activities may fit with the conservation objectives for protected features in MPAs. For some of those protected features that have ‘recover/restore’ conservation objectives, enhancement or restoration may be part of the process to achieve improved condition and status. However, activities should be assessed with respect to the impact on protected features (and their conservation objectives) and restoration may not be the most appropriate action. The first priority must be to protect the extant features and to ensure that, where restoration initiatives are proposed, plans should be based on the best evidence available at the time.
The UK is also signed up to the Water Framework Directive and one of the aims is to prevent deterioration and enhance status of aquatic ecosystems. The UK Marine Strategy sets out a framework for achieving Good Environmental Status (GES) which includes protecting and restoring the marine environment.
European Commission Biodiversity Strategy for 2030 (“Bringing nature back into our lives”) states the importance of achieving good environmental status of marine ecosystems, including through strictly protected areas. The strategy indicates that this should involve the restoration of carbon-rich ecosystems as well as important fish spawning and nursery areas.
Policy considerations for seagrass restoration
Seagrass restoration proposals will be considered on a case-by-case basis and the policy direction will depend on the proposed techniques and location of the site with respect to the legislative processes described above. NatureScot and Marine Scotland Licensing Operations Team should be contacted at the earliest stage of a proposal to ensure all avenues have been considered. Furthermore, seagrass monitoring has been carried out by NatureScot and SEPA (Scottish Environment Protection Agency), therefore unpublished information on seagrass bed from historic records, extent polygons and anecdotal evidence may be available to inform proposals at an early stage.
NatureScot have developed a Marine Enhancement Guidance Framework to assess enhancement and restoration proposals and provide guidance on aspects such as licensing, monitoring, biosecurity and planning. If the proposed site is in a protected area then an assessment will need to be carried out to determine if the activities are likely to have an impact on the protected features of that site. The type of assessment will depend on the type of protected area. The Translocation Project Form may be appropriate to provide detail on all aspects of a seagrass restoration proposal, however, a PMF checklist may also be required to assess the various activities related to the proposal (e.g. seed collection, research trials, large scale restoration etc.).
The Scottish Code for Conservation Translocations provides guidance for proposals that involve moving seeds or adult plants. Some key questions to consider are:
- Is restoration is the best option? Could other conservation actions provide a lower-risk and lower-cost solution?
- Have all the necessary permissions and licences been granted?
- Is the proposal evidence-led? Have the biological and ecological requirements for the species been fully assessed?
- What are the biosecurity risks in terms of introduction and spread of invasive and non-native species and diseases?
- Have all relevant community groups and stakeholders been consulted?
Restoration trials may receive a degree of incidental protection (e.g. from natural coastal barriers such as shallow sand bars and rocky outcrops). If a restoration trial develops into a mature bed in a protected area where seagrass beds are a protected feature then the bed would be assessed to determine its status and may receive protection under the designation. However, the specific conservation objectives for a given site should be carefully considered.
Consents, licences or planning permission may be required for seagrass restoration projects. The exact requirement will vary depending on location and the intended methods to be employed. For example, depositing material on the seabed from a vessel would be a ‘licensable marine activity’ (see section 21 of the Marine (Scotland) Act 2010), although certain ‘licensable marine activities’ do not need a marine licence as they are exempt (e.g. the deposit of scientific instruments). Such activities are described in The Marine Licensing (Exempted Activities) (Scottish Inshore Region) Order 2011 (legislation.gov.uk). If a project for which a marine licence is needed exceeds 1000 square meters, it could trigger pre-application consultation, under The Marine Licensing (Pre-application Consultation) (Scotland) Regulations 2013. This is a minimum 12-week process before a marine licence application can be submitted.
Zostera marina is not a European Protected Species, nor is it listed as a protected plant in Schedule 8 of the Wildlife and Countryside Act 1981 (Scotland). However, section 13(1)(b) of the Wildlife and Countryside Act 1981 (Scotland) states:
“If any person, not being an authorised person, intentionally or recklessly uproots any wild plant not included in that Schedule [Schedule 8], he shall be guilty of an offence.”
Therefore, it is an offence to intentionally or recklessly uproot seagrass unless you have been authorised to do so. If it has been clearly established through the proposal that seagrass will not be uprooted, then the activity is considered legal and does not need approval from NatureScot Licensing or land owner/local authority permission. However, it is likely that the permission from the landowner will be required to access the land to carry out the wider work.
Invasive non-native species (INNS) can have serious negative impacts on native Scottish habitats and species. For example, the carpet seasquirt, Didemnum vexillum is known to smother other animals, which could alter entire habitats and have severe consequences for biodiversity and the economic activity within the marine environment. Invasive species such as Spartina anglica, Sargassum muticum and Codium fragile can also pose a threat to seagrass. Introduction and spread of INNS with seagrass can lead to a downgrade in classification of the entire water body under the Water Framework Directive.
All activities associated with restoration that can lead to introduction or spread of INNS and plant diseases (see Seagrass bed distribution and historic extent in Scotland section) have to be identified in a Biosecurity Plan including the steps needed to minimise this risk. Proposals that include transplanting adult seagrass, seeds or sediment from a different location must carefully consider risk of accidental transfer of INNS and plant disease. have the potential to move biological material between locations should take steps to minimise the spread of INNS and plant disease.
Research at Swansea University suggests that sterilising seagrass seeds can minimise the risk that INNS will spread due to movement of seeds between sites and this does not impact the viability of seagrass growth and development. This process involves soaking seeds in 5% sodium hypochloride (see Churchill, 1992).
However, it is recommended that seeds should be sourced from local beds, where possible, to reduce the risk further.
Carrying out surves for the presence of INNS at proposed sites is highly recommended as INNS can cause problems and reduce the chance of successful restoration. Swansea University are currently examining whether the high prevalence of Sargassum muticum in particular areas may become problematic for the use of hessian bags in seagrass restoration. Continued monitoring for the presence of INNS and pathogens throughout the entire restoration project and even after beds establishment is recommended. The presence of INNS and plant pathogens can be reported to Marine Scotland (MarineNonNativeS@gov.scot) and support with identification of suspicious organisms can be provided.
Biosecurity should be a key consideration for any seagrass restoration proposal, particularly where seeds or adult plants are being moved from one site to another. For any project involving fieldwork, the Check, Clean, Dry procedure should be followed. Marine Scotland and NatureScot can provide specific biosecurity recommendations on a case by case basis depending on location. The Biosecurity Plan should be submitted to Marine Scotland prior to any restoration activities.
Planning and selecting restoration sites
Project aims and objectives
It is important that any seagrass restoration or habitat creation proposal clearly outlines the restoration project aims, for example, some questions to ask are:
- Is the project aiming to restore an entire new bed or enhance an existing bed?
- Is the aim to enhance seagrass bed extent or condition and how will this be measured?
- How will the project determine ‘success’ and over what timescale?
- Is the aim to restore seagrass beds to how they were at a certain point in time?
- Is the aim to restore biodiversity and/or ecosystem services?
- Are there any cumulative benefits – a high value project would improve extent/connectivity over a particular area, potentially in conjunction with other similar or related projects.
- Is the aim to support carbon credits/green credentials or compensate for damage/loss caused by a particular activity?
Objectives are specific, measureable outcomes that are needed to achieve the project aims. The objectives should include site-specific considerations. Measurements could include seagrass density, extent and percent cover but also wider ecosystem effects such as sediment stability, water quality and fish/shellfish biomass. The potential for community involvement should also be assessed and it may be appropriate for goals to incorporate this aspect (e.g. the number of volunteers involved or volunteer hours).
As mentioned above, the first priority is to protect existing seagrass beds. Where seagrass restoration is considered the most appropriate path forward it is imperative that the site is assed for its environmental, biological and social suitability. There should be a justified need for restoration to take place (e.g. has seagrass declined in the area of interest?) and demonstration that other conservation measures have been considered. It must be clearly established whether restoration and the defined aims and objectives are possible. Important factors to consider when choosing a restoration site section highlights the key factors and criteria for selecting a restoration site, however, initial screening should consider barriers such as:
- Environmental and biological conditions at the site
- Policy and legislative restrictions
- Marine activities, such as development proposals, exposure to pollution and fisheries (e.g. is the site in a fishery closed area?)
- Stakeholder and community group engagement – is there buy-in from the local population or potential conflict?
- Are resources and facilities sufficient (e.g. seed storage tanks)?
Important factors to consider when choosing a restoration site
One of the main factors that can cause a restoration effort to fail is unsuitable environmental and biological conditions. Habitat suitability maps can be used as a first step to filter out unsuitable areas. However, it is important to establish baseline measurements of physical and biological parameters alongside modelled habitat suitability at the site level prior to a restoration trial to ensure conditions are optimal. Such baseline measures could include light attenuation, wave exposure, predation and algal cover. It is also important to establish the cause of seagrass decline in the area of interest and ensure that this pressure has been removed prior to restoration efforts commencing. See chapter 6 for more detail on monitoring.
Seagrass beds in Scotland tend to occur in sheltered areas up to 10 m deep, although many are <6m deep. A sufficient amount of water exchange is required to maintain low turbidity levels, but exposure must be low enough for a bed to stabilise and develop. The complex coastline, sheltered sea lochs and firths make Scotland a good place for seagrass to thrive. Habitat suitability modelling and an assessment of the biological and environmental risks to the project are recommended in advance of trial planting.
Environmental envelope analysis (e.g. using Maxent software) is relatively accessible and has been used to model habitat suitability for a number of Scottish PMFs (e.g., Millar et al., 2019; Gormley et al., 2013; Simon-Nutbrown et al., 2020). A wave fetch model for Scotland has been developed by Burrows and is available under a GNU General Public Licence. It is also important to consider predicted changes in the environment at any particular site due to climate change (e.g. sea level rise, changes in turbidity, storminess etc.) to ensure that any restoration attempts are sustainable in the long term.
A study by Huang (2021) used Maxent species distribution model including bathymetry and exposure to determine most suitable, less suitable and unsuitable areas for seagrass to grow in Scotland (Figure 5). However, many other environmental factors such as light and nutrients are likely to influence the distribution of seagrass beds and these were not included due to the poor resolution of layers available. Therefore, while such models can be a useful guide, direct monitoring of physical variables such as light and temperature should be carried out at potential restoration sites. It is recommended that a proposed restoration site should be monitored to determine site suitability for at least one year relative to existing seagrass beds before planting takes place.
Derived from Maxent Species Distribution Modelling (Huang, 2021). GEBCO Bathymetric Compilation Group 2021 (2021). The GEBCO_2021 Grid - a continuous terrain model of the global oceans and land. NERC EDS British Oceanographic Data Centre NOC. doi:10/gn6h.
A common issue for seagrass restoration is poor water quality or low light conditions (Moksnes et al., 2021). However, unsuitable temperature or salinity conditions, growth and drifting algae mats, disturbance from burrowing and grazing animals, exposure to waves and currents and unfavourable geochemical conditions can also contribute to failure (Fonseca et al., 1998, van Katwijk et al., 2009).
As a photosynthetic organism, the maximum depth that seagrass can survive is primarily determined by light levels and low light levels can significantly impact growth (Bertelli and Unsworth, 2018). Light levels vary widely between geographic regions and depend on factors other than depth, such as organic and inorganic input to the water body. The availability of light is partly determined by how quickly the visible light is absorbed into the water, which depends on the type and amount of organic and inorganic particles in the water. See section 7.1 for guidance on measuring light levels.
Seagrass beds are relatively tolerant to changes in temperature, however, extreme summer maximum temperatures can cause extensive algal growth which is likely to have a detrimental effect on seagrass growth.
Current known distribution, extent and condition
Once a geographic area of interest has been defined, the first step is to assess the current status of seagrass beds in the area and to review the data currently available. Scottish seagrass bed records are available from the National Marine Plan interactive and wider records for the seagrass biotopes (at least 25 m2) can be downloaded from Marine Recorder. Records of seagrass species (i.e. not necessarily a bed formation but records of individuals) can be found on the National Biodiversity Network (NBN) database and via SeagrassSpotter. Records can normally be traced back to a report or source of information and it is recommended that historic information should be studied carefully to determine the natural variability, direction of change and likely cause of change to the seagrass bed or waterbody of interest.
Anecdotal information may be available but not necessarily included in the national databases mentioned above. Early engagement with any local community groups or organisations involved in previous work at the site is also recommended.
An early assessment of the need for restoration should consider whether restoration is the best course of action of if other conservation management techniques are better (e.g. to remove pressures and monitor recovery). It is better to avoid restoring areas that would most likely recover naturally in the near future. Management approaches will differ depending on local considerations as well as the health of the bed and connectivity to other beds.
Habitat suitability models typically show that the most suitable areas for seagrass to grow are often close to existing beds. Given the national importance of seagrass beds as a PMF and protected feature in Scotland, efforts should be made to avoid disturbance to natural healthy beds during restoration and enhancement activities.
Connectivity and climate change
An understanding of the location of natural seagrass beds in the area of interest is also important in the context of connectivity. Most seeds only spread a few meters from the bed (although they will likely travel further by storms and when floating in the seed pods; Källström et al., 2008).
Restored seagrass beds should become self-sustaining in the long-term, therefore the beds must be able to persist through sexual or asexual reproduction (see chapter 1). Connectivity with other seagrass populations might be crucial for the survival of a restoration effort. Connectivity and habitat suitability may also change over time due to the effects of climate change and coastal developments for example.
It is important to study the proposed restoration site in detail and to establish the cause of any declines in bed condition or extent. If the pressure causing the decline persists, then it is likely that any restoration efforts will fail. Causes of seagrass decline can range from disease, fishing, aquaculture, trampling, changes in hydrodynamics (see the Feature Activity Sensitivity Tool - FeAST for more information). Seagrass planting should not go ahead if known pressures continue to exist.
Careful selection of a donor population is required both in terms of minimising the impact of collection on natural beds but also the chances of success. Research shows that when transplanting adult plants, it is important to match the conditions of the donor site and the restoration site (Moksnes et al., 2021). This is also important if using a seed-based approach although there is a greater potential for the plants to adapt to the new conditions as they grow. The genetic aspect of donor site seed collection and restoration site selection should also be considered with respect to the chances of success but also the impact on natural genetic diversity. Seagrass beds that are considered to be in a degraded condition should not be used as a donor site - this will be considered on a case by case basis depending on the information available on bed condition.
Seagrass restoration techniques and research
While large-scale habitat restoration is relatively widespread on land (e.g. in forest and peatland ecosystems), the enhancement or restoration of coastal and subtidal habitats is rare and research is in its infancy (France, 2016; Guarnieri et al., 2019). In Scotland, native oyster restoration research and trials (Fariñas-Franco et al., 2018) as well as saltmarsh transplantation (Maynard, 2014) have begun.
Transplantation trials of intertidal seagrass (Z. noltii) in Scotland are reported by Wilkie (2011). In this thesis, Wilke (2011) explored the idea of restoring intertidal seagrass beds to protect saltmarsh from erosion. Seagrass cores were taken from a donor site and planted in experimental ‘plots’ in June 2009. Out of 20 plots, only two survived to the end of the trial in 2010, although these two plots had merged to form a continuous bed and the timeframe for this study provides limited scope to assess long-term success. Adverse environmental conditions and a lack of genetic diversity were given as reasons for failure in the remaining plots (Wilke, 2011).
At the time of writing, a small seagrass (Z. marina) pilot restoration project is underway in Loch Craignish (Scotland) led by the community group, Seawilding (see Annex 1 for the Loch Craignish case study).
In general, an interest in subtidal seagrass (Z. marina) restoration in the UK has taken off in recent years with successful planting trials in both Cornwall and Wales having been recorded (see Unsworth et al., 2019a). A 2 ha project in Dale (Wales) between 2019 and 2021 was followed by the initiation of a 4 ha restoration project in Plymouth Sound (England) which is due to be completed in 2025. At the time of writing, the pilot project in Wales is showing initial signs of success overall with mature plants present across the 2 hectare area, but seed survival is very low (<1%) and shoot density extremely patchy.
There are many different seagrass restoration techniques that have been trialled around the world. Examples provided in this handbook focus around the Bags of Seagrass Seed (BoSS) method(s) as these have been used most widely in the UK (see Seed planting options section for details). This involves planting seeds in hessian bags to buffer exposure and biological disturbance effects. In some parts of the world, seagrass restoration has focussed on the use of transplantation as well as the use of seeds, such approaches do have considerable benefits, particularly in instances where projects need to rapidly overcome physical problems (negative feedbacks) in the system. The collection of seeds has no known negative impacts upon the donor meadow and is low effort relative to plant collection and replanting.
Seagrass restoration trials have started in Scotland but are in early stages therefore information on the best technique to use is lacking. More broadly, research to support evidence-led restoration proposals in Scotland is sparse and particular knowledge gaps in a Scottish context focus around the following themes: natural variability of seagrass beds, habitat suitability and optimal growing conditions, connectivity, biosecurity, interactions between seagrass and invasive non-native species, disease prevalence, ecosystem and societal benefits of seagrass restoration. However, research in this area is gaining momentum with research projects in the pipeline or already underway. Further information on the research available on ecosystem services provided by seagrass beds in the context of restoration can be found in chapter 5.
Worldwide, various techniques to restore seagrass beds have been developed with differing levels of success and this of course depends on the definition of ‘success’. Failed attempts are mostly due to poor site selection and issues of scale. Evidence indicates that small scale projects often fail, as the bigger the project size the greater the collective shelter provided to young shoots, however, success also depends on selecting the most appropriate restoration technique for the environmental conditions.
A range of planting techniques have been tested but they mainly fall into two categories: seed-based or transplantation techniques. There are pros and cons of each but the most appropriate technique to use will partly depend on environmental conditions at any given site and the resources available. In some places a combination of transplants and seed methods have proved most successful.
There is also growing interest in modifying the environment to facilitate improved success (e.g. through altering the sediment) and using particular structures to reduce physical impacts. In addition, there is increasing research interest into the potential role that bivalves can play in a symbiotic relationship with seagrass to enhance recovery.
In the USA, experimental restoration of Z. marina began in the 1970s in Chesapeake Bay (Orth et al., 2006) and a wealth of research exists from this area (e.g. Orth et al., 2002; Stockhausen et al., 2003; Shafer and Bergstrom, 2010). Early trials focussed around transplantation techniques using adult seagrass (Fishman et al., 2004). A comparison of the various transplantation methods reviewed by Orth et al. (2006) include the transplantation of sediment along with the plants (described as “cores” or “sods”), adding fertilisation, mechanised or manual harvesting and the effect of season on planting and survival.
Swedish seagrass restoration guidelines recommend the ‘single shoot method’ where individual shoots are planted by hand without an anchoring system or sediment (Moksnes et al., 2021). Divers simply push the rhizome of the bulkhead obliquely into the sediment and a skilled diver can plant over 300-400 shoots per hour using this method (Figure 6). Compared to transplantation with sediment, the single shoot method is relatively low effort in comparison to other transplanting techniques, however, the diver time required makes this technique resource heavy compared to seed planting.
Seed-based methods focus around three stages - seed collection from donor beds, seed storage and planting or broadcasting. The benefits of this method over transplanting adults include:
- Lower impact on donor beds than extraction of adult plants
- Seagrass produce copious quantities of seeds that would otherwise be lost from the system
- Less time and effort required for planting/broadcasting
- Seeds can be treated to reduce biosecurity risks
Drawbacks of the seed-based method include:
- Seed collection can be resource-heavy
- High mortality of seeds once planted (e.g. due to predation)
- Very low seed germination rate
- Mature beds take longer to establish than transplanted adults and therefore it takes longer to see ecosystem benefits
- Seed storage facilities required
Seed collection normally involves breaking off a reproductive shoot (which contain the seed pods (spathes) from the plant by hand and this is carried out by divers, snorkelers or simply wading if the bed is shallow. This stage can be labour intensive and unpredictable due to the timing and presence of seeds at any given site. Involving community groups can speed up this process with the added benefit of local ecological knowledge (LEK). Mechanical harvesting techniques have been developed (e.g. see Marion and Orth, 2010), although the impacts of such techniques have not been fully assessed. Figure 7 shows seagrass seeds collected in August in Loch Craignish at different stages of development.
Once a suitable donor meadow has been identified, monitoring of the seed development should be undertaken to establish the timing for collections. There is no evidence that seed collection by hand damages the integrity of a seagrass bed significantly. However, as a precaution, seed collection should be favoured from areas where seagrass has been established to be in a healthy resilient state (e.g. see Unsworth et al., 2015).
Seed collection tips
Only mature seeds should be collected. Mature seeds should be rounded and fully formed rather than tapered. If the seeds are too immature (bright green: Figure 7) then they will not continue to mature in the aquaria and would not be viable for restoration.
Each seed spathe may contain up to 20 seeds. Some shoots will have multiple branches of spathes, allowing multiple spathes to be collected at once by picking near to the bottom of the plant. The best way to spot Z. marina seeds is by looking for lighter green, thinner and more cylindrical strands, either above the main canopy or below it. Pick the shoot of a seeding plant at least 10 cm above the seabed (Figure 8). To do this, pinch below the tear point to stop any damage to the rest of the plant. Using one hand, snap off at the tear point and with the other, hold the base of the plant in place to prevent uprooting. Care should be taken to ensure minimal leaf material is taken.
Once collected, the seeds must be extracted from the pods and stored until deployment. Seed pods (spathes) are often stored in aerated tanks, either with natural seawater flow or in an aquarium at a stable temperature in the dark to reduce algal growth. Since seeds are negatively buoyant and the shoots/seed pods are positively buoyant, the seeds naturally separate from the spathes and drop to the bottom of the tanks where they can be collected (Unsworth et al., 2019a; Infantes and Moksnes, 2018).
Seed storage options
Short storage (1–3 days)
Seed material kept in insulated containers (polystyrene and cool boxes) in the shade has been suitable for very short-term storage i.e. for a few days, though it is recommended to add fresh seawater daily.
Long storage (> 3 days)
When storage beyond 3 days is required, the storage container needs to be fitted with an air pump large enough to encourage water circulation. Regular water changes with fresh seawater are still required – as frequently as is feasible. It is important that storage containers aren’t left to heat. The provision of shade may be important during periods of warm weather.
Seagrass seeds collected from a mixture of donor sources must be stored within biosecure aquarium facilities. Where seeds have been taken from non-local sites sterilisation will be used (sodium hypochlorite and sterile seawater) to stop any risk of spread of INNS and pathogens. This is recognised standard good practice. Also abide by Check/Clean/Dry good practice guidelines to minimise vessel-related transfer of non-native species between separate sites. See Biosecurity section for more detail on biosecurity.
Seed planting options
Various seed-based planting techniques have been trialled around the globe, but the most successful large-scale restoration has been in Chesapeake Bay where 3600 ha seagrass has been restored (Orth et al., 2020), largely through the seed broadcasting technique (Marion and Orth 2010). The simplest seed deployment technique is probably hand broadcasting from a boat, which has been used in the USA. Traber et al. (2003) planted seeds by actively pumping seeds suspended in a gel matrix through distribution tubes, while Marion and Orth (2010) describe a system which uses a gravity-driven flow of water to deliver seeds to the seabed through supply tubes to a planting sled pulled behind a boat.
Buoy deployed seeds have also been used as a method for storing and deploying seeds. For example, a technique reported by Pickerell et al. (2005) where seeds were stocked into mesh pearl nets suspended from buoys then as the seeds ripened, they were naturally released from the nets, fell to the bottom and germinated. Other seed deployment methods include the use of biodegradable coconut fibre mats (Sousa et al., 2017).
Bags Of Seagrass Seeds (BoSS) methods
Enthusiasm for seagrass (Z. marina) restoration in the UK has taken off over the past 5 years with successful trials in Wales using a seed-based method called the “Bags of Seagrass Seeds Line (BoSSLine)” developed by Swansea University and Project Seagrass (Unsworth et al., 2019a) as described above. Using this technique, approximately 50 seeds are placed into small (13 cm x 17.5 cm) hessian bags (Figure 9). These bags are then attached to a biodegradable rope for deployment at regular intervals (50-100cm spacing) (Figures 10 and 11). Hessian bags containing seeds can also be planting in intertidal areas (e.g. a method for planting Z. noltii seeds in the Humber Estuary in England has been developed by the Yorkshire Wildlife Trust).
The hessian bags used in the BoSS methods not only keep the seeds from dispersing due to tidal movements, but also protect the seeds from being buried by lugworms (Valdemarsen et al., 2011) or eaten by predators (Infantes et al., 2016). Over time the bags naturally break down, leaving the plants to grow and develop into a bed. Some hessian bag products may include a plastic lining which would not be suitable to use. The mesh size of the bags should also be selected carefully. A fine mesh size may inhibit growth of the blades and rhizome. The development of the BoSS technique makes seed planting a more viable option due to reduced mortality/loss of seeds during the planting stage and therefore reduced labour costs.
Seagrass restoration methods that use hessian bags have been the focus of many restoration efforts in the UK so far and are therefore described in this section in detail. However, other methods may be suitable depending on environmental conditions and local considerations. For example, seed broadcasting may be effective at very sheltered conditions and the single shoot method may be suitable for more exposed sites. There are 2 main hessian bag methods BoSSline and BoSS described below.
The BoSS method also has the advantage of being able to be deployed in any configuration on the seafloor. By planting the bags in a grid pattern, relocation and future assessments should be easier than if the bags a placed haphazardly (e.g. individual bags released from a boat). One option is to plant 100 hessian bags in a pattern shown in Figure 10, (10 bags by 10 bags, approximately 50 cm apart) to make a 4.5 m x 4.5 m grid. Markers can be placed in the corners (to make the entire grid 5 m x 5 m) to help with relocation and a GPS mark can be taken in the centre of the grid. The method for relocation will depend on local conditions and a GPS mark alone might be enough, but it is important to take detailed notes to help with relocation. Figure 10 shows two grids but multiple grids can be used depending on the project objectives (e.g. to test different depths, methods, etc.).
Recent research (Temmink et al., 2020) suggests that positive interactions between plants are more important than the downside of competition. Therefore, clumping hessian bags together may engender higher rates of success. In Loch Craignish, Argyll, two 5 m x 5 m BoSS squares have been planted on the seafloor, with hessian bags located 50 cm apart from each other (100 bags per square).
BoSSLines give the advantage of deploying BoSS with regular spacing over large areas. Deploying the bags on lines makes it easier to relocate the bags in future for monitoring purposes, which is essential for determining the success of a project. The BoSSLines deployed in Dale were 100 m long, with BoSS attached at 1 m intervals. These lines were deployed as close as parallel to each other as possible to attempt an even distribution of BoSS over the seabed. The lines can be weighted either end with a larger hessian bag full of local sediment to act as the weight to minimise the need to recover materials, but the requirement for weight and the exact method used will depend on local conditions (e.g. exposure).
Early indications from monitoring of the Dale project has shown the BoSS method to be successful, but the drivers of success and failure are still poorly understood. Early results suggest that ensuring the BoSS are buried at least 2 cm into the sediment improves success with average shoot counts around 1.5 times higher in buried bags than seabed-surface deployed bags (Unsworth, Pers. Comm.).
It is recommended that the BoSS methods would be the most likely to succeed in Scotland for subtidal seagrass due to the similar environmental conditions as in the Welsh trials. This method is also preferable to ensure relocation of hessian bags and quantitative assessment of success.
Where possible, seeds should be buried at least 2 cm into the sediment for the greatest chance of success.
Seed-based methods have a greater potential for up scaling than transplantation of adults, due to the latter being labour intensive. However, mechanical seed collection is not recommended at present due to the potential impact on donor beds.
Benefits of seagrass restoration
The range of ecosystem services provided by seagrass beds, along with the relatively fast growth of the species, builds a case for the restoration of this habitat. The following section provides a summary of the current evidence base for the ecological, societal and economic benefits of seagrass bed restoration. Research in this area has largely been carried out in countries and waterbodies outside the UK, therefore results should be interpreted with care and particular attention to the seagrass species involved. While these case studies are useful to show the potential benefits of seagrass restoration in Scotland, the overall effect may not be directly transferrable to restoration activities in Scottish waters.
Many seagrass restoration projects quote ecosystem services as a key driver, benefit or reason for investment in such activities. Duarte et al. (2020) review the restoration potential for a number of coastal habitats and highlight key opportunities for seagrass including, blue carbon and coastal defence strategies against storms, enhancing water quality, food provision and biodiversity benefits. Furthermore, Reynolds et al. (2012) found that genetic diversity within the seagrass bed is important for the provision of ecosystem services (invertebrate habitat, increased primary productivity, and nutrient retention). The scale at which seagrass beds provide ecosystem services and societal benefits depends on the species of seagrass and the environmental setting that the beds are in.
Fisheries benefits, including the enhancement of fish nursery grounds, are often used as a primary justification and a benefit of the restoration of seagrass beds (e.g., Thorhaug, 1990; Duarte et al., 2020). Early research on fisheries benefits was based on the assumption that bigger or denser seagrass beds support a greater abundance of commercially important fish and shellfish species, therefore increasing seagrass bed extent and condition through restoration is presumed to enhance such benefits. A review by Unsworth et al. (2019b) suggests that seagrass beds support global food security by providing nursery habitat for fish stocks and creating expansive fishery habitat rich in fauna, which also provides trophic support to adjacent habitats and fisheries. Indeed, UK beds have been found to support commercially important juvenile fish such as plaice, pollock and herring relative to adjacent sand habitats (Bertelli and Unsworth, 2014). However, it is only in recent years where examples of large-scale restoration success have emerged, that it is possible to relate enhanced faunal communities (including commercially important species) to increased seagrass extent as a result of restoration efforts (see Orth et al., 2020).
An increasingly attractive aspect of restoring coastal habitats is the concept of enhancing blue carbon resources as a nature-based solution to the climate crisis, particularly in the context of the climate emergency (Robinson et al., 2020). Carbon sequestration figures quoted for seagrass beds are often based on studies on Posidonia species (Rozaimi et al., 2016; Serrano et al., 2016). However Posidonia seagrass species have a robust, slow growing root structure compared to Zostera species which are relatively fast growing, therefore information on blue carbon stocks are not directly comparable. Röhr et al. (2018) collected samples from 54 Z. marina beds across eight ocean margins and seas. Analysis of above and below ground biomass found organic carbon levels to be in line with estimates for other seagrasses and in fact within the range of other carbon-rich ecosystems (Figure 12).
It is important to put the figures for seagrass blue carbon into a Scottish perspective. Carbon storage capacity of Zostera marina and Z. noltii beds were assessed by Potouroglou et al. (2021) across 10 estuaries in Scotland. The organic carbon of vegetated (seagrass) areas was significantly higher than unvegetated areas (adjacent bare sand or mud), although variation was high. Overall the average organic carbon stocks presented by Potouroglou et al. (2021) are similar to those presented by Röhr et al. (2018).
Greiner et al. (2013) studied carbon sequestration in restored Z. marina beds and found that carbon accumulation rates were 36.68 g C m-2 yr-1 10 years after seeding in the Virginia coastal bays. The restored beds were predicted to accumulate carbon at a rate that is comparable to natural seagrass beds after 12 years, therefore suggesting that Z. marina restoration can enhance blue carbon stocks. However, it is unclear how this would translate to Scottish seagrass and potential restoration projects. It is clear from the scientific literature than the level of hydrodynamic activity at a site can be a strong determinant of seagrass annual carbon storage. Given the highly variable environments in Scottish waters which Zostera spp. live in, it is likely that that storage is also variable. Given the high levels of interest in seagrass restoration in the context of nature based solutions to climate change, further research is required on carbon sequestration rates for restored seagrass beds in the UK.
Röhr et al. (2018) suggest that Z. marina beds enhance sequestration of sediment carbon stocks through the production of epiphytes, microalgae, and macroalgae, trapping of organic particles, and reduction of sediment resuspension and water flow. However, organic carbon stocks varied across the different geographic areas studied and differences were largely driven by sedimentary characteristics (Röhr et al., 2018). Furthermore, experimental work in Florida, USA found that while seagrass beds can sequester carbon, some can be a source of carbon and that rates are driven by light temperature and tidal exchange (van Dam et al., 2021). Sites with limited tidal exchange were net sources of CO2 and sites with large tidal exchange were net CO2 sinks. This highlights the importance for site-specific studies on seagrass ecosystem functions and processes, particularly where there is an interest in restoring seagrass beds as a nature-based solution to climate change. It is critical that seagrass restoration projects assess site suitability before planting to ensure optimal ecosystem functioning.
Regulating services extend beyond carbon sequestration to nitrogen cycling and ammonium retention (e.g. Reynolds et al., 2016). McGlathery et al. (2012) found that nine years after restoration, Z. marina beds had double the organic matter and exchangeable ammonium concentrations, three times more carbon and four times more nitrogen than bare, unvegetated sediments (Figure 13).
Another important functional role of seagrass beds is sediment stabilisation and shoreline protection. The presence of the grass absorbs wave energy and creates turbulence which causes sediment to fall out of suspension while the root system stabilises the seabed sediment, reducing resuspension of sediments and erosion (McGlathery et al., 2012; Moksnes et al., 2021).
Arrows represent sediment suspension or deposition and show that sediment stability increases with meadow development. The relative proportions of sand and silt fractions change with meadow development (indicated by size of circles), with a fining of the sediments over time. The size of the circles for organic matter (OM) and NH4 + concentrations indicates that both pools increase as meadows develop over time (McGlathery et al., 2012).
Greiner et al. (2013) found that after 10 years, restored seagrass beds facilitated the accumulation of 36.68 (±2.79) g C m-2 yr-1. This figure is slightly below the range for natural beds, however, a lag period should be expected due to the continued increase in seagrass density over time as the bed matures.
Evidence around the cultural benefits of UK seagrass beds is lacking, but de los Santos et al. (2020) suggest that UK seagrass beds are important for bird watching and other activities. There is evidence that seagrass beds provide tourism and recreational benefits further afield, for example, recreational fishing in Indonesia (Wahyudin et al., 2018) and tourism in Mexico (de los Santos et al., 2020). The scale of such benefits will be different between tropical and temperate regions and cultural services are difficult to quantify therefore often not included in ecosystem assessments. However, given the accessibility of seagrass beds it is thought that the cultural value of seagrass beds is relatively high.
The 2020 Challenge for Scotland's Biodiversity aims to provide opportunities for everyone to experience and enjoy nature regularly and acknowledges the benefits of connecting people with nature, including health, wellbeing, education, community development and regeneration. In the marine environment, it is not always possible for people to access certain environments, particularly those in deeper, offshore regions. Seagrass is relatively widespread in Scotland and as a shallow habitat often found in sheltered bays, it is relatively accessible to the general public. This makes seagrass restoration projects a key candidate for community involvement, citizen science projects, community monitoring and a way for coastal groups to connect with the habitats on their doorsteps.
Economic costs and benefits
Natural capital is a term for the habitats and ecosystems that provide social, environmental and economic benefits to humans. This term is increasingly being used to ensure the environment is considered as part of more traditional forms of accounting. Economic valuation of natural systems is complex and there are various limitations associated with this type of assessment framework (e.g. lack of scientific knowledge of key linkages, a lack of relevant economic valuation studies, and methodological problems; Hanley et al., 2015). Moksnes et al. (2021) provide a detailed economic assessment of seagrass (Zostera marina) restoration benefits in Sweden including monetary values, inherent values, direct and indirect values, economic goods and intermediate ecosystem services or commodities.
Chapter 7 in Moksnes et al. (2021) provides an assessment of the economic cost of seagrass restoration including site selection and evaluation, harvesting, planting and monitoring. The 10-year cost per hectare is estimated to be SEK 1.2-2.5 million (£105 000 – £220 000) for the shoot method and SEK 2.5-7.2 million (£218 692 - £633 065) for the seed method. The higher cost of the seed method is largely due to the labour required for seed collection. For the first 2 ha restoration project in Dale, Wales, the cost per hectare was approximately £200,000 per year (excluding monitoring costs). A major focus in the restoration projects in Wales and beyond by Project Seagrass and their collaborative partnership with WWF and Swansea University has been the strong element of community engagement due to the relatively novel nature of the work. Such engagement adds additional costs but help secure the longevity of the project as a partnership with local people.
The costs associated with restoration projects can vary depending upon local geographic and economic conditions, with projects in the Solent costing more to run than projects in Wales due to the travel, subsistence and accommodation costs involved (Project Seagrass, Pers. Comm.). Project Seagrass are working on both a mechanical seed harvesting programme (sensu Marion and Orth, 2010), and a seagrass nursery project, which they hope will drive down the cost per hectare over the coming decade (Project Seagrass, Pers. Comm.)
Monitoring and test planting
Baseline monitoring and reference sites
As mentioned above, it is critical that baseline monitoring takes place at a proposed site before planting trials start. By monitoring a site for a year, the seasonal changes in environmental conditions and biological interactions can be assessed as well as site specific ecological information such as the timing of seed production. Baseline monitoring should include assessing the current condition of existing seagrass beds and other species and habitat of conservation interest in the area as well as monitoring environmental parameters. If a project aims to restore ecosystem services related to seagrass beds, then baseline measurements should include ecosystem function indicators (e.g. surveys of mobile fauna, water quality, sediment stability, sediment carbon content etc.).
Effective evaluation of a seagrass restoration project requires reference beds, i.e. natural unaffected seagrass beds that are as close to the restoration area as possible. This is important for demonstrating whether changes observed at the restoration site are due to conditions at the planting site or methods used in the restoration, or to natural variations in seagrass between years or wider impacts (e.g. pollution or changes to hydrodynamics).
It is also important to review existing data for the wider study area or proposed restoration site including the presence of other species and habitats of conservation interest. A survey of the area should be conducted (and informed by the data review) to map the proposed site and surrounding area. The purpose of this is to ensure that restoration activities (e.g. access and planting) as well as bed development do not have a negative impact on any existing natural features. Such monitoring should pay particular attention to protected features but also the wider environment.
The greatest depth that seagrass can survive at a site can be calculated according to the extent that the light is absorbed in the water column. This can be described by the extinction coefficient of light (Kd) and is calculated by taking light measurements from several depths to assess how light attenuates through the water column at any given site. By measuring the light continuously during the growing season at a potential restoration site, the average Kd value can be calculated. This provides important information on both light supply and the length of the growing season to inform project viability. As a rule of thumb, seagrass requires at least 20% of the light supply at the surface to survive. Details for calculating the optimal light conditions can be found in Moksnes et al. (2021), chapter 2.5. Huang (2020) found that the average depth of seagrass bed records (including Marine Recorder, GeMS and Seagrass Spotter datasets) was 4 m below chart datum.
It is recommended that light levels are continuously measured at two different depths using loggers that are placed in each potential restoration site.
Biological parameters should also be assessed at the time of the baseline monitoring phase. These can include:
- The presence and % cover of algae and invasive species such as Sargassum muticum (see Biosecurity section).
- Density of lugworm casts (Moksnes et al., (2021) recommend that lugworm abundance >10 m-2 can have negative effects on seedlings and restoration success).
- The condition of seagrass including potential signs of disease (see Seagrass bed distribution and historic extent in Scotland section)
- Presence of predators, e.g. shore crabs, hermit crabs and urchins.
Evaluating the success of a restoration project is essential and closely linked to the specific objectives of the project. A well-designed monitoring programme is therefore necessary to measure the success of the project and to determine if the goals have been achieved. If the desire is to enhance ecosystem function and services, then baseline monitoring must be carried out to assess such parameters and the timescales should reflect those processes. Monitoring should be considered at an early stage of a proposal and fully budgeted for. Early engagement with NatureScot and Marine Scotland will allow time for guidance and advice on seagrass monitoring on a case-by-case basis.
The results of monitoring a failed restoration project can also be valuable. This is important because the causes of the failure may be identified and such problems can be avoided in future projects. Frequent sampling also allows any problems and disruptions to the planting to be quickly detected and possibly remedied before it is too late.
Seagrass bed surveys
Traditional methods for surveying seagrass bed extent in Scotland include diver transects or spot dives (including Seasearch surveying methods), drop down video, glass bucket observations and boundary walk around for intertidal beds. To assess seagrass condition, quadrats can be used to record percent cover and seagrass density. MNCR (Marine Nature Conservation Review) phase II monitoring is more detailed and designed to assess the entire community. Elsewhere, Seagrass-watch is globally recognised as a seagrass surveying protocol.
The Community-led Marine Biodiversity Monitoring Handbook provides a useful guide to marine survey and monitoring techniques in Scotland, including equipment, planning, protocols and quality assurance. This is the recommended resource for baseline monitoring and assessing seagrass restoration attempts. The Feature Focus techniques (Section 3.3 and 3.4) in the handbook outlines basic survey techniques that are relevant for seagrass restoration, but the following section will describe more detailed monitoring requirements.
The range of methods described here can be considered a ‘toolbox’ for monitoring and the exact experimental design for each project will differ depending on expertise and resources. However, it is recommended that a range of techniques should be used to build a picture of the overall ecological changes overtime.
The difficulty with surveying and monitoring seagrass extent is that it is often very patchy, therefore, particularly for subtidal sites but also intertidally, it can be very difficult to determine where the edge of the bed is because there may be another patch further along which is out of sight. Furthermore, calculating an accurate extent over the seabed is almost impossible by swimming or walking around a bed because surveyors would have to walk or swim around every patch. This has led to inaccuracies in assessing change in extent over time because in one year, the surveyors might walk around every small patch and in other years, surveyors might simply walk around the wider extent. Drop down or towed video transects across a bed can be useful in determining the limits of the bed, but there is still likely to be a large error in the estimation of total extent due to patchiness.
Obtaining an accurate measure of seagrass extent is critical in determining success of restoration efforts because the outer edge boundary may not change significantly over the first few years, but there may be a change in patchiness, with the bed becoming thicker and more continuous over time.
Using a combination of satellite imagery and UAV (unmanned aerial vehicles) or drone surveys can offer a good solution to the issues involved in assessing seagrass extent as the entire area can be included and images compared over time. Guidance for using UAVs for marine monitoring has been produced by JNCC (2019). Ongoing work at NatureScot on aerial monitoring of intertidal habitat indicates such an approach to be a promising option, particularly if seagrass can be automatically mapped using computer algorithms to produce polygons, as this removes the bias between individual assessors. Aerial images can be obtained from satellite imagery, plane or drone surveys and also ground-truthed using snorkelers or divers. Figure 14 shows stitched together drone images with snorkelers swimming in a transect along the shore at the same time as the drone was in the air (NRW unpublished data). Snorkelers can hold up a black or white board to indicate a change in habitat and seagrass polygons extracted post-survey. The drone image shows seagrass forming circular patterns on the seabed while seaweed appears in a linear band along the shoreline.
A variety of parameters have been used to evaluate whether restored seagrass efforts have achieved structural and functional goals across the globe. For example, shoot density, percentage coverage, leaf length, biomass, abundance and diversity of epifauna and infauna, light conditions, nutrient content, chlorophyll concentration in the water, etc. have been reported (Fonseca et al., 1998, Short et al., 2000, Orth et al., 2012). Natural capital accounting techniques and indicators could also be considered in the context of seagrass restoration success.
However, the best way to measure success will depend on the aims of the project. For example, is the aim to enhance biodiversity or improve the provision of ecosystem services? Guidance for restoration projects in the USA suggest that evaluations should primarily focus on how plants survive and grow. This is because the seagrass bed structure correlates with ecosystem functions (e.g. sediment stabilisation; Fonseca et al., 1998) and such seagrass bed metrics are relatively simple to collect compared to indicators of ecosystem function. However, the recovery of ecosystem services will be site specific.
The deployment of BoSS enables the monitoring of each plot as a discreet ‘unit’ and if deployed on a line then this can help to relocate the bags. Monitoring should be conducted in the first summer after BoSS deployment to check for successful germination. The site can be monitored by snorkellers / divers up to 12 months after planting to record signs of seagrass growth. Any dives should be undertaken following HSE Diving at Work Regulations 1997 (Approved Code of Practice and guidance for scientific and archaeological diving projects).
Details of each bag’s condition can be recorded alongside the presence (or absence) of seagrass. Other signs of life (such as the presence of crabs or other notable fauna, including epiphytes) should also be recorded. Where seagrass is detected, the number of shoots and the length of the longest leaf should be recorded.
Seagrass growth from seed takes as little as 12 months, but development into mature bed takes many years and the knock-on ecosystem effects may not be measureable for a decade or so. Since seed planting started in 1999, well-developed meadows now exist in the coastal bays of Virginia with the recovery of corresponding ecosystem services (Orth et al., 2020).
A key time point for assessing restoration success is 6 years after planting and suitable control sites should be identified (e.g. degraded seagrass vs healthy seagrass or vegetated vs unvegetated) depending on the survey objectives. Near and far control sites may be required for a robust analysis to determine the impact of restoration (e.g. on ecosystem functions and services). The level of replication required will depend on variability of the parameters being monitored.
It is recommended that monitoring seagrass restoration or enhancement projects in Scottish waters should continue for at least 10 years.
Blue carbon analysis
The usual sampling technique for blue carbon involves taking cores (50 cm x 5 cm volume) using a 70 cm x 5 cm corer. At each site five cores should be taken over a transect through the central portion of the bed from the landward to the seaward extent ay extreme low water. Core samples are frozen for subsequent analysis (and often separated into 5 cm subsamples). Analysis techniques of the sediment cores can include granulometry, particulate organic carbon (POC), loss on ignition (LOI) and carbon dating.
Checklist for site selection:
- Inform NatureScot, Marine Scotland and the Crown Estate Scotland – advice on licensing, biosecurity, policy considerations and site/methods
- Collect background information – environmental variables at the sites, explore existing habitat suitability models, collate historic seagrass records, identify causes of decline
- Contact local community groups
- Visit potential sites
- Select a range of possible sites. The number will depend on the scale of the project. For larger projects, 8-10 potential restoration sites and 4-6 reference beds could be identified initially.
- Spring-time baseline monitoring
- Aerial photography, depth, light, temperature, exposure, salinity, turbidity monitoring over a tidal cycle, granulometry sampling
- Note the presence of drift algae, lugworm casts, shore crabs, INNS and signs of disease
- Autumn baseline monitoring
- Repeat springtime monitoring
- Fish and shellfish surveys
- Process samples and analyse data to assess habitat suitability at the site level, then rule out unsuitable sites and narrow down to 2-4 restoration sites (depending on the scale of the project)
- Ensure permits and licences are in place
Test planting and monitoring plans
Test planting is the best way to check if a site is suitable for restoration and a strong indication of the chance of success. Failure of the seeds to germinate and low survival rates of plants may indicate sub-optimal environmental conditions (e.g. not enough light), while damage to seagrass shoots from predation (e.g. by shore crabs) is relatively easy to identify according to Moksnes et al. (2016).
A monitoring plan should be developed at the start of a project and NatureScot can provide advice on survey planning, data processing and data archive. A possible outline for a seagrass restoration monitoring plan in Scottish waters is outlined below. However, this should be considered as an example as the exact timescales and metrics will depend on the project aims and objectives. The actions described assume that the relevant licencing requirements have been met.
- Data mining and review
- Set up permanent monitoring stations for data loggers – start collecting physical data – e.g. temperature, light, turbidity etc.
- Community and stakeholder engagement
- Seed/flowering observations
- Baseline habitat mapping and species monitoring of the wider area
- Baseline faunal surveys (e.g. Baited Remote Underwater Videos – BRUVS or fish and shellfish counts on a transect)
- Seagrass condition surveys including data on INNS and signs of disease
- Test planting
- Data analysis and write up
- Habitat suitability modelling - exclude unsuitable sites
- Seagrass surveys at test sites – shoot density, % cover, max blade length, aerial surveys
- Clean loggers, change batteries, download data, redeploy
- Seed collection and storage
- Biodiversity and ecosystem function surveys
- INNS surveys
- Refine sites and plant seeds at test sites
- Community engagement
- Repeat monitoring surveys at the same time of year to minimise seasonal variation
- Assess success parameters and carry out investigation into failed sites if necessary.
- Scale up restoration efforts if appropriate
Year 6 and year 10
- Repeat monitoring surveys at the same time of year as previous surveys and assess long-term success and wider benefits.
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Case Study – Seawilding seagrass restoration in Loch Craignish
Authors: Katherine Knight, Eric Holden, Will Goudy
During the summer of 2021, the first seagrass restoration project in Scotland was initiated by the community of Loch Craignish on the west coast. The lead partner of this project is Seawilding with Project Seagrass providing technical support, and the Scottish Association of Marine Sciences (SAMS) conducting research and environmental monitoring to assess changes in biodiversity over time. This case study provides an example of the techniques that can be used for a seagrass restoration project and at the time of writing this project is well underway, although not at the full planting stage.
The local community have been surveying Loch Craignish since 2020 using methods from the Community-led marine biodiversity monitoring handbook. Twenty-five meter transect lines with quadrats every two meters have been placed over the seagrass beds to record seagrass coverage. Biodiversity abundance and richness of macro flora and fauna have been recorded along the transects using the SACFOR scale. The surveys were undertaken by snorkel at or near low water depending on seagrass depth. Baited remote underwater video cameras (BRUVs) were deployed to assess the seagrass bed nursery function. Project Seagrass has carried out echosounder surveys of the Dunvullaig Bay bed. SAMS are using traditional methods, such as sediment coring, alongside eDNA to track the health of the seabed over time.
The seagrass beds were mapped by paddle boarders using GPS. The use of paddle boards at low water and in calm conditions provides excellent visibility of the seagrass. Mapping seagrass is carried out in pairs where the lead paddle boarder acts as a guide along the seagrass boundary and the second paddle boarder follows to record the track using a GPS enabled smart phone or watch. In deeper water the lead paddle boarder may need to use a snorkel mask or occasionally enter the water for a better view of the seagrass boundary. Dive surveys were also carried out and have shown to be efficient, but ground truthing via paddle board or snorkel was still required.
Seed collection and processing
Seeds were harvested by hand between the beginning of August and the end of September 2021. Spathes containing approximately 200,000 seeds were collected by snorkelers from the largest beds in Dunvullaig Bay. These were then transferred and held in the on-site processing tanks, allowing the organic matter of the spathes to break down, releasing the seeds for collection and replanting. Throughout this process the seed processing tanks were tended to by members of the Seawilding team who monitored the process and protected the health of the seeds with regular water changes.
Born out of the desire of community members to protect the health of the loch, the community is the heart of this project. Volunteers from the local community and beyond have taken part in all stages of the project from seed collection and processing to bagging and planting. Along with the aim to restore seagrass to the loch, the project also aims to engage people with the ocean on their doorstep. Community seagrass restoration provides an opportunity to enhance ocean literacy through education on the role of seagrass in enhancing ocean ecosystems and promoting carbon storage. An open day was held over a weekend in August (21st to 22nd) during which approximately 60 people engaged in seagrass restoration activities. Those who attended had the opportunity to snorkel across the seagrass bed, soaking up the wonder of this unique habitat as well as the chance to learn about the restoration process and have a go at seed harvesting, bagging, and planting.
Sites for restoration were selected through a collaborative process between Seawilding, Project Seagrass and SAMS. Existing seagrass bed polygons were overlaid onto charts to determine the preferred depth of the habitat and substrate type. Other factors included in the decision making included access from the shore, natural protection along the coastline from islands, skerries and outcrops as well as local activities.
The first three 5 m x 5 m test plots were planted over the weekend of the 25th and 26th August 2021 to trial direct planting of seeds without putting them through the processing unit. If shown to be successful, this method will provide a low cost method of seagrass restoration which can be used by other community groups. Seeds were planted in hessian bags, approximately 25 cm apart, and buried into the sediment by snorkelers.
Following processing, the majority of the remaining seeds will also be planted in hessian bags, on 100 m lines with bags placed every 1 m.