Eelgrass restoration is the process of intentionally restoring or creating new eelgrass meadows – a key coastal habitat for biological diversity that provides a wide range of ecosystem services.
Coastal ecosystem before NbS have been implemented
Coastal ecosystem after NbS have been implemented
Eelgrass restoration should be applied in areas where:
Previous meadows are lost, or their ecological state is reduced.
In areas with low biodiversity and reduced ecological functions due to human impact, such as in urban areas.
Eelgrass meadows need shelter from waves and soft sediment, and sufficient light. If possible, areas where eelgrass has previously been documented, should be selected for restoration, as these areas are likely to have favorable conditions.
Shelter and suitable substrate can be provided applying a landscape approach in forming the restoration site. Sufficient light will depend on the depth of the restoration sight. Suitable depth depends on the surroundings as well as water quality. In urban areas, high houses; and in the wild, steep mountains, provide shadow and minimal light hitting the sea water surface. Turbidity on the other hand reduces underwater light availability.
Eelgrass meadows are found in shallow coastal areas throughout the Nordic region. Thus, eelgrass restoration could be a relevant NbS in many places along the Nordic coasts. Loss of eelgrass meadows has been documented in Denmark, Sweden and Norway. Even though eelgrass has been known to occur in a certain area the habitat quality may have been reduced to such an extent that eelgrass cannot grow here anymore. In such cases, the causes of the eelgrass loss need to be mediated before any restoration action can be successfully implemented (e.g. reducing the nutrient discharges if eutrophication is an important driver of habitat loss).
Eelgrass meadows provide habitat supporting a range of species, including juvenile cod and other commercially important species. They also serve as popular fishing sites for recreational and tourist fishing. Thus, eelgrass meadows restoration is important for biodiversity enhancement, food security and economic development. Furthermore, these ecosystems are natural hotspots for carbon sequestration, and they therefore play a role in climate-change mitigation. They also play a role in climate-change adaptation as they reduce coastal erosion and reduce resuspension of sediment caused by currents and waves. Eelgrass meadows clean water by plants uptake of nitrogen and phosphate from the water and the substrate, thus mitigating eutrophication, and reducing the growth of opportunistic macroalgae and phytoplankton, which is beneficial for a range of ecosystem services, including economic development and human health and well-being. They may also play a role in reducing pathogens in seawater, another human health and well-being effect. Eelgrass meadows function as an important indicator species of eutrophication in Denmark and Sweden (i.e., in clear waters with low nutrient concentrations they grow deeper and are more abundant, compared to more turbid and nutrient-rich ecosystems), and is thus relevant for water management.
Eelgrass restoration requires careful planning, execution and monitoring, preferably using an adaptive approach that includes the opportunities for trial and error. The stepwise process:
1) assessing site suitability
2) determining restoration methods
3) conducting pilot restoration: if pilot restoration is not successful, then site-suitability and methods could be assessed again, and a new pilot study carried out.
4) full scale restoration
5) evaluating results
Restoration methods
Passive eelgrass restoration entails modifying site conditions to allow for natural regrowth or expansion of eelgrass. This can be carried out by:
Removing debris (i.e., clearing the area of any debris that may hinder eelgrass growth or establishment)
Modifying the substrate (e. g. removing excess sediment or adding sediment to create appropriate depth). Using local sand has been shown to result in greater survival than artificial sand
Reducing/removing other threats to eelgrass such as high nutrient levels (eutrophication), physical disturbances and marine wildlife (e.g., crabs).
Active eelgrass restoration refers to the use of an active transplant method. There are two main methods to active restoration – transplanting shoots and using harvested seeds. See below for a description:
Transplanting shoots with intact sediment cores or transplanting shoots with bare roots and rhizomes, with and without anchoring, are the most common transplanting techniques. Transplanting shoots is the recommended approach in the Nordics. This approach is the focus of a guide to eelgrass restoration for Oslofjorden. It has also been recommended for large-scale restoration along the Swedish NW coast. A focus on transplantation-based restoration has also been encouraged for Danish estuaries.
Planting seeds can be an effective method. Seed-based approaches can enable large-scale restoration. It is costly, however, and obtaining seeds that can be planted along with the planting itself is elaborate. Though a seed-based approach can be successful, heavy seed losses have been reported along the Swedish NW coast and in Danish Estuaries. Seed-based methods are discouraged in national eelgrass restoration guidelines for Sweden, based on based on the high costs and risks of failure using seeds compared to the single-shoot method.
Restoration in urban areas
In urban areas, the landscape and substrate must be modified to allow the eelgrass plants to live, if suitable habitats do not exist. This includes creating shelter for waves, providing soft sediment that is minimum 20 cm thick, and a flat, shallow terrain, formed in a manner that prevents the sediment from being lost. After the landscape is sufficiently modified, both passive and active restoration can be applied in the existing or facilitated habitat to populate the meadow.
Documented positive outcomes of eelgrass restoration include:
Climate mitigation and reduced eutrophication through carbon and nitrogen sequestration. Scientists have explored the changes in carbon and nitrogen content in the sediment when an eelgrass meadow is lost. They found high levels of carbon and nitrogen in Swedish eelgrass sediments down to > 100 cm depth confirming the role of eelgrass meadows as carbon and nutrient sinks. Loss of eelgrass meadows drastically reduced carbon and nitrogen content. Wave exposure was found to reduce the storage capacity.
Enhanced biodiversity by restoring a key habitat for a range of marine species. In a a recent study, scientists followed faunal colonization of a restored site in southwestern Sweden. They document rapid faunal colonization after 3 months of eelgrass transplantation which equates to one growing season and found that the faunal community was fully restored after 15 months (i.e., two growing seasons). Similarly, in another study, rapid restoration of faunal communities was also found following eelgrass meadow transplantation in Denmark and substantially higher species richness, abundance and biomass in transplanted meadows compared with bare bottom areas.
Coastal protection through reduced wave attenuation and erosion. Scientists have looked at how eelgrass meadows in Skåne, southern Sweden, a region with problems with coastal erosion, attenuate waves. They found that wave heights were reduced by 35-40% in dense eelgrass meadows during storms compared with 5-10% in sparsely vegetated areas.
The greatest risk associated with eelgrass restoration is the prospect of restoration failure and thus a lack of return of investment. Failure is often attributed to environmental conditions often cited as the primary cause. Factors such as water quality, hydrodynamics, algal blooms, sedimentation, and light are found to be particularly important. On the other hand, unsuccessful attempts provide valuable knowledge that can be used to improve methods and future success rate.
Protecting eelgrass meadows should always be the first priority as restoring the ecosystems is costly and difficult, as per the mitigation hierarchy (see above). Restoring eelgrass meadows is demanding and has had a low success rate. A key to succeed is to have a good understanding of the causes of their disappearance, implementing mitigation actions to reduce the negative drivers, and to have a holistic approach in the restoration process that include both biotic and abiotic factors influence on the survival and growth of the eelgrass as well as on its associated diversity.
1) Identify why eelgrass meadows have been lost in the first place, where they used to occur and whether the threats have been removed or reduced. Optimally, threats such as eutrophication must be mitigated prior to restoration. If the habitat has changed substantially (e.g., the sediment is destabilized or large abundances of drifting algae are present) it might be difficult to restore the eelgrass meadow.
2) Select areas with suitable habitat for eelgrass. Environmental conditions should be within acceptable ranges for eelgrass in terms of depth, light and water quality, current speed, erosion rate, wave attenuation, sediment quality and biological disturbance from for instance sandworms (Arenicola marina), shore crabs (Carcinus maenas), seabirds, and drift algae.
3) Avoid restoring areas where eelgrass meadows are likely to come back on their own. A rule of thumb is to not restore areas closer than 100 meters from a healthy eelgrass meadow.
4) Full-scale restoration should only be carried out in areas where test planting shows positive growth after one year and where other variables indicate good conditions for long-term regrowth of eelgrass. If the plants survive in several sites, the sites with the highest growth rate should be selected for full-scale restoration.
5) Careful coastal planning and management is needed to protect existing and restored eelgrass meadows.
Shading from docks and marinas have been shown to pose a threat to eelgrass meadows along the NW coast of Sweden. In the same area, studies have also found that eelgrass habitats are rarely assessed or considered when planning and deciding to construct docks. Presence of protected areas is also not necessarily a guarantee for the protection of eelgrass meadows from infrastructure development.
Coastal planners should also be conscious about potential negative effects of creating artificial beaches on the restoration potential of eelgrass meadows. Research has shown how artificial sand from a nearby beach was transported to deeper waters and thus potentially limited the extent of the eelgrass meadow by burying plants.
A key concern should be to avoid carbon and nitrogen emissions from eelgrass meadow loss as this is negative for climate mitigation and can contribute to eutrophication.
In terms of aquaculture impacts on eelgrass, a recent review found that shellfish aquaculture can have positive, neutral, and negative effects on eelgrass. Positive interactions include improved water quality and nutrient provision, while negative effects like shading and sedimentation are more common near shellfish farms. In contrast, limited research on finfish aquaculture's effects on eelgrass in temperate regions exists, with clear negative interactions reported in Mediterranean seagrass species studies.
6) Spread risk by restoring multiple sites over multiple years. Large natural interannual variation in growth and distribution of eelgrass meadows occurs and random events such as heavy storms, sea ice, freshwater discharges, algal blooms and high summer temperatures can result in low survival of transplanted eelgrass. Restoration may be more successful when planting occurs at multiple sites over multiple years.
7) Restoring large areas is better than restoring small areas. Large eelgrass meadows function as "ecosystem engineers," transforming the seabed's hydrodynamics and biogeochemistry. Once a restored eelgrass meadow reaches a critical size, it establishes a self-reinforcing effect, stabilizing the seabed, enhancing water quality, and promoting favorable conditions for its own and other species' growth.
Research has assessed whether small-scale eelgrass restoration could be facilitated by the use of biodegradable establishment structures (BESEs) that stabilize sediments. It has been shown that using BESEs increased early eelgrass survival and the success of small-scale eelgrass (and bivalve) restoration across case studies in northern Europe (i.e., Denmark, Finland, Norway and Estonia). Hence, small scale efforts to stabilize the sediments has the potential to increase the success rate of eelgrass restoration.
Eelgrass is the most studied seagrass species globally. The number of studies testing the efficiency of restoration methods at higher latitudes has increased in recent years and several guidelines for planners and managers have been developed. Thus, compared with many other marine nature-based solutions, eelgrass restoration has come a long way and can be readily implemented in coastal planning and management.
Eelgrass meadow loss is costly. For instance, one study report that the ecosystem services provided by seagrass meadows has an economic value of 34,000 per hectare per year US$, while another found that historic eelgrass meadow loss on the Swedish NW coast and the subsequent carbon and nitrogen release had an estimated cost to society of app. 8 000 US$ per hectare and 140 000 US$ per hectare, respectively.
Eelgrass meadow restoration costs. Restoring one hectare of eelgrass using recommended methods in Sweden typically costs between 1.2 and 2.5 million SEK. This estimate includes expenses for site selection, as well as a decade of monitoring costs, which remain constant per year regardless of project size. The costs of harvesting and planting vary directly with meadow size and shoot density. Should anchoring techniques be necessary, planting costs may double.
Location: Oslo Fjord
Which ecosystem type(s): Coast, fjord ecosystem
Title/ name of the NbS: Eelgrass meadow restoration
Contact: Kristina Kvile and Eli Rinde NIVA and Oslo municipality
Relevant links:
Short summary of the project: Vil få tilbake livet i fjorden med restaurering av ålegrasenger (klimaoslo.no)
Report on the potential for eelgrass restoration in the Oslo Fjord Potensialet for restaurering av ålegrasenger i Oslofjorden (klimaoslo.no)
Guide to eelgrass restauration tailored to the Oslo Fjord Restaurering av ålegrasenger. En praktisk veileder utviklet for Oslo kommune. (unit.no)
Location: Swedish coastal ecosystems
Contact: Louise Eriander Louise Eriander | University of Gothenburg (gu.se), Eudardo Infantes Eduardo Infantes | Researcher at the University of Gothenburg, Per-Olav Moksnes https://www.gu.se/en/about/find-staff/permoksnes
Relevant links: https://www.havochvatten.se/en/our-organization/publications/swam-publications/2021-03-16-handbook-for-restoration-of-eelgrass-in-sweden.html
Location: Horsens Fjord
Which: cosystem type(s): Coast, fjord ecosystem
Title/ name of the NbS: Eelgrass meadow restauration
Contact: Mogens Flindt Mogens Flindt - SDU, Rune Steinfurth rune-steinfurth - SDU
Relevant links: Finally, the eelgrass is coming back - SDU
Further reading (eelgrass meadow restoration guidelines):
Gundersen, Hege (Niva), Tanya (GRID-Arendal) Bryan, Wenting (Niva) Chen, Frithjof E. (Imr) Moy, Antonia N. (AquaBiota) Sandman, Göran (AquaBiota) Sundblad, Susi (Niva) Schneider, Jesper H. (Niva) Andersen, Sindre (Niva) Langaas, and Mats G. (Niva) Walday. 2017. Ecosystem Services. Nordic Council of Ministers.
Infantes, Eduardo, Eli Rinde, and Kristina Øie Kvile. 2022. Restaurering av ålegrasenger. En praktisk veileder utviklet for Oslo kommune. NIVA Report. 7693–2022.
Moksnes, Per-Olav, Lena Gippert, Louise Eriander, Kristjan Laas, Scott Cole, and Eduardo Infantes. 2021. Handbook for Restoration of Eelgrass in Sweden - National Guideline. 5. Swedish Agency for Marine and Water Management.
Rinde, Eli, and Elin T. Sørensen. 2023. Manual for villgjøring av urbane sjøområder - Miljødirektoratet. Urbant HAV, NIVA.
Eeelgrass meadows distribution:
