Handbook leads the way for large-scale restoration of eelgrass
Eelgrass planting has advanced from experiment to success. Even though swans and crabs can cause problems, the method is now established for large-scale restoration in both the Skagerrak, Kattegat, and the Baltic Sea. This is shown in a new handbook from researchers at the University of Gothenburg.
Eelgrass meadows are some of the most valuable ecosystems in the sea. They serve as nurseries for fish, bind carbon, and counteract erosion. However, in recent decades, large areas of eelgrass have disappeared in Sweden, especially along the west coast. Restoration has therefore become an important measure to preserve biodiversity and mitigate the effects of human activities.
The Zorro research group at the University of Gothenburg has played a central role in developing methods for eelgrass restoration. On behalf of the Swedish Agency for Marine and Water Management, they have now, together with the Västra Götaland County Administrative Board, produced an updated version of the 2016 Handbook on Eelgrass Restoration.
“Since the first handbook was published, we’ve learned incredibly much. Over the past eight years, eelgrass restoration has developed from small-scale, experimental trials to an established and effective method for large-scale restoration,” says Louise Eriander, researcher at the Department of Marine Sciences.
Crabs cause problems
The researchers have encountered several relatively unexpected problems along the way. One example is swans, which naturally graze on eelgrass, but have now been found to damage newly planted shoots in sparse plantings. The solution has been to plant the shoots at a depth of at least 1.5 metres – out of reach of the swans.
Another unexpected problem encountered by researchers is the high density of green shore crabs. These small but powerful animals have proven to be surprisingly destructive. A prominent example is a large eelgrass planting carried out in Kosterfjorden in 2020. Despite careful preparations, almost the entire planting was lost – the shoots had been snipped off, and there is much to suggest that the crabs were behind the damage.
“It seems that the crabs attack the eelgrass shoots and that the damage increases in the autumn when food becomes scarce. We don't know why. Analyses of stomach contents show that they can eat the shoots, but above all they seem to be provoked and snap off the shoots,” says Louise Eriander.
To reduce the problem with crabs, the researchers are testing a method where crabs are removed from areas where the eelgrass is about to be planted. Researchers have also tested planting eelgrass in denser groups, which has shown positive results.
Great international interest
An important conclusion in the report is that genetic variation and how different meadows are linked through the spread of shoots and seeds, must be taken into account in eelgrass management and restoration. Some meadows have a high dispersal capacity and can contribute to regrowth in large areas, while others are more isolated and therefore extra vulnerable.
“By selecting donor meadows with high genetic variation, we can increase the chances of eelgrass establishing itself and spreading to new areas,” says Louise Eriander.
Sand capping of the seafloor has proven to be a promising method for improving the growth conditions for eelgrass. It’s an expensive method, but it has proven effective in areas where historical eelgrass losses have led to turbid water that prevents recolonisation. In a restoration project at Lilla Askerön outside Stenungsund, large-scale sand capping has been a prerequisite for improved light conditions and a dramatic increase in eelgrass growth. There, the number of eelgrass shoots has increased 26-fold after four growing seasons.
“However, it’s important to remember that even though we now have effective methods for restoring eelgrass meadows, it’s always much better to protect eelgrass meadows in the first place,” says Louise Eriander.
There is considerable international interest in the restoration of eelgrass meadows. The handbook is therefore being translated into English so that projects in other countries can benefit from the lessons learned in Sweden.
Text: Karl-Johan Nylén
The Handbook for Restoration of Eelgrass in Sweden is an update of the handbook of the same name published in 2016. The report presents results from 59 plantings at 37 different locations in the Skagerrak, Kattegat, and the Baltic Sea, between 2016 and 2024.
The report was produced by researchers at the University of Gothenburg and the County Administrative Board of Västra Götaland on behalf of the Swedish Agency for Marine and Water Management.
