Vanishing eelgrass

An underwater regime change has been in progress along the Bohuslän coast for several years now. Ecosystems are undergoing fundamental changes, and the eelgrass meadows that provide a breeding ground for cod are gradually being replaced by algal mats in many places.
“Up to 150 km2 of eelgrass has disappeared,” says Per-Olav Moksnes, a researcher at the Department of Marine Sciences.

The marine flowering plant eelgrass, or glass wrack, grows on shallow soft bottoms from the west coast of Sweden all the way up to the Sea of Åland, and is often referred to as the ‘nursery of the sea’.

“This is because of cod, for example,” explains Per-Olav. “Eelgrass meadows are an important breeding habitat for cod. They are rich in food in the form of small predatory fish and crustaceans, and they also provide protection.”

In addition to providing breeding grounds, eelgrass also carries out a number of other so-called ecosystem services. For example, it absorbs nutritive salts, thereby reducing eutrophication, and dampens waves, reducing erosion and the build-up of sediment in the water. Eelgrass also absorbs carbon, helping to counter the greenhouse effect and ultimately climate change.

In recent decades, there has been a dramatic reduction in the amount of eelgrass along Sweden’s coastlines. No one knows for certain how much has actually disappeared, as no overall inventorying has ever been carried out, but Per-Olav says it could be in the region of 80-150 km2. Commonly cited reasons for this reduction include eutrophication, overfishing and the construction of harbours and bridges.

“Overfishing leads to what is known as a trophic chain reaction. When the cod disappear, there will be more small fish, crabs and prawns that the cod eat left in the water. These creatures in turn eat the small creatures that eat algae, leading to algae growing too much, blocking out the light and ultimately choking the eelgrass.”

The Swedish Agency for Marine and Water Management is currently investigating whether restoration can be used to reduce the great loss of eelgrass. In recent years, there have been several cases where the Environmental Court has ruled that eelgrass meadows destroyed during harbour building works, for example, should be compensated for by transplanting eelgrass to new areas. Until recently, one obstacle to this type of measure has been a lack of restoration methods for Scandinavian areas. During the last few years, Per-Olav’s research team has developed methods that work in Swedish waters, but he also notes that eelgrass restoration is both costly and difficult.

“In practice, the eelgrass has to be planted shoot by shoot. And it can’t currently be restored in all areas. In places where large eelgrass meadows have been lost, a ‘regime change’ is thought to have occurred whereby the build-up of sediment and drifting algal mats prevent the natural re-establishment and restoration of eelgrass, despite eutrophication in the area having been reduced. It is therefore incredibly important to protect the remaining eelgrass meadows in Bohuslän.”

Since 2011, Per Olav has worked together with environmental lawyer Lena Gipperth at the University of Gothenburg to coordinate the multidisciplinary Zorro research programme, which aims to develop methods for restoring eelgrass ecosystems and to draw up better regulations for the protection and management of shallow coast ecosystems in Sweden.

“We are now in the final stage of the work to produce a handbook to support eelgrass management. The real challenge for the future is to identify methods for getting the ecosystem back on track, and to understand how we can bring the cod and the eelgrass back.”

Find out more

www.gu.se/zorro