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Carbon capture, storage and utilisation


First large scale transplantation of eelgrass

26. March 2024

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Eelgrass has unfortunately disappeared from the seabed along the coast in many places. But eelgrass is essential for biodiversity as it creates living places for several species. Eelgrass also has a great potential to improve water quality and capture CO2. A new project shows, that large scale transplantation of eelgrass can help bringing it back.

Facts about eelgrass

Eelgrass is one of the most important plants in the inner Danish waters, serving various functions. It counteracts coastal erosion, provides habitat for many species, and is a crucial tool against climate change. Eelgrass can quickly absorb large amounts of CO2, up to 35 times faster than tropical rainforests. In this way, eelgrass is the ocean’s own super tool for capturing and storing carbon. Additionally, the plant also binds nutrients from the water, keeping it clear.


Source: Nature Description, WWF.


WSP attempts a new method, where seagrass cuttings are planted on coconut mats underwater to test if entire mats can be moved to another location without causing the seagrass to perish. If successful, this could make it much easier in the future to replant seagrass along the Danish coasts.

In collaboration with WSP and HedeDanmark, By & Havn has initiated Denmark’s, and perhaps the world’s, largest eelgrass experiment at Lynetteholm in the harbor of Copenhagen. The project aims to investigate the conditions and possibilities for spreading eelgrass along the coasts in the future. In the initial trial, eelgrass is being harvested from the seabed and transferred to the new canal at Trælastholmen in Nordhavn. If successful, the canal could become a home for flourishing eelgrass, benefiting both the marine environment and underwater biodiversity.

Large scale transplantation

Until now, planting eelgrass has been done with individual shoots, and it is extremely time- and resource-intensive. But if this project is successful, we can conduct seagrass transplantations on a large scale in the future, making it possible to restore habitats and improve water quality to a significantly higher degree than before.

Isolated events such as the establishment of a new beach, burying a gas pipeline, or constructing a tunnel underwater can make life difficult for eelgrass locally, and here, a seagrass transplantation can make a difference.

More eelgrasses not only improve water quality but also serves as a crucial habitat for many animals, micro- and macroalgae, contributing to restoring balance in the marine environment.

Soon, the new canal at Trælastholmen may be filled with eelgrass sourced from the seabed east of Trekroner in Copenhagen Harbor, where the construction work for Lynetteholm Phase 2 begun in October 2023. The relocation of seagrass is an unusual experiment conducted by By & Havn in collaboration with WSP as advisor and HedeDanmark as entrepreneur. The purpose is to secure and store the eelgrass that would otherwise perish during the construction of Lynetteholm, so it can later be planted in new areas. If the experiment succeeds, it is estimated that in the future, it will significantly ease the process of spreading eelgrass to other regions.

By & Havn had prepared the new canal at Trælastholmen to house the eelgrass from the area near Trekroner.


A Canal Filled with Seagrass

If the experiment goes as experts hope and expect the new canal at Trælastholmen will soon be teeming with eelgrass, and subsequently, individual coconut mats with eelgrass will be removed and relocated to new planting areas. It is expected that the remaining eelgrass in the canal will spread again over time, providing ongoing opportunities to harvest new supplies of the beneficial underwater grass. This way, a completely new solution to a well-known problem will be bound.