Eelgrass

Eelgrass (Zostera marina) is an underwater plant with stems that are 50-100 centimeters long. It grows in shallow areas of fjords and marine regions in fine gravel, sand, or mud, and when plants grow closely together they form underwater forests (eelgrass beds). Eelgrass and other seagrasses create the sea meadows, which play a key role in the ecology of coastal waters. They have high productivity and store carbon and nutrients in the seabed. A variety of small animals and fish live in eelgrass beds. The plants also create a natural coastal barrier by dampening wave impact and stabilizing the seabed. Additionally, they increase the deposition of particles, helping to keep the water clear, which benefits their own growth and extent. Denmark’s extensive shallow coastal waters with soft, sandy bottoms, moderate salinity, and typical summer temperatures around 20 degrees offer exceptionally favorable conditions for eelgrass.

Eelgrass plays an important role in shallow marine areas. Eelgrass beds have a large biomass and retain and store carbon and nutrients in the seabed. In areas with seagrass, the organic matter in the sediment is dominated by slowly degradable plant material, which provides a foundation for a food web with many benthic species, many of which have long lifespans. Eelgrass beds thus function as nursery and living areas for fish and benthic animals, creating the foundation for a rich community of animals and plants with high biodiversity.

Additionally, eelgrass acts as a natural coastal barrier because the leaves dampen the movement of waves. As the plants move with the water’s motion in the surf zone and bend with the current, they also slow the water flow over the seabed, reducing sediment erosion.

Eelgrass binds nutrients and carbon in the plant biomass, thus reducing their content in the water. The breakdown of the living plant and the formation of dead organic material occur slowly and are therefore temporally displaced from production. The readily available nutrients are thus removed from the cycle dominated by plankton algae, where production and decomposition are closely linked in time, and where nutrients are continually made available again. The plankton cycle, therefore, becomes less significant. While perennial seagrasses do not immediately remove nutrients, they can delay part of the release to times of the year when light and other factors, rather than nutrients, determine plankton growth. Furthermore, nutrients and carbon are bound in large amounts as the slowly degradable dead plant material accumulates in the fjord bed.

A hundred years ago, there were dense eelgrass meadows throughout the inner Danish waters, from very shallow water down to depths of 5-6 meters in the fjords and all the way down to 8-10 meters in the clear waters of the Kattegat and the Belt Sea (though only sporadically along the west coast of Jutland). However, in the early 1930s, a fungal disease killed around 90% of the eelgrass, and though some of the meadows were restored during the 1950s and 60s, they did not fully reach their former extent. From the early 1950s and for the next 30 years, the eutrophication of coastal waters increased and phytoplankton bloomed, making the water less clear and shading the eelgrass. As a result, the distribution of eelgrass has been significantly reduced, and today it only grows as scattered patches in narrow belts in shallow water.

In many fjords and coastal areas, eelgrass vegetation has been reduced to only a few percent coverage and no longer forms large and dense underwater meadows. Natural reestablishment of the original meadows from the current scattered growth is not realistic within the next 100 years, as many processes, including physical stress, and sandworms hinder the spread of eelgrass to such an extent that natural recolonization on a large scale is not possible.

Reestablishment of smaller eelgrass meadows through transplantation of eelgrass shoots harvested from still existing donor beds is a possibility to "assist" natural recolonization. From the reestablished eelgrass meadows, colonization by seed dispersal and subsequent growth of new shoots would ensure the actual large-scale reestablishment.

Therefore, plantings have been initiated along various parts of the Danish coasts to actively assist eelgrass recolonization. SDU has developed a transplantation program that holds potential for eelgrass restoration in Danish fjords. However, challenges remain in eutrophicated areas, where eelgrass is quickly overgrown by epiphytes. This clearly shows that a prerequisite for sustainably reestablishing larger eelgrass areas is a significant reduction in nutrient input to the marine environment.

Transplanting eelgrass shoots is a labor-intensive process which limits the size of transplanted areas. However, the existing method is recommended if the whish is to secure seed-producing meadows that can subsequently lead to natural reestablishment as environmental conditions improve. It would, however, be beneficial to develop new transplantation techniques that are more efficient.