Throughout the world, there has been a massive decline in the prevalence of the sea grass meadows, and this has made them some of the planet's most endangered ecosystems. Eel grass is the most widespread sea grass in the northern hemisphere, and previously it covered very large areas in Danish fjords, belts and the Kattegat. A Danish research team has now carried out an analysis of unique Danish data, and this reveals that the last 100 years have been critical for eel grass.

Sea grasses on the seabed need light to grow – just like plants on land. But over the years, discharges of nutrients from the land have caused eutrophication and unclear seawater, which obstructs sunlight from reaching as deep into the sea as it could before. This is an important reason for the reduced spread of Danish eel grass meadows. However, despite the reductions in nutrient discharges in recent decades, the eel grass has not yet recovered nationally.

The authors of the article, Century-long records reveal shifting challenges to seagrass recovery,which has just been published in the journal Global Change Biology, are leading researchers from Aarhus University and the University of Copenhagen. They have reviewed the extensive Danish data and demonstrated the negative impact of eutrophication, but the study also indicates that bottom trawling, for example for mussels, along with sea warming, have also hampered the prevalence and living conditions of seagrass.

Blue forests

The data behind the researchers' analysis stretches back to before the year 1900, and covers Danish coastal areas where, at one time, there were up to 6,700 km2 of eel grass meadows, forming a kilometre-wide belt around Danish coasts. Today, it is estimated that this area has dwindled to less than a third, while there is an increasing awareness of the positive ecological benefits of eel grass and the world's other seagrasses for the marine environment and the climate.

Species of grass have a wide range of functions in the natural world, where they act as wave dampers and thus constitute natural coastal defences. They are also important breeding grounds for a number of species of fish with both ecological and commercial value. They absorb carbon and nutrients, much of which becomes buried in the seabed beneath the meadows.

"Seagrass species can be compared with the forests on land, which also contribute to biodiversity and at the same time constitute a carbon sink. You could call the seagrass meadows blue forests, which make positive contributions to biodiversity, the marine environment and the climate. Our study suggests that the meadows are under pressure on several fronts, and this calls for better management," explains Professor Dorte Krause-Jensen from the Department of Bioscience, who is one of the Danish researchers behind the project.

More pressure factors

Among the group’s findings is that, at around the turn of the 20th century, in many places along open coasts, eel grass could grow down to depths of 10-11 metres, while deep populations are now a rarity that only occur in the most undisturbed areas. The study shows that now the meadows grow at an average depth 3.4 metres shallower than the historical populations.

While unclear water is part of the explanation, eel grass is also scarce at depths where there is enough light to encourage growth. The study indicates that bottom trawling is a contributory factor to the absence of the grass. If the meadows are pressed towards shallower water, they also become challenged by rising sea temperatures, explains Dorte Krause-Jensen.

"We’ve demonstrated a correlation between several factors that play a negative role in the spread of eel grass. Overall, this means that protecting and restoring the eel grass meadows will require continued reductions in discharges of nutrients, and it means that the combination of several other stress factors also restricts re-establishment of the lost meadows. This underlines the importance of a holistic approach to managing eel grass meadows in the form of regulation of both eutrophication and trawling at local level and meeting global climate goals."

Contact:

Professor Dorte Krause-Jensen