At last: Mysterious ocean circles in the Baltic Ocean explained

January 30, 2014
This shows eelgrass rings growing in shallow water off the cliffs of island Mon in Denmark. Credit: Jacob T. Johansen

Are they bomb craters from World War II? Are they landing marks for aliens? Since the first images of the mysterious ocean circles off the Baltic coast of Denmark were taken in 2008, people have tried to find an explanation. Now researchers from the University of Southern Denmark and University of Copenhagen finally present a scientific explanation.

The first pictures appeared in 2008, taken by a tourist and showing some strange circular formations in the shallow waters off the famous white cliffs of chalk on the island Møn in Denmark. In 2011, the circles came back, and this time there were so many that they made it to the media.

Investigating biologists then concluded that the circles consisted of eelgrass growing on the bottom of the shallow water. But only now scientists can explain why the eelgrass grows in circles here – eelgrass usually grows as continuous meadows on the seabed.

"It has nothing to do with either bomb craters or landing marks for aliens. Nor with fairies, who in the old days got the blame for similar phenomena on land, the fairy rings in lawns being a well known example", say biologists Marianne Holmer from University of Southern Denmark and Jens Borum from University of Copenhagen.

The circles of eelgrass can be up to 15 meters in diameter and their rim consists of lush green eelgrass plants. Inside the circle there can be seen only very weak or no eelgrass plants.

This is an eelgrass plant growing off the cliffs of Island Mon in Denmark. The plants create a circle formation. Credit: Ole Pedersen

"We have studied the mud that accumulates among the eelgrass plants and we can see that the mud contains a substance that is toxic to eelgrass", explain Holmer and Borum.

The poison is , a substance that accumulates in the seabed off the island of Møn, because it is very calcareous and iron-deficient.

"Most mud gets washed away from the barren, chalky seabed, but like trees trap soil on an exposed hillside, eelgrass plants trap the mud. And therefore there will be a high concentrations of sulfide-rich mud among the eelgrass plants," explain the researchers.

Sulfide is toxic enough to weaken the old and new eelgrass plants but not toxic enough to harm adult and strong plants. And since eelgrass spreads radially from the inside out the oldest and weakest plants are located in the center of the growth circle.

Jens Borum and Marianne Holmer say: "Eelgrass populations grow vegetatively by stolons which spread radially in all directions and therefore each plant creates a circular growth pattern. When the sulfide begins to work, it starts with the oldest and thus the inner part of the population because here is an increased release of toxic sulfide and uptake by plants due to accumulation of mud. The result is an exceptional circular shape, where only the rim of the circle survives – like fairy rings in a lawn".

The waters off Møn's chalk cliffs are not the only place where sulfide destroys eelgrass. Sulfide poisoning of eelgrass is a major problem worldwide. Sulfide is often created where oxygen disappears from the seabed. This can happen when the seabed is fed nutrients from agriculture.

Underwater meadows of eelgrass and other seagrasses grow in many parts of the world where they serve as home to a variety of small animals, filter the water and trap carbon and nutrients. But the meadows are threatened in almost all regions of the world, and in several places, including Denmark, researchers and authorities work to prevent seagrasses from disappearing.

Explore further: Global warming may impede eelgrass growth

More information: Eelgrass fairy rings: sulfide as inhibiting agent . Borum , Holmer , et al. Mar. Biol. Published online 12 October 2013.

Related Stories

Global warming may impede eelgrass growth

July 26, 2009

Scientist Ron Thom probably knows more than anyone else about the growth of eelgrass, the humble marine plant commonly found in sheltered bays, inlets and other shallow waters.

Loss of species makes nature more sensitive to climate change

September 26, 2012

High biodiversity acts as an insurance policy for nature and society alike as it increases the likelihood that at least some species will be sufficiently resilient to sustain important functions such as water purification ...

Study reveals that animals contribute to seagrass dispersal

December 19, 2012

Look out the window and you're likely to see the dispersal of seeds—dandelion tufts in the wind, a squirrel burying an acorn, a robin flying off with a dogwood fruit. You might even have a burr "velcroed" to your sock.

Recommended for you

Scientists overcome key CRISPR-Cas9 genome editing hurdle

December 1, 2015

Researchers at the Broad Institute of MIT and Harvard and the McGovern Institute for Brain Research at MIT have engineered changes to the revolutionary CRISPR-Cas9 genome editing system that significantly cut down on "off-target" ...

Study finds 'rudimentary' empathy in macaques

December 1, 2015

(—A pair of researchers with Centre National de la Recherche Scientifique and Université Lyon, in France has conducted a study that has shown that macaques have at least some degree of empathy towards their fellow ...

Which came first—the sponge or the comb jelly?

December 1, 2015

Bristol study reaffirms classical view of early animal evolution. Whether sponges or comb jellies (also known as sea gooseberries) represent the oldest extant animal phylum is of crucial importance to our understanding of ...

Trap-jaw ants exhibit previously unseen jumping behavior

December 1, 2015

A species of trap-jaw ant has been found to exhibit a previously unseen jumping behavior, using its legs rather than its powerful jaws. The discovery makes this species, Odontomachus rixosus, the only species of ant that ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Jan 30, 2014
"Most mud gets washed away from the barren, chalky seabed, but like trees trap soil on an exposed hillside, eelgrass plants trap the mud. And therefore there will be a high concentrations of sulfide-rich mud among the eelgrass plants," explain the researchers.

That answer doesn't say much at all, in fact it begs more questions. It is a bit like the old question, what came first, the chicken or the egg. If the sulfur was already present at that location, then how did the grass grow? And if it is the grass that is trapping the mud/sulfur, then why in a ring formation? Doesn't there have to be a point of entry?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.