Washington: Images of the mysterious ocean circles off the Baltic coast of Denmark have left people baffled since 2008.
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 Mon 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 plants 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", biologists Marianne Holmer from University of Southern Denmark and Jens Borum from University of Copenhagen, said.
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.
"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", Holmer and Borum said.
The poison is sulfide, a substance that accumulates in the seabed off the island of Mon, 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," the researchers said.
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.
"Eelgrass populations grow vegetatively by stolons which spread radially in all directions and therefore each plant creates a circular growth pattern," the researchers said.
"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," they added.