Strange “Fairy Rings” in Seafloor Explained

Here’s one mystery that has come full circle: Scientists have figured out the meaning behind ocean “fairy rings.”

In 2008 a tourist photographed some bizarre circles on the seafloor in shallow waters of the Baltic Ocean off the chalky cliffs of Møn, Denmark.

Photo of the eelgrass circles in the shallow water off the coast of Denmark.
Circles seen in shallow water off the coast of Denmark. Photograph by Jacob T. Johansen

The images piqued public interest, with people offering intriguing guesses that included World War II bomb craters and alien crop circles. (Also see “New Theory Explains Africa’s Mysterious Fairy Circles.”)

Now scientists have an answer: Sulfide, a toxic substance that accumulates on the ocean bottom, is stunting vegetation called eelgrass, creating rings of healthier plants around these diseased zones.

Biologists Marianne Holmer at the University of Southern Denmark and Jens Borum at the University of Copenhagen studied samples from five of the circular patches, which ranged from 6.5 to 49 feet (2 to 15 meters) in diameter, as well as the mud accumulating among the eelgrass plants.

The team found that on the inner parts of the ring, the eelgrass roots and leaves were shorter, less dense, and overall less robust. They also found the mud was high in sulfide, a chemical compound of sulfur that’s released when plants die.

Sulfide usually bonds with iron, which is naturally found in the ocean. But in the chalky, iron-poor sediments in this area of Denmark, sulfide accumulates in the sediment, where it’s eventually taken up by older eelgrass plants, according to the study, published February 2014 in the journal Marine Biology.

Eelgrass grows radially, or out from a center point, so the older plants that are more exposed to the sulfide and thus weaker are in the middle, with healthier, younger plants around the perimeter—explaining the rings.

Ebbing Eelgrass 

Sulfide is also common in waters low in oxygen, which is increasingly a reality due to pollution. For instance, runoff from lawn fertilizers and industrial waste shuttles nutrients like phosphorus and nitrogen into the oceans, causing blooms of algae that eat up available oxygen in the water. Not only that, the algae also block the sun and kill seagrass. (Read more about ocean pollution.)

Photo of eelgrass growing in the shallow water off the coast of Denmark.
Eelgrass provides habitat for marine life. Photograph by Ole Pedersen

Due to these causes, as well as outright destruction, seagrasses are in decline the world over, the scientists noted.

That could be bad news for the ocean, since eelgrass is “a very important habitat for all kind of sea creatures,” noted Laura Murray, a marine ecologist at the University of Maryland Center for Environmental Science, who was not involved in the study.

“Smaller fish or molting crabs will hide in eelgrass to escape predators, and the grass will grow algae on its leaves that are eaten by other critters,” she said.

So eelgrass has “its own sort of food web right there.” (See pictures of marine species under threat.)

Eelgrass also protects shorelines from erosion and filters particles and sediments to help keep the water clear.

The University of Southern Denmark is a partner and coordinator of NOVA GRASS, a research project that’s working on restoring eelgrass meadows to their former lushness.

As for those rings, we finally have a perfectly good explanation—either that or those wily fairies have staged one awesome cover-up.

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Liz Langley is the award-winning author of Crazy Little Thing: Why Love and Sex Drive Us Mad and has written for many publications including Salon, Details and the Huffington Post. Follow her on Twitter @LizLangley and at www.lizlangley.com

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