Mystery of strange Baltic Sea underwater circles finally explained

These strange circles keep cropping up at the bottom of the Baltic Sea. (Credit: LiveScience.com)

A mystery in the Baltic Sea has been puzzling scientists for more than five years. Strange circles have been reported on the sea floor, photographed in both 2008 and 2011, but biologists now have a scientific explanation for why they're showing up.

These large rings — up to 15 metres wide — of green eelgrass on the sea floor off the Danish island of Møn have inspired theories about aliens, bomb craters and even fairy creatures. However, the answer — now published in the journal Marine Biology — is much more mundane, and it highlights the potential damage that agricultural runoff can do to aquatic environments.

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"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 in a statement.

"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," they explained in the statement.

What Holmer and Borum found was something that is actually affecting eelgrasses all around the world — sulfide. Although it's not toxic enough to harm adult eelgrasses, newer plants are inhibited and it causes older plants to weaken and die. Eelgrass patches typically take root in a location and then spread out in a circle. Mud accumulating around the edges of the patch acts on younger sprouts, inhibiting their growth, while at the same time the sulfide in the mud near the centre of the patch is able to kill the older plants. This leaves behind just the ring of healthier plants, creating the strange patterns seen in 2008 and 2011.

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It's the chalky sea floor off the island of Møn — an extension of the famous white chalk Cliffs of Møn — that makes this area more susceptible to sulfide buildup. However, this can happen in many different places, typically where agricultural runoff ends up in coastal waters. The nutrients in the runoff feed tiny microorganisms in the water, which consume all the oxygen, leaving behind an environment where sulfide can accumulate. In the case of algal blooms growing from the runoff, these cover the water surface, keeping aunlight from reaching the seagrasses.

Since sea grasses are a benefit to the oceans, tending to keeping waters clean and clear (likely why such good photographs were taken of the 'mysterious' rings), they also host many forms of sea life that use patches for their homes. Researchers like Holmer and Borum are working on ways to address this sulfide problem in the hopes of reversing the current decline in seagrass meadows.

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