In summer 2010, European marine biologists discovered conspicuous and mysterious ring formations in shallow waters off the Island of Møn in Denmark. Suggestions on what could have formed these rings were plenty ranging from World War II bomb craters to crop circles formed by UFOs.
Further examinations revealed that the distinct circles are formed by Zostera marina – a species of eelgrass, growing at water depths from 1.5 to 2.5 m.
In 2013, a team of Danish and Australian scientists set out to explain why the eelgrass grows in circles there.
“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”, said Dr Marianne Holmer of University of Southern Denmark and Dr Jens Borum from the University of Copenhagen, co-authors of a paper published in the journal Marine Biology.
The circles range from about 1 to 15 m in diameter and consist of narrow fringes of dense eelgrass shoots.
“We have studied the mud that accumulates among the eelgrass plants and we can see that it contains a substance that is toxic to eelgrass,” the scientists said.
The poison is sulfide, 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 high concentrations of sulfide-rich mud among the eelgrass plants.”
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. 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 scientists said.
“The result is an exceptional circular shape, where only the rim of the circle survives – like fairy rings in a lawn.”
Jens Borum et al. 2014. Eelgrass fairy rings: sulfide as inhibiting agent. Marine Biology, vol. 161, no, 2, pp. 351-358; doi: 10.1007/s00227-013-2340-3