Mysterious Fairy Rings

Oct 9, 2018

In this month's episode of Field Notes, Susan Knight of UW-Madison’s Trout Lake Station describes an unusual growth pattern of an aquatic plant, reminiscent of mushroom fairy rings.

Over a year ago, a colleague sent me a fuzzy photo of a lake with what looked like two rings, or rather, doughnuts, of floating plants in the middle of a lake.  The plant was probably water shield, and while water shield often forms a ring around the perimeter of a lake, I had never seen a ring out in the middle of a lake. My colleague asked if I knew what could cause this weird pattern. I first thought there might be a spring in the middle of the ring, with water too cold to allow plants to grow, and the plants were growing far enough away from the spring where the water was warmer. “Where is this lake?”, I asked. “It’s a lake in the Huron Mountain Club”, he responded.  “No kidding!”, I thought.  The Huron Mountain Club is a mythical place. The Club owns about 20,000 acres on the south shore of Lake Superior, west of Marquette, MI. The club is beyond exclusive, with only a few dozen members allowed on to the property. Reports of virgin old growth forest are legend and I had been dying to see this property for decades. I hadn’t known, but learned, that the club has a research foundation that awards small grants to natural resource scientists. A mystery involving aquatic plants and the Huron Mountain Club!  It was destiny!  I wrote a grant proposal to investigate the rings and invited three aquatic plant colleagues, Carol, Michelle and Barb (who, like me, were just busting to go) to help research the rings. We named our quarry “the Fairy Rings”.

We were awarded the grant, and our designated time to visit finally arrived. We loaded up with gear and food we would need for our short stay.  Since our hypothesis was that there was a spring spewing cold water at the center, we brought a temperature and dissolved oxygen probe.  We also brought equipment to collect and filter water samples to measure chlorophyll and calcium levels, both of which might be unusual near a spring.  We brought a long rake to sample the plants and assess the bottom sediments. We brought two canoes (freshly steam cleaned so as not to contaminate the lake) we might need. At this point, we were not even sure if the rings would still be there.  What if they were there just that one time? 

We found the lake, which was an adventure in of itself, and, though the lake was completely undeveloped, it had its own boathouse with five sweet rowboats, all with two sets of oars. We loaded the gear into two boats and set out for the far northwest end of the lake where the rings had been seen.  And, there they were!  We hooted and hollered.  The Fairy rings were real! There were actually three rings, each about 30 yards across, with the band of the plants about 2-3 yards wide.  The plant was indeed water shield, a very common floating plant around here.  It has football-shaped leaves about 3-5” long and has copious jelly-like mucous on the underside of the leaf. The three rings were not touching each other and had absolutely no plants in the middle.  In fact, there were no other plants anywhere near the rings of water shield.  The water was only about 3 feet deep, but the underlying sediment was a thin gruel of soupy muck, the kind you can put your oar into and meet with utterly no resistance. Carol and I beached our boat and clambered up a high ridge overlooking the lake, and we could tell this is where the original photo had been taken.  It offered a spectacular view of the rings, and we snapped photos as the other boat moved into the center of the ring.  We thought this was particularly brave of Michelle and Barb, since it seemed possible that the boat would disappear, Bermuda Triangle-style into the Fairy Ring. Luckily, they survived.

We took our measurements and quickly concluded the ring pattern was not due to a cold-water spring at the center. I am still waiting for the chlorophyll and chemistry data to be concluded, but it now seems more likely that the ring of water shield is exhausting the nutrients in one spot and sending roots outward to find new, more nutrient-rich sediments. However, this doesn’t explain why the entire bay isn’t already full of water shield in the part of the lake where the plants are expanding.  It also doesn’t explain why the rings, which are large, and likely expanding, have not yet bumped into each other.

Only one thing to do.  Write up our results, make it sound as mysterious and compelling and cool as we believe it is, and ask permission to visit again. I’ll never be a member of the Huron Mountain Club, but I am thrilled to call it my research site.

For Field Notes, this is Susan Knight of UW-Madison’s Trout Lake Station.