Didymo in a river looks like many gross things: trailing wisps of toilet paper, goopy brown blobs, a coating of slime. It’s commonly referred to as “rock snot” and for good reason. If
you are brave enough to touch it, you will note it feels like slippery wet wool or cotton.
Didymo (Didymosphenia geminate) is a single-celled algae found in the upper Connecticut River in Vermont and New Hampshire; and in the White, Mad and Batten Kill Rivers in Vermont.
That Didymo blooms have occurred in the region is bad news for two reasons: First, people don’t want to swim, tube, wade or boat near the stuff; and, second, during “nuisance blooms,” mats of the algae can form several inches thick on the river bottom, covering anywhere from a few square feet to a mile or more of the river.
It can smother wildlife living on that bottom, including stonefly, mayfly and caddisfly nymphs, says Leslie Matthews, an environmental scientist with the Vermont Department of Environmental Conservation. These nymphs, living among the cobbles, depend on fresh clean water for food and oxygen. If the water flow is stopped by Didymo mats, the fly nymphs die.
Now generally, people don’t seem to care much when bugs are in trouble. But this particular trio of nymphs is held in very high esteem by anglers. They are the bugs that are consumed by every species of trout found in New Hampshire and Vermont. They are the flies that put the “fly” into fly-fishing.
Didymo is new to the region and relatively new to scientists. It was first identified only about 100 years ago in the Faroe Islands of Scotland, according to a report from the U.S. Geological Survey. About that time, Didymo also was reported on Vancouver Island, British Columbia, the biggest island off the west coast of North America.
Didymo was not considered a problem until about 15 years ago when Vancouver Island started experiencing “nuisance blooms.” (A “bloom” occurs when free-floating Didymo cells attach together on rocks, becoming small clumps, but a “nuisance bloom” is said to occur when the clumping is widespread enough to bother people or disturb an ecosystem.)
The nuisance blooms have affected more than two-thirds of Vancouver Island’s rivers and have spread to otherwise pristine trout streams throughout the Rocky Mountain West, mostly in the United States.
In Northern New England, Didymo was first spotted in the northern-most part of the Connecticut River early last summer, covering patches from the headwaters south to Lunenburg, Vermont, says Matthews. It was then found in the White River in Bethel and in the Mad River in Warren and Waitsfield. A nuisance bloom in the New York section of the Batten Kill, which has its headwaters in Vermont, was discovered in 2006.
“It seems to be showing up in popular fishing destinations,” says Matthews. “There is circumstantial evidence that Didymo is being spread by felt-soled waders.” Didymo can survive for weeks in a moist environment, Matthews notes, and the felt soles that give anglers—and river scientists—such a good grip on river rocks can stay damp for months, providing the rock snot with a hospitable medium.
Vermont’s Department of Environmental Conservation has made some changes: “We completely re-outfitted ourselves in light of this discovery,” Matthews says. “Last year we replaced all our felt-soled waders, got neoprene-sock waders and Korkers (a brand name) boots with removable soles.” The new wading gear dries quicker and is much easier to disinfect – using detergent and very hot water or a bleach solution, which also helps guard against the spread of various fish diseases and aquatic invasive species.
There have been no new reports of blooms of Didymo this summer, says Matthews. And that may be because blooms seem to come in cycles of a year or more, or because blooms tend to occur in spring and fall. In the Vermont section of the Batten Kill, for example, only individual Didymo cells have been identified floating free in the water. The river is being surveyed again this month to identify any blooms that may form in response to the cooler weather.
Scientists have not found a way to eradicate Didymo once it is in a river. For now, prevention appears to be the best answer. Matthews hopes the outbreak in Vermont will lead to a new form of hygiene among river users. “We need to develop habits … (of) cleaning gear and not moving water between water bodies,” she says.