Tennessee Wildlife Resources Agency
Region 4
Stream Management
Information pertaining to the invasive algae Didymosphenia geminata "Didymo" found in East Tennessee trout tailwaters
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Tyler Baker
Tyler Baker
Invasive algae 'Didymo' found in Tennessee River


By Owen Schroeder

Officials from the Tennessee Valley Authority and TWRA have confirmed the finding of an invasive algae in the waters below Watauga, South Holston, Norris and Cherokee Dams on the Tennessee River. It is the first confirmed incident of the algae to be found east of the Mississippi River.

The algae, Didymosphenia geminata, is commonly called "Didymo" and it does not present a health hazard to humans, although some reports indicate that people often experience irritated eyes while swimming in waters downstream from contaminated areas.  Anglers find the algae a nuisance in that it fouls fishing line and lures with a gooey mass of material.

The real danger of Didymo lies in the impact that it may have on our fish populations, especially the trout fishery of East Tennessee. The algae thrives in cool, nutrient-poor water where it forms blooms that result in massive mats. It crowds out the native organisms in streams and rivers that trout and other fish feed on resulting in the decline of their populations.

In South Dakota, brown trout populations have been severely reduced in the Rapid Creek area of the Black Hills downstream from Pactola Dam. Up to 90 percent of the stream bottom in the area is covered with the algae.

Didymo is most often found on the bottoms of streams and rivers where it attaches itself by stalks to the gravely bottom of the stream or river bed, smothering out rocks and other submerged plants.

"This is a concern for a number of reasons," said TVA scientist Tyler Baker. "Didymo” reduces the area of clean substrate upon which fish nest and lay eggs. The resulting change in habitat could conceivably cause a shift in the types of aquatic insects present. It also tends to out-compete and limit the growth of native algal species, many of which are food sources for aquatic insects, which in turn are preyed upon by fish and other creatures.

Baker noted that the algae has also caused problems by clogging water intakes in Canadian streams and rivers. The TVA is not sure of how Didymo may have arrived in East Tennessee. The algae has been documented in Arkansas, Montana, South Dakota and British Columbia in North America. It is also found in New Zealand and Poland.

"No one is sure how it got here, but it could be that human activity or waterfowl was the unwitting culprit," Baker said. "However it happened, it's not good news, and we will be watching it closely to see how it affects Valley waterways."

The TVA is asking anglers and boaters on the Tennessee River system to report any suspicious algae growth that they might encounter. Didymo appears as a thick white, light gray, pale yellow-brown, or beige (not green) mass, which cover more than 90 percent of the river bottom in places.

Once established, it may look like a brown shag carpet covering the whole river or stream bottom. It frequently forms flowing "rat's tails" that often turn white at their ends and look like a length of toilet paper.

Although is may look slimy, it is actually spongy and feels scratchy, much like wet cotton wool.

Boaters and anglers can help to prevent the spread of Didymo by observing the following precautions:

•  Before leaving a stream, river or reservoir, check your shoes, waders, life vest, boat hull, trailer tires and other equipment for clumps of algae, taking care to look in all creases and compartments of your boat, trailer and equipment. If algae is found, remove it and leave it at the site.
•  If you discover algae on your boat, trailer or equipment after you leave the area, do not wash it down the drain. Treat it with a two percent solution of household bleach, or a five percent solution of salt, antiseptic hand cleaner or household detergent. Infected equipment can be treated by letting it completely dry for at least two days.
•  Under no circumstances should fish, plants, or other items be moved from an infected waterway to an uninfected waterway.
If you spot an algal mat in a stream or river that you suspect might be Didymo, report the incident to Baker by calling (423) 876-6733.

The Tennessee Valley Authority contributed to this report.
Originally published September 1, 2005


Additional information can be found at these sites or downloaded
"Didymo" in Wilbur Tailwater, Carter County. Photo courtesy of Tyler Baker, TVA.