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Invasive algae show up in some TVA tailwaters

TVA scientists have recently confirmed the presence of a freshwater diatom (a single-celled algal species) in the reaches of river below Watauga and South Holston Dams.

Its taxonomic name is Didymosphenia geminata, but it is commonly referred to as “Didymo.” No one’s exactly sure how it got here, but it could be that human activity or waterfowl were the culprits — unwittingly transferring it from one waterbody to another. However it happened, it’s not good news, according to TVA Scientist Tyler Baker: “Any time a non-native species is introduced to an aquatic environment, there are likely to be implications. We will be watching closely to see how Didymo affects Valley waterways.”

Didymo thrives in cool, clear, nutrient-poor water, where it tends to form massive blooms that result in algal mats. Believed to be native to northern Europe, Canada, and other regions in the Northern Hemisphere, it appears to have recently exhibited a greater tolerance for different conditions, gradually expanding its geographic range. While it has been found in some streams in the Western U.S. — as well as in the White River in Arkansas — this is the first confirmed record of the diatom occurring east of the Mississippi River.

Most often found on the bottom of streams and rivers, Didymo attaches itself by stalks to the gravelly bottom of the stream or riverbed — smothering rocks and submerged plants. “This is a concern for a number of reasons,” explains Baker. “It 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 outcompete 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.” Didymo also has caused problems by clogging water intakes in British Columbia streams, according to Baker.

Didymo seems to become easily established in lake-fed or regulated rivers (below dams), where stable water currents are likely to promote further growth by transferring plenty of nutrients to the mat surface. Cold tailwaters (the areas immediately downstream of dams) and streams are the most likely candidates, says Baker: “Apalachia is at the top of my list — given Didymo’s habitat preference. We’ll also be closely watching other reservoir tailwaters and streams throughout the Valley. It could easily find a home it likes at Norris, Blue Ridge, Nottely, and possibly Chatuge or Tims Ford tailwaters.”

The good news is that TVA monitoring efforts have not yet indicated any recent declines in fish or aquatic insect populations in the Watauga or South Holston tailwaters. “We conduct annual sampling in these tailwaters,” says Baker, “and we have quite a bit of historical data that should tell us a lot about densities and composition. That’ll be a big advantage when it comes to spotting any trends that might be attributable to the proliferation of Didymo.”

How can you tell if an algal bloom (a common occurrence this time of year) is actually Didymo? First of all, it’s much more likely to be found in a stream or river; algal blooms in reservoirs are usually indicative of another species. It appears as a thick white, light gray, pale yellow-brown, or beige (not green) mass, which may cover over 90 percent of the river bottom in many reaches. 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 a lot like lengths of toilet paper. Although Didymo looks slimy, it’s actually more spongy and feels sort of scratchy — rather like wet cotton wool. While it does not appear to affect the safety of drinking water, taste and odor problems may be a concern.

You can help slow the spread of Didymo in Valley waters by taking these precautions:

• Before leaving the river, check your shoes, waders, life vest, boat hull, tires, and other equipment for clumps of algae, taking care to search within creases or compartments. Leave clumps at the site.
• If you discover clumps of algae after leaving the area, do not wash them down the drain. Treat them in a 2 percent solution of household bleach or a 5 percent solution of salt, antiseptic hand cleaner, or dishwashing detergent. Infected equipment can also be treated by drying completely for at least two days.
• Under no circumstances should fish, plants, or other items be moved from an affected waterway to an unaffected waterway.

If you spot an algal mat in a stream or river that you believe might be Didymo, please contact Tyler Baker at 423-876-6733.