Didymo Outbreak in Delaware River

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In April, 2012, an aquatic biologist with the Delaware River Basin Commission (DRBC) found extensive mats didymo, an invasive type pf aquatic alga. According to biologists, Didymosphenia geminata (also known as didymo or “rock snot”) is not a threat to human health, but can be a threat to rivers and streams.

Dr. Erik Silldorff discovered large Didymo blooms in the Delaware River over a 40-mile stretch extending from the area near the confluence with the Lackawaxen River (river mile 279) downstream to the vicinity of Dingmans Ferry Bridge (river mile 239).

The invasive algae was found within two National Parks: the Upper Delaware Scenic and Recreational River and the Delaware Water Gap National Recreation Area.

Didymo covers rock surfaces in cold, moderate to fast flowing water. Since 2007, Didymo has been found at low concentrations during the summer months from around Hancock, N.Y., downstream to the area around Dingmans Ferry, Pa., with high-density patches frequently observed in the cold-water zones of the East and West branches of the Delaware River, as well as in the colder zones of the upper main stem river.

Scientists with the National Park Service and the Pennsylvania Department of Environmental Protection independently documented Didymo blooms extending north of the area discovered by Silldorff to Callicoon, N.Y. (river mile 303) as well as from Long Eddy, N.Y. (river mile 315) upstream into the East and West branches of the Delaware River (upstream of river mile 330).

Biologists observed dense coverage in some areas, while other locations contained relatively small patches. More than 100 miles of river bottom is said to contain the invasive didymo.

Didymo is associated with a number of problems. Thick mats of Didymo can crowd out aquatic insects or smother native algae growing on the riverbed, thereby significantly altering the physical and biological conditions within a stream.

Additionally, Didymo can easily attach to any fishing equipment, especially felt-soled boots, and the chance of it hitchhiking its way into nearby streams or rivers that currently lack this unwanted invader is cause for alarm. The recent didymo bloom coincides with the beginning of trout season, when anglers appear along the river in large numbers.

Most state wildlife agencies recommend that anglers clean all gear after an outing. Boats, paddles, propellers, tackle, clothing, and any other gear used in areas where didymo occurs can be cleaned using a detergent and/or bleach solution. After cleaning, items should be dried thoroughly for at least 48 hours before using in another body of water.

Because of their ability to harbor didymo, felt-soled fishing waders have been banned in several New England and Mid-Atlantic states.The National Park Service also advises against the use of felt-soled waders.

For additional information on how to prevent the spread of didymo, visit: