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Large blooms of didymo discovered in Delaware River


April 24, 2012

DELAWARE RIVER BASIN – The Delaware River Basin Commission (DRBC) today announced that a staff scientist last week found extensive mats of the aquatic alga Didymosphenia geminata (also known as Didymo or “Rock Snot”), an invasive species, in the Delaware River. Anglers are encouraged to vigilantly clean equipment to prevent the spread of this invader.

Dr. Erik Silldorff, an aquatic biologist with the commission, on April 18, 2012, 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). This section of river includes portions of two National Park units: the Upper Delaware Scenic and Recreational River and the Delaware Water Gap National Recreation Area.

“We knew Didymo occurred in the river,” Dr. Silldorff said, “but the spatial extent and intensity of this bloom is alarming given its potentially detrimental effect on ecosystems and the ease in which it can be spread to nearby tributaries.”

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.

Following the recent discovery, scientists with the National Park Service and the Pennsylvania Department of Environmental Protection over this past week 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). Each biologist noted that the intensity was variable, with some areas having dense coverage, while other sites or locations only having relatively small patches. Regardless, these findings indicate that the blooms of Didymo now extend across more than 100 miles of river.

While Didymo is not a public health hazard, there is great ecological concern with discovering the invasive alga to this extent and in these concentrations. Thick mats of Didymo can crowd out or smother more biologically valuable algae growing on the riverbed, thereby significantly altering the physical and biological conditions within a stream.