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Concern Over "Rock Snot" Stimulates Additional Study By DRBC

By Jessica Rittler Sanchez, Delaware River Basin Commission


On April 18, 2012, Dr. Erik Silldorff, an aquatic biologist with the Delaware River Basin Commission (DRBC), was surprised to see extensive mats of the invasive, aquatic alga Didymosphenia geminata (also known as Didymo or "Rock Snot") in the Delaware River near Matamoras, Pike County, Pa.  Further examination led to the discovery of large blooms of Didymo occurring over a 40-mile stretch of the Delaware River, extending from the area near the confluence with the Lackawaxen River downstream to the vicinity of Dingmans Ferry Bridge.  Subsequent surveys have confirmed that that Didymo is present throughout the entire 200-mile non-tidal portion of the Delaware River and into several tributaries.

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. Additionally, Didymo can easily hitchhike its way into nearby streams or rivers that currently lack the unwanted invader.  This is alarming given that there are many cold, low-nutrient streams in the Delaware Basin and surrounding areas.  Keeping Didymo out of such streams is critically important because once Didymo is found in a body of water there is no known way to fully eradicate it.

DRBC received an award from Pennsylvania Sea Grant in August 2012 to help delineate the threats from the expanding Didymo invasion and provide the global community of scientists with a better understanding of how nutrients may impact the alga’s morphology.  During surveys, Dr. Silldorff noticed that while the diatom was extensive throughout the non-tidal Delaware River, its form was notably different in the higher nutrient waters below the Lehigh River, lacking the long stalk seen in the lower nutrient waters upstream.  Starting in February 2013, DRBC will perform additional surveys and transplant colonized rocks to investigate the impact of different water chemistry on stalk morphology.

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