VDH warns of algal mats on the Shenandoah River

Published: Aug. 8, 2022 at 10:48 PM EDT
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STRASBURG, Va. (WHSV) - The Virginia Department of Health is asking water recreators in Shenandoah County to be on alert for potentially toxic algal mats along the North Fork of the Shenandoah River in the Strasburg area.

VDH sent out the alert at the end of last week for an 11.5-mile stretch of the river from Deep Hollow Lane to Long Meadow Road. The algal mats can be widespread or patchy throughout the area.

Algal mats are large collections of algae that can carry cyanotoxins that can cause skin rashes at the touch and make people or animals who drink the water nearby very sick.

“Dogs are especially susceptible to these algal mats if they drink water in the vicinity of some of these mats where there’s a high concentration of these toxins they can rapidly absorb into their system,” said Mark Frondorf, the Shenandoah Riverkeeper.

Frondorf said that the algal mats can range in size anywhere from a few yards across to several hundred yards across. He said that the warm weather, high levels of nitrogen and phosphorus runoff in the area from poultry and cattle farming, and the residency time of the water create the river’s algae problems.

“Residency time is the amount of time that the water sits in a system or flows through an area. So when that water residency time is low, often just due to low water conditions, then you have all the ingredients necessary for a harmful algal bloom or at least a large algal mat,” said Frondorf.

Algal mats can also cause some problems for the river’s aquatic ecosystem even if they aren’t toxic. When the mats die off they consume a large amount of oxygen in the water which can cause hypoxia or a lack of oxygen for fish.

“When hypoxia sets in as a result of the decay of an algal mat those fish either have to swim to get out of there along with other creatures or there is a real risk of them dying off,” said Frondorf.

Frondorf said he has been going out and testing water samples for toxins that were common in the river during last year’s algae bloom. He said that there are warning signs that a spot on the river may be high in toxins from algal mats.

“Trust your senses. If it smells like ammonia let your senses tell you that there is a potential danger that is there. Often times the ammonia smell is very strong and very pungent so if it is get out of the water there,” he said.

Frondorf said he anticipates the algal mats will continue to be an issue on the river into the fall.

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