The Maryland Department of Natural Resources (DNR) is urging pet owners to exercise caution around freshwater bodies this summer due to the presence of benthic algae mats, which can pose health risks to dogs and other animals. These dense, fibrous accumulations of algae, often green, yellow, brown, or black, grow in rivers, creeks, lakes, ponds, and reservoirs, adhering to submerged surfaces or floating freely. While not all mats are toxic, some contain potent cyanotoxins, prompting DNR to issue warnings following detections in Maryland waters over the past two years.
Benthic algae mats, particularly those containing cyanobacteria, can produce neurotoxins such as saxitoxin and anatoxin-a, as well as the liver toxin microcystin. These toxins are harmful when ingested, with dogs being especially vulnerable due to their tendency to drink from natural waterbodies, eat algae mats, or groom their fur after swimming. DNR reports that anatoxin-a has caused dog deaths in over 20 states, while microcystin has led to canine mortalities in Maryland. Symptoms of anatoxin exposure range from muscle twitching, salivation, and loss of balance at low doses to rapid death from respiratory paralysis at high doses.
“DNR’s environmental program manager, Cathy Wazniak, noted that benthic algae mats are still being studied and may be more prevalent in the area than previously realized,” according to a DNR statement. Recent findings from an Environmental Protection Agency project identified cyanotoxins in the Patapsco and Patuxent rivers, adding to known detections in the upper Chesapeake Bay, Potomac River, and 15 Maryland lakes. In 2023, DNR issued a pet health caution for a 17-mile stretch of the non-tidal Potomac River, followed by a 54-mile alert in 2024.
To protect pets, DNR advises owners to bring drinking water for animals, avoid areas with green or scummy water, keep pets on leashes near freshwater, and rinse pets with tap water after exposure. Dogs showing tremors or breathing difficulties should be taken to a veterinarian immediately. The public is encouraged to report suspected algal blooms via the bloomWatch app, DNR’s Eyes on the Bay website, or the Fish Health Hotline at 866-633-4686.
Toxic mats cannot be identified visually, requiring lab analysis to confirm the presence of toxins. The Maryland Department of the Environment tests for anatoxins, while an external lab analyzes for saxitoxins. DNR’s Resource Assessment Service monitors algae in the Chesapeake Bay, Maryland’s Atlantic coast, and 12 state-owned lakes, while responding to public reports of suspected benthic harmful algal blooms (HABs). Unlike water-column HABs, which turn water green or reddish-brown, benthic mats are a distinct phenomenon, and Maryland operates a response-based program rather than active surveillance for them.
The risk to humans is low, as adults are unlikely to ingest mats, but children and dogs may be attracted to their musty smell. Individuals experiencing symptoms of exposure should contact the Maryland Poison Control Center at 1-800-222-1222. DNR notes that toxin production and exposure risks are under ongoing research, with high mat coverage increasing the likelihood of contact during recreational activities.
Benthic mats are most abundant in mid to late summer, thriving in warm conditions. In recent years, detections in the Susquehanna Flats, North East River, and Potomac River have heightened concerns. DNR’s monitoring efforts, coordinated with the Maryland Department of Health and Department of the Environment, aim to track and respond to these risks. The public can help by reducing nutrient pollution, which contributes to algal blooms, and reporting sightings promptly.
The department emphasizes that when benthic mats are present, people and pets should stay away. Images of mats are available on DNR’s website to aid identification. As research continues, DNR remains focused on protecting public and animal health while studying the prevalence of these potentially hazardous algae formations.
