A potentially poisonous algae bloom never before seen in the Chesapeake Bay watershed has forced state officials to temporarily ban oyster harvests from infested portions of the Potomac River.
Scientists say the bloom detected last week is probably linked to the drought afflicting the Chesapeake region and could be the harbinger of other effects on the bay's aquatic life.
The algae, Dinophysis acuminata, was spotted along a 30-mile swath of the lower Potomac. It does not harm oysters but can cause gastrointestinal illness in people who eat shellfish harvested from bloom-infested waters, state officials said.
No illnesses linked to the Potomac bloom have been reported. Officials at the Maryland Department of the Environment said they closed the area's oyster bars as a precaution.
The tiny, colorless, toxic cell comes from a family of potentially harmful microorganisms that have infested mussel beds in the cold seas along the Nova Scotia coast. It has been seen before at very low levels in Maryland's coastal bays, which are typically as salty as the sea, but large blooms had never before been detected in the Chesapeake's fresher waters.
Scientists say it might be growing because the drought has made the water in many rivers and streams much saltier than usual for this time of year.
"We are setting new high-salinity records in December, January and February," said Robert Magnien, director of the state Department of Natural Resource's tidewater ecosystems division. "It's pretty widespread."
This month, salinity in the Potomac is usually 10 parts per thousand, less than one-third as salty as seawater, Magnien said. DNR samples show the river's water now is 18 parts per thousand salt, about half as salty as the ocean.
If rains don't start soon, the drought could have unseen effects underwater that are nearly as far-reaching as those on land.
The bay's salt levels usually rise and fall with the seasons, linked to the amount of rainwater that flows into the region's rivers. Those changes are important cues for aquatic plants and animals, especially in spring, when fish and shellfish are preparing to spawn and dormant underwater meadows are about to re-sprout.
Yellow perch, a favorite catch for anglers, seek fresher water as they begin spawning this month, followed by white perch and rockfish next month and in April. The higher salinity could limit spawning areas, and that could result in fewer than usual hatchlings, DNR biologists said.
Magnien said some underwater grasses that can't tolerate high salt levels might not grow back as they usually do in spring. That would deprive young fish of shelter from predators. And the diseases that have ravaged bay oysters seem to thrive in saltier water.
"We're starting behind the eight-ball," Magnien said. "It's really been the worst [season] we've seen" as far as freshwater flow is concerned.
Scientists don't know how spring weather will affect the bloom of Dinophysis, the latest harmful algae species to be discovered in Chesapeake waters in the past five years. A toxic Pfiesteria outbreak in 1997 prompted state officials to step up their water-sampling program.
"Honestly, we don't know enough about it to make predictions," Magnien said. "We're just going to have to see how it runs its course."
Richard Eskin, who supervised the MDE's testing, said sampling Thursday showed a bloom under way from Nanjemoy Creek in Charles County to Ragged Point on the Virginia shore, across from a sparsely settled stretch of St. Mary's County.
Last week's samples of the algae are being tested to determine whether the microorganisms are producing toxins or are in a harmless stage of their life cycle. New samples are to be taken tomorrow.
Eskin said shellfish take the algae into their bodies without harm. "It's food to them," he said. Once the bloom dissipates, the creatures gradually shed the toxins, making the shellfish safe for people to eat, though scientists aren't sure how long that process takes.
The algae's toxin, which can't be eliminated by cooking, causes an illness called diarrhetic shellfish poisoning. Symptoms include nausea, vomiting and stomach upset. The disease is neither fatal nor permanently harmful.