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Mid-Chesapeake Bay algae bloom turns water rusty-red, but is not harmful to humans

Severn River Association Board Member Ted Delaplaine uses a Secchi disc to record water clarity on Forked Creek. An algae bloom is hurting water clarity in tributaries. - Original Credit: Tom Guay
Severn River Association Board Member Ted Delaplaine uses a Secchi disc to record water clarity on Forked Creek. An algae bloom is hurting water clarity in tributaries. - Original Credit: Tom Guay (Severn River Association / HANDOUT)

A common algae species has turned Chesapeake Bay tributaries rusty-red, and while it isn’t harmful to humans these types of algae blooms carry the potential to irritate gills, suffocate fish and shade critical underwater grasses.

Fortunately, Chesapeake Bay Foundation Maryland Scientist Doug Myers said, they are seeing signs that the “mahogany tide” is residing. He said usually this time of year he sees this algae to appear in one river, dissipate, and appear in another. This year the salinity levels and temperatures sparked blooms in multiple tributaries all at once, from the Patapsco down to the West and Rhode, he said.

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Myers said he was concerned about how widespread the bloom was, and that if it persisted it could kill underwater grasses by blocking out sunlight. The decomposing grass and the decomposing algae would strip the water of oxygen, potentially killing fish if they can’t flee the area.

“Right up until a week or so ago I was afraid we were going to lose a lot of that grass, then you can really see some fish kills,” he said. “We may have dodged the bullet on that.”

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Typically dinoflagellate Prorocentrum minimum blooms in April, Jay Apperson with the Maryland Department of Environment said in an email.

“It usually dies off when water temperatures are rising in early to mid May, but because it’s been a little cooler than usual this year, a later die off is likely,” he said.

A sample collected from the bay between the West and Rhode Rivers found a concentration of 24,000 dinoflagellate cells per milliliter. That is a moderately heavy bloom, Apperson said, and is enough to turn the water red.

While the algae is naturally occurring, such blooms are made bigger by nutrient pollution. Myers said the state has made significant progress is reducing pollution from the waste water treatment plants that manage sewage from homes and businesses, but more work is needed.

“The bay is still dangerously out of balance,” he said. “We have a long way to go until we’re in a self-sustaining position where we don’t have mahogany tides.”

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