By: Jeremy Cox,

A reddish-brown patina has overtaken several Maryland rivers that feed into the Chesapeake Bay.

Yep, themahogany tideis back.

The algae bloom, caused by the speciesProrocentrum minimum, often shows up around this time in May as temperatures warm and spring rains flush nutrients into waterways. This year’s outbreak, though, appears more intense and widespread than most, scientists say.

“We often see it pop up in one river, and it dissipates before it pops up in another river. But this year, it seems to have come up all at once,” said Doug Myers, a Maryland scientist with the Chesapeake Bay Foundation.

A brownish wake trails a boat as it motors through Curtis Creek south of Baltimore along the Chesapeake Bay’s western shore on an overcast May 27. (Photo submitted by Alice Volpitta)

Since early May, the Maryland Department of Natural Resources has identified five waters samples containing blooms from the Patapsco, Patuxent, South and West rivers. Environmental groups and other state sources have reported more in the Mid-Bay and Baltimore’s Inner Harbor, as well as the Magothy, Rhode and Severn rivers. It also has been spotted on the Eastern Shore in the Chester and Wye rivers.

P. minimumturns water a trademark rusty color. It isn’t toxic to humans, but it can send an aquatic ecosystem into a tailspin, experts say.

If it’s extensive and concentrated enough, it can block sunlight from reaching underwater grasses that are vital for fish habitat and preserving the Bay’s water quality. When the algae die off, the ensuing biological reaction can rob the water of oxygen, triggering “dead zones” where fish cannot live.

As of May 28, no fish kills had been linked to the bloom. In 2012, a mahogany tide wasblamedfor the deaths of more than 100,000 fish in the Patapsco and its adjoining waters.

Analysis of satellite images has revealed levels of chlorophyll, an algae bloom indicator, in this year’s blooms at levels well above 50 micrograms per liter and, in some places, possibly greater than 100, said Cathy Wazniak, a DNR environmental program manager.

Deep red splotches show the greatest intensity of a “mahogany tide” on May 10, 2020, in the Mid-Chesapeake Bay and its tributaries in this National Oceanic and Atmospheric Administration satellite image. (Photo submitted by Rick Stumpf/NOAA, derived from Copernicus Sentinel-3 data)

“A bloom of this level is likely impacting the light penetration in the water needed for Baygrasses to grow and will certainly impact oxygen levels when it dies off,” she said. The bloom was so dense in waters off Annapolis and in the Chester River, she added, that scientists couldn’t estimate the chlorophyll levels from the satellite imagery.

Myers also worries about troubles ahead. “How much damage it will do is based on how long the bloom persists and over how large of an area,” he said.

The algae’s rampant growth is likely fueled by lawn fertilizers running off into nearby waterways and sewage overflows during heavy rain, said Alice Volpitta, the Baltimore Harbor Waterkeeper. Such pollution has remained stubbornly high despite millions of dollars invested across the watershed as part of a state-federal cleanup partnership that’s well into its fourth decade.

“We’re still in an out-of-balance ecosystem because of the nutrients,” Volpitta said. Her own water samples have confirmed chlorophyll levels of up to 450 micrograms per liter in Curtis Creek, just south of Baltimore.

Tom Guay, executive director of the Severn River Association, fears that river’s bloom will lead to a repeat of last year’s dead zone, which covered 5 square miles of the Severn’s bottom. The mahogany tide this year stretched the entire length of the 14-mile river.

“It just looks like you’re plowing through sewage,” said Guay during a brief break from his regular river patrols to fill his boat up with gas. “It’s ugly.”

By May 28, though, the water’s hue had begun to turn a greenish-brown.

“I think that this is a positive sign this thing is abating,” Guay said.

David M. Higgins II, Publisher/Editor

David M. Higgins was born in Baltimore and grew up in Southern Maryland. He has had a passion for journalism since high school. After spending many years in the Hospitality Industry he began working in...