A “mahogany tide” of algae, including one species known to cause fish kills, appeared in the Baltimore harbor this week. The algae bloom is an unusual occurrence at this time of year when waters are cool.

Monitoring equipment showed an explosion of chlorophyll, the molecule plants use to turn sunlight into energy, in waters near the National Aquarium on Thursday. Tests revealed two types of algae, said Allen Place, a professor at the University of Maryland Center for Environmental Science — one known for its brown color, and the other for emitting toxins.


Scientists and water-quality advocates said they have not seen any signs of a fish kill but will be watching the bloom closely. A similar cool-weather algae bloom caused a massive fish kill in the Middle River in November 2015.

Baltimore’s Patapsco Wastewater Treatment Plant released four times the amounts of nitrogen and phosphorus permitted in 2016, according to a new report by an environmental advocacy group.

“What the effects are going to be depends on the persistence” of the bloom, Allen said. “My guess is that the fish in the harbor can avoid it.”

Alga is a natural part of the ecosystem, but when large blooms sprout quickly, they can be a sign of pollution.

Algae blooms are common during the spring and summer, but become less common as waters cool.

The blooms can be fueled by unnatural sources of nutrients, such as runoff from lawn or farm fertilizers or leaks from sewage systems or wastewater treatment plants.

Given little rainfall in recent weeks, the bloom may have been fueled by constant leaks of sewage from Baltimore’s aged wastewater system or some unknown source of pollution, said Alice Volpitta, water quality manager for Blue Water Baltimore, an advocacy group.

“That combined with the warmer temperatures we’ve had might have been enough to spark this type of thing,” Volpitta said.