WESTERLO — A mass of a toxic type of bacteria, known as a harmful algae bloom, was spotted last Friday in the Basic Creek Reservoir in Westerlo.

The state’s Department of Environmental Conservation reported on Tuesday that a large, localized suspicious bloom was spotted on Aug. 2, said DEC spokesman Kevin O. Frazier, meaning the bloom is within an entire region of the body of water. “Suspicious” refers to the fact that laboratory analysis has not been done to confirm if this is a harmful algae bloom.

“Bloom conditions can change rapidly,” Frazier wrote in an email on Tuesday. “We encourage the public to follow our guidance to avoid blooms and check DEC’s online reporting system for updates.”

The state’s notification page can be found on the DEC website.

A small, localized suspicious bloom was reported on June 28 at the reservoir, meaning it was limited to one area of the water, according to DEC records.

Those who have been in contact with a harmful algae bloom generally report allergic reactions such as trouble breathing; skin or throat irritation; or gastrointestinal distress such as nausea, vomiting, or diarrhea.

The Basic Creek Reservoir is used by the city of Albany as a backup water supply when the Alcove Reservoir, located in Coeymans, runs low, said Joe Coffey, the commissioner of the city’s water department. He said that the Basic Creek Reservoir has not been used for this purpose since April 2018.

The city also monitors the water, said Coffey, often reporting suspected harmful algae blooms, measuring levels of toxins in the water, and putting up signs warning of the blooms. Fishing is allowed on the reservoir, and he encouraged people to avoid areas of water with suspected harmful algae blooms.

According to DEC records, which go back to 2012, harmful algae blooms have been found at the reservoir in the summer or fall of 2013, 2014, 2016, 2017, and 2018. Coffey said that the city generally does not use the Basic Creek Reservoir in the late summer and early fall because of its tendency to have algae blooms, both toxic and nontoxic.

The cause of harmful algae blooms is not fully known, but Frazier wrote that likely culprits include excess nutrients like phosphorus and nitrate that are often found in fertilizer; lots of sunlight; and warm, calm water. Coffey said that the Basic Creek Reservoir is surrounded by farms and other agricultural operations, and is shallower than the Alcove Reservoir, which is surrounded mainly by forests.

Erin Silk, a spokeswoman for the state’s Department of Health, said that their department monitors algae blooms throughout the state and looks at potential impacts to drinking water; though in this case the city of Albany’s water is not impacted.

“Water treatment plants and treatment plant operators have the technology to test for the harmful toxin microcystin in source water, and if detected, use existing equipment, treatment and disinfection to ensure treated drinking water meets health advisory levels,” she wrote in an email on Tuesday.

While Albany’s water department has not had to do this, Coffey said that the department is able to augment its treatment process should harmful algae blooms be found in water supplied to the city.