Health officials have closed the Mohegan Colony Association Beach and the Mohegan Beach Park District -- the only county-regulated beaches around Mohegan Lake.
Harmful cyanobacteria, or blue-green algae, can discolor water or produce floating scums -- and it looks like green paint floating on the water.
Algae blooms often occur in lakes and conditions have been ideal: the water is warm, there hasn't been much wind which allows the bacteria to thrive in still water, and Lake Mohegan contains high levels of phosphorus.
"Conditions can change quickly, but as long as we have the kind of heat we're having, and the levels of phosphorus in the lake, we're always in danger of having blooms," said Ken Belfer, of Mohegan Lake Improvement District.
Lake Mohegan is a mile long and spans 104 acres. There are seven beaches run by homeowners associations.
"There's 240 members to this association, they live in the district area and people come down here all the time to swim," said Tony Rose of Mohegan Beach Park District.
Last year an aeration system with 43 diffusers was installed to try and circulate oxygenated water on the surface to the bottom of the lake.
The concern is that algae can release toxins, which can cause skin, eye and throat irritation.
In order for the beaches to reopen, the algae must be completely cleared from the swim area.
At one point there was so much algae, people complained of a natural gas smell.
In addition to the beaches closed in Westchester County, there are also some beaches that are closed in Putnam County due to harmful algae blooms.
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