Cyanobacteria advisories are in place at Elm Brook Park beach in Hopkinton and Silver Lake State Park Beach in Hollis.
State officials are urging visitors to stay out of the water if they observe blue-green scum or clumps suspended in the water column. Cyanobacteria can be toxic to both humans and dogs.
Researchers say this spring's weather may be contributing to an increase in blooms in local lakes. Cyanobacteria respond to nutrients from fertilizers and other sources that run into lakes after rainstorms. Warm temperatures also encourage blooms.
The New Hampshire Department of Environmental Services keeps an updated map of advisories on its website.