Blue-green algae, (cyanobacteria) are a common and natural component of the microscopic plants (plankton) in Lake Champlain. Some kinds of blue-green algae produce natural toxins or poisons. When these algae die and break down, toxins can be released into the water.

If animals ingest the toxin, they can be quickly paralyzed and die. Signs of poisoning include weakness, staggering, difficulty breathing, convulsions and death. Although there are no documented cases of human illness related to blue-green algae in Lake Champlain, during the summers of 1999 and 2000 the death of two dogs was attributed to blue-green algae poisoning, after drinking large amounts of contaminated water directly from the lake.

Blue-green algae can become very abundant in some sections of Lake Champlain once the water warms up in mid-summer. Particular problem areas are Missisquoi Bay and St. Albans Bay. Under calm conditions, blue-green algae can accumulate in thick layers at the surface or along the shoreline. These accumulations are frequently referred to as “blooms” or “scums.”

While blue-green algae toxins have been detected at many locations in Lake Champlain, the highest concentrations of toxins are usually found in blooms and shoreline scums. These dense accumulations pose the greatest potential risks to people and pets.

Weather influences where blue-green algae will accumulate in a given location. During extended periods of calm and sunny days, blooms can accumulate at the surface in any location. Wind and waves may cause them to form along shorelines or in protected areas. Shifts in wind direction can move a bloom from one location to another. Periods of cool rainy weather can often lead to the disappearance of a bloom.