Harmful algal blooms (blue-green algae) on lakes

A harmful algal bloom, often called blue-green algae, is a rapid growth of cyanobacteria. Some blooms produce toxins that can make people sick and can kill dogs that swim in or drink the water.

When and where they happen

Blooms are most common in late summer and early fall, in warm, calm, nutrient-rich water. Shallow bays, coves with little flow and lakes fed by farm or lawn runoff are the usual spots. A few calm, hot days in a row can turn a clear cove green.

How to spot one

• Water that looks like pea soup or spilled green paint
• Scum, mats or streaks on the surface, sometimes blue, green, brown or red
• A musty or rotten smell
• Dead fish along the shore

Not every bloom is toxic, and toxic water does not always look bad. You cannot tell by looking.

What to do

• When in doubt, stay out. Keep kids and dogs out of discolored water.
• Do not let dogs drink lake water or lick their fur after a swim near a bloom. Rinse them with clean water.
• Avoid boating or paddling through scum, which throws spray.
• Check your state health or environmental agency. Most states post a map of current bloom advisories and beach closures.

Lakeglance does not track blooms directly. Wind matters, though: a steady breeze can push surface scum into the downwind shore and coves, while rough water mixes it out.