What Is Blue-Green Algae? (Cyanobacteria Explained)
Blue-green algae are not algae at all — they are cyanobacteria, ancient photosynthetic bacteria that have been on Earth for 3.5 billion years. They are naturally present in most lakes at low levels. When phosphorus, warm water, and calm conditions stack up, they can multiply explosively into visible blooms that look like spilled green paint or pea soup. Some species produce toxins capable of causing skin rashes, GI illness, and liver damage.
Cyanobacteria, not algae
The "blue-green algae" name is a hold-over from when biologists classified them by appearance. Genetically, cyanobacteria are bacteria that happen to photosynthesize like plants. They produce their own oxygen — in fact, cyanobacteria are responsible for the oxygenation of Earth's atmosphere two billion years ago.
There are dozens of bloom-forming species in North America. The most common are Microcystis, Anabaena, Aphanizomenon, and Planktothrix. Each can produce different toxins.
How to identify a bloom
A blue-green algae bloom usually looks like one of these:
- Water that resembles spilled green paint or pea soup
- A thick, scummy floating mat on the surface, especially against the downwind shore
- Bright blue-green or turquoise streaks in the water
- A strong musty, earthy, or sewage-like odor
- Dead fish along the shoreline (a late-bloom symptom)
The toxins
Cyanobacteria can produce three main classes of toxins. <strong>Microcystins</strong> are liver toxins, the most common in North American freshwater blooms. <strong>Anatoxin-a</strong> is a neurotoxin — fast-acting and the cause of most dog deaths attributed to lake water. <strong>Cylindrospermopsin</strong> is a kidney/liver toxin produced by some species.
Symptoms in humans include skin rash, nausea, vomiting, diarrhea, headache, and in severe cases liver damage. Symptoms in dogs include vomiting, drooling, weakness, seizures, and death within hours of ingestion. Children and pets are at the highest risk.
Reading lake data for cyanobacteria risk
Two numbers predict cyanobacteria bloom risk on a LakeQuality lake page:
- Chlorophyll-a above 20 µg/L — algae are already abundant; conditions can shift toward cyanobacteria fast.
- Phosphorus above 30 µg/L — bloom fuel is loaded.
- Trophic State Index above 60 (eutrophic) — chronic bloom risk; above 70 (hypereutrophic) is bloom-prone every summer.