Blue-green algae vs green algae

The key difference

Green algae are true algae, simple plants that grow as threads, clumps and films. They are a normal part of most lakes and ponds and aren't known to make toxins. Blue-green algae are cyanobacteria: tiny bacteria that float in the water and can multiply into dense blooms in warm, calm weather. Some of them make toxins that can harm people and kill dogs.

That difference in form is the clue. Green algae grows as structures you can pick up. Blue-green algae is made of tiny particles suspended in the water, so it spreads like paint or makes the water look like soup.

Side by side

The stick test

A rule of thumb many lake managers use: push a stick into the growth and lift it out. If strands drape over the stick like wet hair, it's likely green algae. If the stick comes out coated as if dipped in paint, with no strands, it may be blue-green algae. Use gloves or a long stick, don't touch the water, and keep dogs back while you do it.

The test only hints at what kind of growth it is. It can't tell you whether toxins are present, and the two often grow together. If there's any doubt, treat the water as unsafe and check the latest report for the lake.

Other lookalikes

• Duckweed: tiny floating plants with separate round leaves you can see individually. Often covers still ponds completely.
• Pollen: a yellow or yellow-green dusty film in spring that collects along shorelines.
• Plant debris and sediment: brown water after storms is usually mud, not algae.

For what a toxic bloom looks like in more detail, read what does toxic algae look like?

Questions

Is blue-green algae actually algae?

Not quite. Blue-green algae are cyanobacteria, a kind of bacteria that photosynthesise like plants. They are called algae because they live in water and look like it. Some kinds make toxins; true green algae don't.

Does the stick test prove algae is safe?

No. Lifting the growth on a stick is a rule of thumb for telling stringy green algae from blue-green algae, but it can't tell you whether a bloom is toxic, and both kinds can grow together. Only a lab test can.

Is green algae harmful to dogs?

True green algae isn't known to be toxic, but it often grows alongside blue-green algae and you can't easily tell them apart. Keep dogs away from any thick algae, scum or mats.

Why is my pond green?

Most green ponds are coloured by tiny green algae or floating plants like duckweed, which are usually harmless. A pond that suddenly turns bright green or blue-green, smells bad or has a paint-like scum may have a blue-green algae bloom.