Six things turn a lake green. Only one of them is dangerous, and it is not the one that looks worst.

Every summer, somebody stands on a dock, looks at water that has gone an unfamiliar colour, and asks the internet whether it is safe. The internet says algae. That is technically true and practically useless, because "algae" covers everything from a harmless dusting of tree pollen to a bloom that can hospitalise a dog.

Here is how to tell them apart.

1. Pollen

Late spring, a yellow-green film that collects along the downwind shore and in corners. It sits on the surface, moves with the wind, and disappears in a week. If you can see individual grains or it dries to dust on the dock, it is pollen. Harmless.

2. Filamentous algae

Green, stringy, and attached — mats growing on rocks, docks, and the bottom in shallow water. Often called pond scum. It floats up in clumps as it decays. Unsightly, sometimes a nuisance for swimmers, not a health risk.

3. Duckweed and watermeal

Tiny individual green plants floating on the surface, each smaller than a pencil eraser. Duckweed has a visible leaf; watermeal looks like green sand. They cover sheltered coves entirely and are frequently mistaken for algae. Not toxic.

4. Green algae bloom

The water itself goes green and turbid, like weak paint. Clarity drops. This is planktonic algae suspended throughout the water column, and it signals a lake running high on nutrients. It is a real ecological signal and worth measuring. It is not a toxin risk.

5. Cyanobacteria bloom

This is the one that matters. Cyanobacteria — historically called blue-green algae, though they are bacteria — can produce toxins that affect the liver and nervous system.

Signs to look for: the water looks like spilled paint rather than suspended particles. It may have a blue-green, turquoise, or unpleasant brown-green cast. It often forms a surface scum that collects on the downwind shore. There can be a musty or septic smell. Dead fish or a sick dog are late signals.

6. Something that is not algae at all

Iron bacteria produce a rust-coloured or oily-looking sheen that people sometimes read as a bloom. Sediment suspended after a storm can make water look green-brown. Certain dissolved organics from wetlands tint water tea-green.

Here is the uncomfortable part

You cannot reliably distinguish four and five by looking. Neither can we, and neither can most professionals, from a photograph.

Some cyanobacteria blooms look exactly like ordinary green algae. Some produce no toxin at all. And two blooms with identical appearance can differ by orders of magnitude in microcystin concentration, because toxin production depends on which strains are present and what conditions they are in — not on how bad it looks.

This is why the visual guides you find online stop short of an answer. They are not being coy. Appearance genuinely does not determine toxicity.

What to actually do

If it looks like paint, has a scum layer, or smells wrong: keep people and dogs out until you know. Dogs matter disproportionately — they drink the water and lick their coats, and they are usually the first casualty.

Then test. A microcystin test returns a concentration you can compare directly against your state's recreational advisory threshold. It is the only thing that converts a judgment call into a decision you can defend to your board and your neighbours.

And once the bloom passes, keep measuring. A single bloom is an event. Blooms in the same bay three summers running are a pattern with a cause, and the cause is usually something you can do something about.