Why does a summer pond turn paint-green, and is it safe to touch?
― The most dangerous day is when the green piles up on the downwind shore
That green sheet covering the water is called an algal bloom. It's actually made of tiny organisms that can float themselves upward. Not all of them are toxic. But some species store a toxin inside their bodies that doesn't break down easily, even with boiling. And the real danger doesn't come on the day the green looks thickest — it comes on the day the green has been blown onto the shore.
It's the end of summer, and you're walking past a park pond. The surface looks like someone spilled a can of green paint across it. On the shore where the wind is blowing toward, the green has gathered into a thick, syrupy mass.
Your dog leans in and sniffs that shore. Just the day before, this was plain brown water.
This green isn't dirt or dye staining the water. Every speck of it is alive — the result of countless organisms floating themselves up.
There are only two reasons
The cyanobacteria that make up an algal bloom carry tiny gas-filled chambers inside their bodies. Like a submarine that has dropped its ballast, they rise slowly to the surface. That's why the green doesn't scatter through the water — it collects in a thin layer right at the top.
Anything floating on the surface gets pushed by the wind toward the downwind shore. Green that was once spread thinly across the whole pond becomes dozens of times more concentrated right at the water's edge. Since the toxin is inside the cells, concentration is danger.
Let's take these in order, starting with why the green collects at the surface at all.
Why does the green only gather at the surface?
Most of the cyanobacteria that form algal blooms carry tiny gas-filled compartments inside their cells, called gas vesicles. Because of these, their bodies are lighter than water. Anything lighter than water moves upward through it — so even if wind stirs them through the water column overnight, once the wind dies down, they drift back to the surface within a few hours.
As calculated in the collapsible section below, this rising speed works out to a bit over one metre per hour. In a pond roughly two metres deep, that's fast enough for most of the cells to reach the surface in a single night. This is thought to be why a pond often looks greenest first thing in the morning.
Then wind gets added to the mix. Look at Figure 1. Cells that have floated up get pushed by the wind toward the downwind shore. What looks thin in the middle of the pond becomes as thick as paint right at the water's edge. It's usually that concentrated shore that people and dogs end up touching.
Why is thick green dangerous?
Among the cyanobacteria that form algal blooms, several species produce toxins. The best known damages the liver. This toxin is said to resist ordinary boiling, so boiling the water does not make it safe.
The tricky part is that most of the toxin stays locked inside living cells. So the color of the water and the amount of toxin don't line up neatly. Even within one pond, only the shore where the green has piled up becomes extremely concentrated.
And it's usually dogs who are affected first. Dogs drink straight from the water at the shore, and after swimming they groom themselves, licking the green off their fur. Because they're smaller, the same dose hits them harder. Overseas, there are reports of dogs dying within hours after swimming in a lake with an algal bloom. In people, symptoms more often show up as itchy skin, sore eyes, nausea, or diarrhoea.
It's not unusual for a bloom to be made of a strain that doesn't produce toxin at all. But you can't tell whether toxin is present just by looking or smelling, and there's no way to test it on the spot. So the rule isn't "learn to tell them apart" — it's "keep your distance."
Nutrients (nitrogen and phosphorus) washing in from farmland and homes build up in a pond and give the cyanobacteria plenty of food. Add strong summer sunlight and a stretch of calm, windless days, and numbers can explode. Ponds that go a long time without rain or fresh water flowing through are especially prone to it.
So what should you do?
- Never put your hands or feet in green waterThe thickest green collects right at the shore. Don't stir it with a stick either.
- Keep dogs away from green waterDon't let them drink it or swim in it. If they get wet, rinse them off with clean water right away.
- If you touch it, wash your hands with clean water immediatelyDon't rub your eyes or mouth — wash first.
If nausea, diarrhoea, severe fatigue, or skin redness appear after contact with green water, see a doctor and tell them when and where you touched the water. If someone becomes confused or starts convulsing, don't hesitate to call emergency services. If a dog shows similar symptoms, get it to a vet immediately. The most reliable approach is simply not to go near a pond or lake with a thick green bloom in the first place. And of course, don't jump in. Where local authorities have closed off an area, follow their instructions.
Summary
That green is made of tiny organisms that can float themselves upward. It gathers at the surface because it floats, and it thickens on the shore because the wind pushes it there. You can't tell by looking whether it's toxic — but you can tell where the danger is, just by checking which way the wind is blowing.
The danger isn't the pond with the thickest green —
it's the shore the green has been blown onto.
For another visible hazard at the water's edge, see Why shouldn't you go near the white foam on a river? And for how nutrients end up flowing into ponds in the first place, see Why does the same rain turn city streets into rivers so fast?
- Photograph a pond with a bloom from both the upwind and downwind shores and compare them. Even in the same pond, the density should look completely different.
- Check the wind direction using a flag or the movement of leaves, and see whether it matches the thicker side. Check again the day after the wind changes direction, to see whether the thick side has swapped.
- Photograph the same spot in the morning and evening and compare. On calm days, the green is often more distinct in the morning.
Please don't scoop up the water or stir it with a stick. Observe from the shore, using only your eyes and a camera.
For readers who want more — terms, formulas, and where this fits in the curriculumWe've labelled each section by level, from middle-school science through university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school "Physics Basics / Biology Basics"
- HS+Advanced high-school content, or textbook sidebar material
- UnivNot covered in high school — university specialist content (fluid dynamics, limnology)
- ResearchNot yet settled even at university level — an open question researchers are actively studying
MSTerminology: this phenomenon has a name
- Algal bloom (aoko): the state where cyanobacteria multiply massively in a lake or pond and cover the surface, as well as a term for the algae itself.
- Cyanobacteria (blue-green algae): a group of bacteria that photosynthesize. They aren't plants, but they do release oxygen.
- Gas vesicle: a tiny gas-filled chamber inside a cyanobacterium's cell. It lightens the body enough to let the cell float upward.
- Eutrophication: the buildup of nitrogen and phosphorus in water, which makes it easier for algae to multiply. It's the foundation an algal bloom builds on.
MSHSWorking it out with a formula: how many metres does cyanobacteria rise in an hour?
This article's main question was "why does the green gather at the surface?" So let's actually work out the rising speed. We'll use the formula for the speed of a small particle moving slowly through water.
| In symbols | v ＝ 2 × g × r² × Δρ ÷ ( 9 × μ ) |
| In words | Rising speed ＝ 2 × gravitational acceleration × radius squared × density difference ÷ (9 × water's viscosity) |
| Where it comes from | It comes from the balance between the upward buoyant force and the downward drag from water's viscosity (Stokes' law). Once they balance, the speed becomes constant and settles at this value. |
| Radius r of a cyanobacteria cluster | 0.0001 m (0.1 mm, a typical value for colony-forming species) |
| Density difference from water Δρ | 20 kg/m³ (roughly 2% lighter than water) |
| Water's viscosity μ | 0.001 Pascal-seconds (water at around 25°C) |
| Gravitational acceleration g | 9.8 m/s² |
| Coefficient times gravity first | 2 × 9.8 ＝ 19.6 |
| Multiply by the density difference | 19.6 × 20 ＝ 392 |
| Radius squared | 0.0001 × 0.0001 ＝ 0.00000001 |
| Multiply that in | 392 × 0.00000001 ＝ 0.00000392 |
| Build the denominator | 9 × 0.001 ＝ 0.009 |
| Divide, giving speed per second | 0.00000392 ÷ 0.009 ≒ 0.000436 |
| Convert to a per-hour figure | 0.000436 × 3600 ≒ 1.57 |
Units: speed in m/s (metres per second), radius in m, density difference in kg/m³, viscosity in Pascal-seconds. The answer is about 1.6 metres per hour. In a pond two metres deep, that means a surface layer can form within a few hours of the wind dying down. A cluster with ten times the radius would rise a hundred times faster — reaching the surface in just a few minutes.
HSHS+What it means that speed scales with the square of the radius
HSBuoyancy is determined by Archimedes' principle and is proportional to volume. Volume grows with the cube of the radius. Viscous drag, on the other hand, is only proportional to the radius to the first power. Working out the balance between them, speed ends up proportional to the square of the radius. That's why species that form larger colonies can rise faster.
HS+Cyanobacteria are thought to adjust their own buoyancy. When light is too strong, they store sugar made through photosynthesis inside the cell, becoming heavier and sinking for a while. Once that sugar is used up in the dark, they lighten again and rise. This daily up-and-down cycle is thought to be part of why a bloom can look thin in the evening and thick again by morning.
UnivWhy the "slowly moving particle" formula applies here
This calculation uses Stokes' law, which only holds in viscosity-dominated flow — that is, where the Reynolds number is much smaller than 1. Plugging in our values gives a Reynolds number of about 0.00004, well within that range. In a real lake, this rising motion is layered on top of wind-driven currents, density stratification caused by temperature differences, and daytime vertical mixing. Research that models the distribution of algal blooms typically builds this rising speed in as one term within an advection-diffusion equation.
📖 For the derivation and further reading: Stokes' law (Japanese Wikipedia) ／ Reynolds number (Japanese Wikipedia)
ResearchWhat's still not fully understood
- Why do they make toxin at all? Whether it's to fend off other organisms, or a tool for handling metals, isn't settled. It's also not fully explained why toxin-producing and non-toxic strains can coexist in the same pond.
- When does the toxin actually enter the water? Most of it stays inside the cells, but it's released once a cell breaks apart. There's evidence that treating a bloom with algicide can actually spike toxin levels temporarily.
- What happens as the climate warms? Higher water temperatures are thought to favour cyanobacteria, but changes in rainfall patterns and nutrient inflow are also involved, and regional predictions aren't settled.
In other words, everything in this article reflects "what's understood so far." For safety decisions, always defer to the latest information from your local authority.
Where this fits in the curriculum (by level)
| Level | Subject / unit | Where it appears in this article |
|---|---|---|
| MS | Science — structure and function of living things / nature and humans | What cyanobacteria are, how eutrophication works |
| HS | Physics Basics (buoyancy) · Biology Basics (ecosystems) | Balance of buoyancy and drag, nutrients and population growth |
| HS+ | Advanced physics (viscous drag) | Why speed scales with the square of the radius |
| Univ | Fluid dynamics · limnology | Stokes' law, Reynolds number, vertical mixing |
| Research | Environmental toxicology · lake ecology | Why toxin is produced, effects of climate warming |
| ― | Everyday relevance | Staying away from the downwind shore, keeping dogs out of the water |
- Ministry of the Environment, Water and Atmospheric Environment Bureau (環境省 水・大気環境局), "Materials on Water Quality Conservation in Lakes and Ponds" (conditions for eutrophication and algal blooms)
- World Health Organization, Guidelines for Drinking-water Quality (chapter on cyanobacterial toxins)
- National Institute for Environmental Studies (国立環境研究所), "Research Report on Algal Blooms in Lakes and Their Toxins"
- Algal bloom / aoko (overview of terminology and mechanism, Japanese Wikipedia)
- Reynolds, C. S., The Ecology of Phytoplankton (chapter on rising speed and vertical distribution)
※This article is a general-audience science explainer. The figures given are approximate, meant to help illustrate the underlying mechanism. If you feel unwell, see a medical professional; for access to bodies of water, follow the instructions of your fire department or local authority.