A cyanobacterial bloom - what to do and how to prevent it
In summer, a greenish “scum” on the water and streaks by the bank are often a cyanobacterial bloom - a phenomenon that can threaten the health of people and animals, because cyanobacteria produce toxins. It is worth knowing what to do in the moment and how to reduce the risk over the longer term.
An informational article. On matters of bathing sites and safety, follow the announcements of the services (including the sanitary inspectorate) and local managers.
How to recognise it
- a green/blue-green scum or streaks, water like “pea soup”, an unpleasant smell,
- a sharp fall in transparency (see the Secchi disc),
- it is favoured by warm, calm, sunny weather and fertile water - see bloom monitoring.
Not every green water is cyanobacteria - and what is floating decides whether people have to be taken out of the water or the bank simply needs clearing. Four things are confused most often:
| What you see | It is probably | Field test | Risk |
|---|---|---|---|
| Suspension like “pea soup”, scum or streaks on the leeward bank, unpleasant smell | a cyanobacterial bloom | a stick dipped in the scum cannot be wound - the mass breaks up and runs off | toxins; take people and animals out of the water |
| Green threads, “wool”, tufts on the bottom and on plants | filamentous algae | the threads wind around the stick | usually no toxins; an aesthetic and oxygen problem |
| Tiny green leaves covering the surface | duckweed (a plant) | a single leaf has a rootlet, you can lift it in your hand | no toxins |
| Yellow dust by the bank, in spring | tree pollen (e.g. pine) | disappears in a few days, does not grow | none |
The glass test (indicative): leave water with the bloom overnight in a glass in the fridge. Cyanobacteria have gas vesicles and will gather in a green layer at the surface; green algae will settle to the bottom. This is a field hint, not an identification - certainty comes from a microscope or a laboratory.
What to do in the moment
- Limit contact - do not bathe and do not let animals bathe in water with a bloom; do not drink or use it without treatment.
- At a bathing site - follow the announcements and bathing bans; report a noticed bloom to the manager.
- In a pond - aeration and improving the mixing of the water can help; avoid “miracle” chemicals without consultation (they can make the situation worse, e.g. by releasing toxins). More on algae in small water bodies: algae in a pond.
- Observe - a bloom can move (wind); note and document it.
If you have a measurement. For recreational waters WHO uses a three-step alert scheme (Alert Levels Framework, 2nd ed., 2021): the vigilance level is 4 mm³/l of cyanobacterial biovolume or 12 µg/l of chlorophyll-a, and Alert Level 2 - the level at which contact with the water is restricted - is 8 mm³/l or 24 µg/l of chlorophyll-a (the thresholds are derived from the recreational guideline value for microcystin-LR: 24 µg/l). Chlorophyll is a proxy for the toxin here, so it only counts when cyanobacteria dominate. Treat a visible scum where people bathe as an exceedance regardless of the measurement; more in bloom monitoring.
How to prevent it in the long term
Cyanobacteria are driven by an excess of nutrients, so the key is limiting the inflow of nutrients:
- less fertilisation and run-off from fields, efficient sewage management,
- strips of vegetation along the banks (a buffer), protection of the catchment,
- in water bodies - measures limiting the release of phosphorus from sediments.
This is a fight with the cause (eutrophication), not just with the symptom.
Sources
- Recognition and response to blooms - Toxic Cyanobacteria in Water, 2nd ed., WHO 2021 (the reference handbook for public health; it describes, among other things, the release of toxins after a bloom collapses). Chapter 5, with the Alert Levels thresholds, is openly available: cdn.who.int - chapter 5 (PDF) (checked 19 August 2026). ⚠️ Always check the year of the edition - the 1st edition of 1999 circulates under the same title, with thresholds that WHO has replaced.
- Bathing waters in the EU are governed by Directive 2006/7/EC (checked 2026-08-13); decisions on bathing bans are taken by the competent national or local health authority - follow its notices rather than your own measurement.
- The field tests (stick, glass) are long-used recognition hints of water and environment agencies, not methods of determination - they give a lead, not a result.
In practice
Early detection gives an advantage. Systematic monitoring of transparency, chlorophyll and weather lets you anticipate the risk. In LimnoLog you can keep these indicators together (with weather from connectors), and share the current picture via a public link - e.g. with a bathing-site community.
Calculate it yourself
- 🧮 Carlson TSI calculatorTrophic state index from transparency, chlorophyll-a and total phosphorus.
See it in the LimnoLog app
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