Cyanobacteria and algal toxins
Blooms of cyanobacteria in nutrient-enriched lakes and reservoirs can release potent toxins, principally microcystins. Blooms are becoming more frequent as warmer water and continued nutrient loading combine.
Key facts
- Contaminant group
- Biological
- World Health Organization — guideline
- 1 µg/l for microcystin-LRFor lifetime exposure; a higher short-term value applies during bloom events.
- European Union — legal limit
- Monitoring required where blooms occur
Where it comes from
- Nutrient enrichment from agricultural runoff and wastewater
- Warm, calm, stratified conditions in reservoirs and lakes
- Low flow conditions in slow-moving rivers
How it behaves in water
Toxins are largely contained within cells while a bloom is healthy, and are released in bulk when cells rupture. This makes treatment that lyses cells — such as aggressive pre-oxidation — actively counterproductive if applied before the cells are physically removed.
How it is measured
Microcystin-LR measured in micrograms per litre by ELISA or LC-MS. Cell counts and chlorophyll-a serve as operational early indicators.
Regulatory position
| Jurisdiction | Value | Status | Notes |
|---|---|---|---|
| World Health Organization | 1 µg/l for microcystin-LR | guideline | For lifetime exposure; a higher short-term value applies during bloom events. |
| European Union | Monitoring required where blooms occur | legal limit | — |
Table sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- European Commission — Drinking Water Directive (EU) 2020/2184 and reporting. Open — attribution required · European Commission reuse policy (Decision 2011/833/EU)
- Drinking Water Inspectorate — Drinking water quality in England and Wales. Open — attribution required · Open Government Licence v3.0
- United States Environmental Protection Agency — Safe Drinking Water Information System. Open · US Government work — public domain
Health context
Microcystins are hepatotoxic. Acute exposure through drinking water has caused serious illness, and the most severe recorded incident involved contaminated dialysis water in Brazil in 1996.
What removes it
| Row | Effective | Not reliably effective |
|---|---|---|
| 1 | Removing intact cells by coagulation and filtration before any oxidation step | Boiling, which releases intracellular toxin and concentrates the dissolved fraction |
| 2 | Activated carbon for dissolved toxin | Chlorination alone at typical doses |
| 3 | Ozone and advanced oxidation for dissolved toxin | — |
| 4 | Reservoir management to reduce nutrients and disrupt stratification | — |
Related intelligence
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- Peer-reviewed scientific literature — Open-access hydrology, water chemistry and water use research. Public but restricted · Publisher terms — public access, reuse not clearly granted