Decision support tool

Quality targets

The occurrence of cyanotoxins is the last symptom in a chain of causes (Fig. 1). Cyanotoxins are produced by cyanobacteria, the occurrence of which depends on nutrient concentrations (primarily phosphorus, rarely nitrogen) in a water body. The concentration of nutrients in the water body is often determined by the nutrient loads from the catchment.

Therefore, reducing high phosphorus loads, being the fundamental cause of the cyanotoxin risk, is the most sustainable control measure. However, this cannot always be achieved to the necessary extent or sufficiently quickly, and then control measures on the following levels of the chain of causes are necessary. To avoid hazardous cyanotoxin concentrations the following quality targets on the single levels are suitable:


For microcystins in drinking water a provisional guideline value of 1 µg/L has been published by the WHO. For bathing water usually higher microcystin concentrations are acceptable. For other toxins no guideline values have been established, although for cylindrospermopsin a guideline value of 1 µg/l has been proposed. More information here.


1 mm³/L biovolume [1] of cyanobacteria and/or 3 µg/L chlorophyll-a [2] when cyanobacteria are dominant, derived from a toxin (µg) to biovolume (mm³) relation of 1 or toxin (µg) to chl-a (µg) relation of 0.3. More information here.

Nutrient concentration (phosphorus)

The target concentration for total phosphorus (TP) in a water body to control cyanobacterial growth range from 10 — 50 µg TP/L depending on other water body conditions. More information here.

Nutrient loads (phosphorus)

The critical TP-load to achieve a target concentration of total phosphorus can be calculated from the water retention time or water exchange rate and the average depth of a water body. More information here.

If it is foreseeable that the target concentrations for cyanotoxins cannot be met by controlling the chain of causes, human exposure should be avoided: for drinking water the treatment shall be optimized to remove cyanotoxins from the water, and for recreational use of water bodies the public shall be informed in case of mass developments.

[1] Determination of the biovolume

The biovolume is determined as follows:

  1. Microscopic cell counts
  2. Microscopic measurement of relevant cell dimensions
  3. Cell volumes are determined by assuming idealised geometric bodies
  4. Calculation of the biovolume: biovolume per L = cell numer per L multiplied by the avaerage cell volume

[2] Chlorophyll-a

Alternatively, the chlorophyll-a concentration can be used as surrogate for phytoplankton biomass and also as surrogate for cyanobacterial biomass if microscopy indicates that the biomass consists largely of cyanobacteria.