Cyanobacteria (blue-green algae)

Cyanobacteria (commonly known as blue-green algae) are photosynthetic prokaryotic organisms that are integral parts of many terrestrial and aquatic ecosystems. In aquatic environments under favourable conditions, cyanobacterial cells can multiply and form planktonic (suspended in the water column) blooms or dense benthic (attached to the substrate) mats.

An increasing number of cyanobacterial species are known to include toxin-producing strains. These natural toxins, known as cyanotoxins, are a threat to humans and animals when consumed in drinking water or coming into contact with the skin during recreational activities.

Benthic cyanobacteria (phormidium) is very different to blue-green algae. It forms dense black shiny mats, typically on rocks in stony river beds but can also grow on sandy substrates. It is difficult to spot and is very toxic to dogs. Although the risk is higher in summer, algae can be present in Northland waterways all year round.

Planktonic cyanobacteria

Cyanobacteria floating on water.Planktonic (free floating) cyanobacteria at Lake Omapere.

Benthic cyanobacteria

Cyanobacteria on rock in river (Photo Horizons RC).Benthic cyanobacteria growing on a rock. (Photo: Horizons).

Algal activity occurring in Northland

Northland Regional Council has undertaken sampling and advises:

During recent routine monitoring of the Lake Omapere outlet, Northland Regional Council staff observed early signs of algal formation on Lake Omapere. Water samples were taken from the lake and sent for testing on the 23 September 2019.  Results from this testing returned low levels of planktonic cyanobacteria. 

Further monitoring undertaken on Lake Omapere on 7 October 2019 identified that the algal formation has increased significantly, and the lake appears to be experiencing a cyanobacterial bloom.  Recent wind direction has kept the cyanobacteria away from the lake outlet, however a wind change direction predicted later this week may see the wind shift to a easterly direction.  It is likely this wind change will concentrate the cyanobacteria on the western side of the lake, where the lake outlet is located.  Additional sampling is scheduled this week and results will be distributed as soon as they become available. 

Frequently asked questions

Download our Cyanobacteria - FAQs document (PDF, 343KB) 

Report it

If you suspect a waterbody is experiencing a cyanobacterial bloom and there are no signs placed near the waterbody informing there is one – please contact the NRC 24hr Environmental Hotline to report it : 0800 504 639.

Cyanobacteria Monitoring Report - 8 October 2019

Guidelines Location Biovolume (mm3/L) Date sampled

Surveillance (green mode)

  • Situation 1: The cell concentration of total cyanobacteria does not exceed 500 cells/mL.
  • Situation 2: The biovolume equivalent for the combined total of all cyanobacteria does not exceed 0.5mm3/L.

Lake Omapere



Alert (amber mode)

  • Situation 1: Biovolume equivalent of 0.5 to < 1.8 mm3/L of potentially toxic cyanobacteria; or
  • Situation 2: 0.5 to < 10 mm3 /L total biovolume of all cyanobacterial material.




Action (red mode)

  • Situation 1: ≥12 μg/L total microcystins; or biovolume equivalent of ≥1.8 mm3/L of potentially toxic cyanobacteria; or
  • Situation 2: ≥10 mm3/L total biovolume of all cyanobacterial material; or
  • Situation 3: cyanobacterial scums consistently present.



Monitoring sites

Map showing lakes monitored for cyanobacteria.