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

Planktonic cyanobacteria.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).

Lakes contact recreation sampling programme

Northland Regional Council undertakes routine algal sampling during the bathing months, October through to April each year. Routine samples are taken from these lakes: 

  • Lake Ngatu
  • Lake Waihoropita
  • Lake Omapere
  • Lake Waiparera
  • Lake Rotopokaka (Coca cola)

Other lakes are monitored where a bloom is suspected. 

Frequently asked questions

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

Cyanobacteria monitoring report - 29 October 2020

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

Lake Ngatu

Lake Waiparera

Lake Rotopokaka (Coca cola)









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.
Lake Waiporohita 0.13 12/11/2020*
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.

*Further toxin testing for Lake Waiporohita was undertaken and the results came back as below detection limits.

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 our 24/7 Environmental Hotline to report it : 0800 504 639.

Monitored sites

Map of displaying lakes in Northland monitored for cyanobacteria.