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:

Results from routine planktonic cyanobacteria monitoring during March have returned low levels of cyanobacteria. The last day of scheduled cyanobacteria monitoring this season is to be undertaken on 16 April 2019.

Benthic cyanobacteria was identified in the Waitangi river during a routine water quality sampling event. Samples of the benthic cyanobacteria were taken and tested for cyanotoxins. The testing identified the presence of Anatoxin. The levels of Anatoxin found are considered low and below drinking water standards. Rainfall received after the sampling event appears to have flushed the cyanobacteria away, however further monitoring will be undertaken this week.

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 April 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 Ngatu
Lake Waiparera
Lake Waiporohita
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.