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).

Algal activity occurring in Northland

Northland Regional Council has undertaken sampling and advises:

Routine samples are taken from these lakes: 

  • Lake Ngatu
  • Lake Waihoropita
  • Lake Omapere
  • Lake Waiparera

Other lakes are monitored where a bloom is suspected. Latest results are displayed in the monitoring report table below.

Lake Rotopotaka (Coca Cola lake) Cape Karikari

Elevated cyanobacteria levels have been confirmed within Lake Rotopotaka. The lake has been closed by the Northland District Health Board. 

Benthic cyanobacteria

Cyanobacteria mats have been identified throughout Northland due to the low flows currently being experienced. It is likely that as rains come and flows start to increase, these mats will become dislodged where they may become deposited along river/stream banks and more. They are particularly attractive to dogs as they have a strong musty odour and can be fatal if eaten. They can also cause skin irritation to humans.

Frequently asked questions

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

Cyanobacteria monitoring report (Last updated 28 February, 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 Ngatu

Lake Waiparera





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 Omapere

Lake Waihoropita

Lake Rotopotaka (Coca cola lake)







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.



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 Northland displaying sites monitored for cyanobacteria.