Cyanobacteria are an ancient group of organisms with characteristics in common with both bacteria and algae. They are often referred to as blue-green algae. Routine monitoring for the presence of potentially toxic cyanobacteria is carried out at a number of popular recreation sites throughout Canterbury during summer.

Potentially toxic cyanobacteria explained

Cyanobacteria are widespread in many lakes and rivers in New Zealand, and are found in a wide range of water quality conditions, including relatively clean waters.

Under favourable conditions, cyanobacteria cells can multiply and form planktonic (floating or suspended) blooms in slow-moving water such as lakes or thick mats attached to benthic substrates such as river and stream beds.

Some cyanobacteria species have the ability to produce natural toxins called cyanotoxins which potentially affect people and animals if present in drinking water or if people and animals come into contact with water during recreational activities.

Planktonic cyanobacteria blooms are not confined to lakes — nor are benthic mats confined to rivers, but this is where we most often see them.

  • The algae occur naturally all year round, but can increase in numbers rapidly during warmer months.
  • If the water is cloudy, discoloured, has small globules suspended in it, or forms coloured scums, avoid all contact.
  • Not all cyanobacterial blooms are visible to the naked eye and toxins can persist after visible blooms disappear.
  • Cyanobacterial concentrations can change quickly with changing environmental conditions, such as wind.
  • If a health warning is in place, avoid contact with the water.

Planktonic cyanobacteria in lakes

Planktonic cyanobacteria is floating algae suspended in the water column of slow moving water such as in a lake. It often produces bright green or blue-green soup-like scums.

Routine monitoring for potentially toxic planktonic cyanobacteria is carried out monthly for Te Roto o Wairewa/Lake Forsyth and Te Waihora/Lake Ellesmere. This increases in frequency when potentially toxic planktonic cyanobacteria are identified.

For planktonic cyanobacteria lake warnings, click here.


Benthic freshwater cyanobacteria Phormidium

Benthic cyanobacteria in rivers

Benthic cyanobacteria is attached to substrate such as a river bed. Its appearance depends on species. Phormidium looks like a thick and velvety black or brown mat and has a musty, earthy smell. Nostoc forms small globular mats.

Routine monitoring for potentially toxic benthic cyanobacteria is carried out at a number of river sites that are popular for recreation throughout Canterbury.

For benthic cyanobacteria river warnings, click here.