Cyanobacteria are an ancient group of organisms with characteristics in common with both bacteria and algae.
The naturally occurring cyanobacteria are widespread in many lakes and rivers in New Zealand.
Under favourable conditions, cyanobacteria cells can multiply and form suspended planktonic blooms in slow-moving water such as lakes, or thick mats attached to benthic substrates such as rocks in river beds.
Some cyanobacteria species have the ability to produce natural toxins called cyanotoxins, which are potentially harmful to people and animals. If potentially toxic cyanobacteria are abundant, you should presume that the water is unsafe for contact recreation or consumption.
Types of cyanobacteria
While planktonic (suspended) and benthic (stream bed) cyanobacteria share similar traits, their visual appearances are quite different.
Planktonic cyanobacteria blooms are not confined to lakes nor are benthic mats confined to rivers, but this is where we most often see them.
Planktonic cyanobacteria in lakes
Planktonic cyanobacteria are algae that suspend in the water column of slow moving waters such as a lake. The species that cause toxic blooms are Nodularia, Anabaena, Microcystis and Picocyanobacteria.
Nodularia, Anabaena and Microcystis generally form a thick, bright green colouration to water and can produce scums on the water surface.
Picocyanobacteria, which is a major species causing potentially toxic blooms in Te Waihora/Lake Ellesmere, often do not have obvious visual traits.
Benthic cyanobacteria in rivers
Benthic cyanobacteria is attached to substrates such as gravels, cobbles and boulders on a river bed. Its appearance depends on species. Phormidium occurs naturally throughout Canterbury and in flowing water can form thick, dark brown/black cohesive mats. The species may detach from the riverbed and become caught in other debris in the river.
When Phormidium mats die and dry out, they become light brown or white in colour, but may still be toxic. Nostoc is another mat-forming benthic cyanobacteria. It forms small globular mats or gelatinous ‘bobbles’ firmly attached to hard substrates and is relatively translucent in colour.
The more brightly-coloured long filamentous green algae that are commonly found in rivers and streams are harmless as they do not produce toxins.
For more information download our cyanobacteria factsheet (PDF File, 907.85KB).