Toxic algae

Toxic algae, or cyanobacteria, naturally occur in rivers and lakes throughout Otago, including waterways with very good quality water. It is more likely to be present during summer because of warm weather and low river flows.

Mat forming toxic algae (cyanobacteria) Toxic algae

In flowing rivers, the cyanobacteria forms thick dark brown/black mats. The mats may come loose from the riverbed and float downstream to become caught up in other river debris. When the mats die and dry out, they become light brown or white.

The more brightly coloured long green algae commonly found in rivers and streams are harmless algae that don't produce toxins. 

Algal blooms in lakes or still waters are commonly blue-green, but can also be red or yellow. Algal blooms can give the lake a vivid green colour, appearing like 'pea soup', but can also form films or scums on the water's surface. They typically from thick scums on the surface, and may also develop foams at the water's edge. The algae that commonly grown in lakes are a free-floating form (phytoplankton).


Some cyanobacteria species produce natural toxins, which can be a risk to human and/or animal health if water is drunk, through contact with water, or if the algae are eaten by dogs. 

Not all species produce toxins. Those that can aren’t always toxic to people. The presence of toxins is confirmed by laboratory testing. If you are unsure whether cyanobacteria are toxic or not, it is safest to assume they are.

The toxins can be deadly for dogs. To be on the safe side, do not allow your dog near water with blue-green algae.

You should avoid skin contact with the water and avoid swallowing it – especially water that is bright green, or where surface scums or films are present.

To be on the safe side, avoid swimming. Wearing a wetsuit will not protect you and may cause severe irritation around the collar and cuffs.

Fish caught in waters with abundant cyanobacteria are likely to have a muddy/earthy taste. While you can eat them, it is recommended you do so in moderation (less than one meal per week). You should avoid eating the fish’s liver as toxins may have accumulated there.

Protect your dog from cyanobacteria

Keep your dog safe by avoiding areas where cyanobacteria are present. If you are on a waterway with abundant cyanobacteria, keep your dog out of the water and stop it from eating any algal material from the water or on the banks.

If you suspect that your dog has eaten cyanobacteria, you should treat it like an emergency and contact your veterinarian immediately.

What should I do if I see a cyanobacteria bloom?

If you see anything that you think could be a cyanobacteria bloom, call the ORC Pollution Hotline on: 0800 800 033 (answered 7 days a week) or email us to let us know what you have observed.

Any photographs of suspected algal blooms that you can send us of will help us  to quickly identify the type of algae and notify people of any risk that may be present.

View our toxic algae factsheet

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