Algal blooms

Summer has arrived. While we get to enjoy warmer weather (for the most part), the rise in temperature causes an increase in the amount of toxic blue-green algae in our waterways, which can be harmful to humans and dogs. Health warnings of algal blooms have already been issued in Selwyn, Waimakariri and Bay of Plenty rivers.

While referred to as toxic algae, it is not algae at all but cyanobacteria, an ancient group of organisms. Cyanobacteria are naturally present in all our waterways, however, when conditions are perfect the cyanobacterial cells multiply and form planktonic (suspended in the water) blooms or dense benthic (attached to rocks) mats. With the increase in cyanobacteria levels there is an increase in cyanotoxins which are released during growth. Consuming or coming into contact with water that contains cyanotoxins is a major health issue for both humans and pets.

“Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips,” says Dr Brunton, Canterbury Medical Officer of Health.


Unfortunately the algae odour can be very attractive to dogs who may consume washed up benthic mats. It can take as little as a teaspoon of potentially toxic cyanobacteria to cause a fatal outcome. Dogs that have been in an area where there is an algal bloom and who start to show signs of illness should be taken to a vet immediately. Treat any potential ingestion or contact as an emergency. In extreme cases, death can occur in just 30 minutes after the first signs of illness. Typical signs of poisoning in your dog may include lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsions.

“People and animals should remain out of the waterways until the warnings have been lifted,” states Dr Brunton.

A good rule of thumb is stay out of water that looks green. If you or your dog come into contact with toxic algae, rinse off in fresh water as quickly as you can and monitor for symptoms. Don’t let your dog lick or nibble at its fur after swimming in a possibly contaminated river or lake either!

Although district or city councils will place warning signs, these may not be seen at the numerous river access points, hence the need for people/dog-walkers to treat every low-flowing river cautiously. Here is what to look out for.


• Black, green or brown slime on rocks, or brown or black mats at the river’s edge that have a velvety texture and earthy/musty smell.
• Mats that come loose can wash up on the river bank. If this happens the mats may dry out and turn a light brown or white colour.
• Check for alerts on the LAWA website, which provides live updates on where it is safe to swim.


• If the water has a pea soup appearance’, it could contain toxic algae. Discoloured, cloudy water with small green blobs suspended in it should be avoided.

Summer algal bloom in river