ORC warns of possible emergence of toxic algal bloom

The Otago Regional Council (ORC) is warning people to be wary of the possible presence of toxic algae in rivers and lakes throughout the Otago region this summer.

ORC director of environmental information and science John Threlfall said toxic algae occurs naturally in a wide range of water quality conditions, including relatively clean waters. It is more likely to be present during the summer months than at any other time of the year.

In rivers, Phormidium is a common toxic algae which typically forms a thick brown-black slimy algae mat found on large rocks, stones, and cobbles, Dr Threlfall said

If the mats detach, they may float downstream to become caught up in other debris such as sticks. When the Phormidium mats die and dry out, they become light brown or white. Either fresh or dried, it can be highly toxic and harmful to humans and animals.

This is especially true for dogs, as they are particularly susceptible to poisoning from both mat-forming and free-floating toxic algae.

“Many dogs enjoy being in the water and can eat these algae intentionally or by accident, which can have tragic results,” Dr Threlfall said.

Symptoms of poisoning in animals exposed to the type of toxins present in Phormidium mats include lethargy, muscle tremors, fast breathing, twitching, paralysis, and convulsions. In extreme cases, death can occur within 30 minutes after signs first appear.

Toxic algal blooms are commonly blue-green in lakes or still water, but can also be red or yellow.
They typically form thick scums on the surface, and may also develop foams at the water's edge. In Otago toxic algal blooms in lakes do not occur regularly.

If an algal bloom is present, people have been known to develop allergic reactions such as asthma, eye irritations, rashes, blistering around the mouth and nose, and gastro-intestinal disorders, including abdominal pain, cramps, and diarrhoea.

Dr Threlfall said any reaction depends on the type of toxic algae, the type of toxins present, and the concentration of the toxin in the water.

“The higher the concentration of algae toxins and the longer the contact with the water, the more severe the symptoms are likely to be,” he said.

Where humans become ill or are suspected of being ill after swallowing water containing algal bloom, medical advice should be sought immediately. Likewise, if animals become sick, a veterinarian should be contacted.