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Potentially toxic algae found in Whakatane River

Potentially toxic algae found in Whakatane River around Ruatoki Bridge

Routine monitoring by the Bay of Plenty Regional Council has identified potentially toxic algal mats covering the bed of the Whakatane River, near Ruatoki Bridge in the Eastern Bay of Plenty. The local council has placed warning signs at the area most affected.

People are advised to avoid contact with this section of the Whakatane River and be on the lookout for algal mats elsewhere in the river.

Toxic algae (cyanobacteria) often multiply to excessive levels during periods of warm, dry, sunny weather when rivers levels are low. In rivers they can form extensive black or brownish slime-like mats that cling to rocks and logs. Significant clumps sometimes break off and float free, eventually collecting on vegetation at the water's edge.

“The algae that form the mats can release toxins that are potentially harmful to people and animals,” says Dr Jim Miller, Medical Officer of Health. Cyanobacteria toxins can trigger asthma and hayfever attacks in susceptible individuals, as well as causing skin rashes, stomach upsets, and even neurological effects such as tingling round the mouth, headaches, general breathing difficulties and visual problems.

People are advised not to paddle, wade, swim, or participate in any recreational activity that might involve significant contact with the cyanobacteria mats or swallowing of raw river water. The health warning also includes people keeping their pets and livestock out of the river. “Highest risk areas tend to be shallow river margins where infants and dogs are most likely to come in contact with the mats,” says Dr Miller.

As a similar problem could occur in other areas of the river at any time, Dr Miller advises people to make their own visual assessment of the river and avoid diving in if black and brown algal mats are found covering large areas of the river bed. People should also avoid swimming in or drinking river water that has a strange musty smell. “If in doubt, go somewhere else,” he says.

Anyone suffering illness after contact with the Whakatane River should seek medical advice.

For more information and images of toxic algae mats go to: or

Cyanobacterial mats vary from dark brown/black and are moss like in appearance, thickness and colour (black to dark green) but have a much slimier texture and glisten when exposed to air. In shallow areas the mats may appear bleached and take on a golden brown colour. The mats are easily dislodged from the riverbed and form floating ‘rafts’.


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