A warning has been issued after potentially toxic algae was found in the Whakatane River around Ruatoki Bridge.
Routine monitoring by the Bay of Plenty Regional Council identified potentially toxic algal mats covering the bed of the river. Warning signs have been placed at the area most affected.
People are advised to avoid contact with that section of the 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," Medical Officer of Health Dr Jim Miller said.
Cyanobacteria toxins can trigger asthma and hayfever attacks in susceptible people, 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 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," Dr Miller said.
As a similar problem could happen in other areas of the river at any time, Dr Miller advised people to make their own assessment of the river and avoid diving in if black and brown algal mats were 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 said.
Anyone suffering illness after contact with the Whakatane River should seek medical advice.
For more information and images of toxic algae mats go to www.ttophs.govt.nz/recreational - water or www.boprc.govt.nz/environment/rivers-and-drainage/rivers-health-warning.