A health warning has been issued for Lake Rotorua and the Ōhau Channel, with the Bay of Plenty Regional Council reporting a bloom of potentially toxic blue-green algae.
Currently Holdens Bay, the Ōhau Channel and the eastern side of the lake are most affected, but the bloom could rapidly shift or affect other parts of the lake.
Sampling and testing is under way for the algae, also referred to as Cyanobacteria.
"This health warning means that people should avoid any activity which results in contact with the lake water or channel water," Toi te Ora medical officer of health Dr Neil de Wet said.
"With high levels of blue-green algae in the lake, activities such as swimming or water skiing could expose people to toxins when in the water, as well as through inhaling droplets of water spray."
Contact with water affected by blooms of blue-green algae can cause asthma and hay fever attacks in some individuals.
Contact with the blue-green algae can also cause skin rashes, stomach upsets and in some cases neurological effects such as tingling around the mouth, headaches, breathing difficulties and visual problems.
Signage is being erected at Lake Rotorua advising potential lake users about the algal bloom.
As we head into the warmer months, Dr de Wet advised people to keep an eye out for signs of algal blooms if using lakes in the Bay of Plenty and Lakes districts.
"Algal blooms can develop rapidly and unpredictably and can sometimes produce toxins that are harmful for people as well as animals.
"If the water looks discoloured, has an unusual smell or has green or brown particles floating in it then it is best to avoid contact with it."
The combination of the recent warmer temperatures and an out-of-action dosing plant has likely contributed to the potentially toxic algae bloom, the Bay of Plenty Regional Council believes.
Bay of Plenty Regional Council Rotorua catchments manager Helen Creagh said people should take care if they were fishing for trout near the affected locations and follow advice from Toi Te Ora Public Health around whether it is safe to swim in the lake.
"At this point, we would advise the community to avoid fishing for trout near the affected locations," Creagh said.
Cyanobacteria are a natural occurrence but can form blooms in response to warmer temperatures and excess available nutrients.
Some species produce toxins as they die, which may cause illness in humans and pets that drink or swim in affected water. Symptoms range from nausea and diarrhoea to more serious conditions such as liver damage.
Bay of Plenty Regional Council manages two alum dosing plants, located near the Utuhina and Puarenga streams.
Maintenance work is taking place on the Puarenga plant. So far, the dosing plants have been successful at improving water quality in Lake Rotorua.
Creagh said the plant would be operational before Christmas and dosing would be increased in response to this type of bloom, however, until the plant is operational more blooms were likely.
Alum dosing refers to treating incoming streams with alum, which helps reduce the impacts of phosphorus in certain lakes.
The continual low dose of alum binds phosphorus in the incoming streams so when the streams reach the lake, plants (like algae) cannot use the phosphorus to grow.
Creagh said it was likely with climate change predictions of higher ambient temperatures, this type of bloom could occur regularly.
"We will continue to monitor the situation and any updates will be available on the Rotorua Te Arawa Lakes Programme's website, as well as Toi te Ora Public Health," Creagh said.
Up-to-date information on health warnings for the Bay of Plenty and Lakes districts is available through these channels:
• Phone: 0800 221 555
• Website: www.toiteora.govt.nz/health_warnings
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• Twitter: www.twitter.com/toiteora
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