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Lake Ngā Roto

A public health warning has been issued for Lake Ngā Roto in Te Awamutu due to a significant rise in cyanobacteria levels. Click here for more information.

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Ngā Roto Public Health Warning

December 4, 2024: Ngā Roto - significant rise of cyanobacterial levels

A Public Health Warning was issued for Lake Ngā Roto in Te Awamutu by Health New Zealand, Te Whatu Ora on December 4, 2024, due to a significant rise in cyanobacteria levels.

We strongly recommend that people avoid any recreational activities that involves contact with the water. People using boats are also advised to stay off the lake, because of the risk of contact with the water.

Cyanobacteria can release toxins in the water that may be harmful to humans and animals. If people come into contact with the water while warnings are in place they should shower and change clothing. Dogs should be kept away from the water, as dogs are particularly sensitive to cyanobacteria.

Seek medical or veterinary advice immediately if illness occurs.

Waikato Regional Council are regularly monitoring and testing the water quality of the lake.

Up-to-date information on cyanobacterial cell counts is available from local councils, Waikato Regional Council or by visiting the Land Air Water Aotearoa (LAWA) website at www.lawa.org.nz.

Background

Ngā Roto is a peat lake, formed over thousands of years from unique communities of plants that exist in few other places and are adapted to low nutrient conditions and water logged soils. Peat lakes are important for this uniqueness, for their cultural and spiritual value to tangata whenua, and the recreational opportunities they provide. However, Ngā Roto has changed considerably, and is now very different than it was 100 years ago.

In terms of lake health, Ngā Roto is currently super-enriched, high in nitrogen and phosphorus, which stimulates boom and bust cycles of blue-green algae (cyanobacteria). As large numbers of algae die, lake water is depleted of oxygen and the chemistry of the water changes, which can trigger mass fish deaths as seen in the summer of 2021/22. Algae produce toxins that are harmful to humans and animals, especially dogs.

A further complexity is the bacteria such as E. coli that enter the lake from run off and farm drains in the catchment. High algae and bacteria levels can lead to the lake being closed for contact recreation such as swimming, sailing or rowing.

The lake is also turbid, or brown and cloudy from sediment that has entered the lake in run off from surrounding land. Pest fish such as catfish and koi carp feed, disturbing the lake bottom and margins, stirring the sediment and preventing it from settling. It has been estimated that Ngā Roto has 8.9 tonnes of catfish, koi carp, rudd, gambusia, and gold fish. As well as their effect on water quality, these fish eat and compete with native fish and tuna. Cloudy water and the pest fish prevent any aquatic plants from growing. These plants could capture nutrients and release oxygen which would assist in improving the water quality if they were able to flourish.

More information can be found here: www.waikatoregion.govt.nz/lake-ngaroto

Restoring Ngā Roto

Over the past 25 years, Waipā District Council and partners have invested time, money and resources towards improving the current water quality at Ngā Roto.

For more information on what we have done visit here.

Potentially toxic algae, known scientifically as cyanobacteria, are naturally occurring in rivers and lakes in New Zealand, including waterways with good water quality. Most of the year it is present in lakes at low levels that are not much of a danger. But algae can be a problem during the summer months, when warm temperatures and nutrients in the lake create an environment where it can thrive, forming extensive blooms which can be toxic to humans, dogs, livestock and wildlife.

Scientists do not yet understand when and why this alga turns toxic, so to be safe, always treat it as toxic.

Dogs are particularly susceptible to poisoning from toxic algae as they may drink lake water or play in the water.

Optimal conditions for cyanobacterial blooms occur when a perfect storm of nutrients from surrounding farmland combine with sunlight and warm water temperatures in the shallow peat lake. This environment also promotes bacteria such as E. coli to multiply.

Cyanobacteria are a form of microscopic organisms that can produce toxins harmful to the health of humans and animals exposed to or swallowing the water where the organisms are growing.

You can use the walking track at Lake Ngā Roto but Water based activities should not be undertaken during this time.

If people come into contact with the water while warnings are in place they should shower and change clothing. Dogs should be kept away from the water, as dogs are particularly sensitive to cyanobacteria.

Symptoms in humans may not appear until sometime after contact with the affected water. Symptoms include tingling and numbness around the mouth and fingertips. Rashes, skin and eye irritation; allergy symptoms such as hayfever and asthma; nausea and possibly stomach upsets, including diarrhoea and vomiting. If you think you have come into contact with cyanobacterial blooms, shower and change your clothes as soon as possible. Long-term exposure to cyanobacterial toxins may cause additional health risks.

Dog owners should prevent their dogs from drinking or swimming in the water or interacting with wildlife in and around the lae. Dogs are particularly sensitive to cyanobacteria. If dogs swallow cyanotoxins, it can cause illness or be fatal. Signs that a dog might have consumed toxic cyanobacteria include lethargy, muscle tremors, fast breathing, twitching, paralysis, and convulsion. Animals showing signs of illness after visiting this lake should be taken to a vet immediately.

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