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North Waikato Lakes, Ngaroto and Rotoroa (Hamilton Lake)

North Waikato Lakes, Ngaroto and Rotoroa (Hamilton Lake)

Health warnings remain in place for three of the five regularly tested lakes outside Hamilton. The warning was lifted for Whangape in June and following the most recent monitoring it is now possible to lift the warning for Waahi as well.

No update is possible at this stage for Ngaroto as it was not tested in the last sampling round. The two remaining North Waikato lakes, Hakanoa and Waikare have shown improvement but the biovolume measurement needs to be lower than the guideline on two occasions before the warning can be lifted.

The current bloom in Rotoroa (Hamilton Lake) remains a significant health risk, especially for small children. The warning is still in force because biovolume remains well above the guideline at the sampling point in this lake although, as with all the lakes, wind causes algae to become concentrated in certain areas. At present the algae are starting to break down, which is creating unpleasant smells and appearance of the water as well. Toxin levels in the water may rise during this phase as the cells break down and release their contents.

Rotoroa has had a health warning since mid February.

Waikato Regional Council is no longer routinely testing Lake Kainui but caution is always advised for users of this lake, because of its history of cyanobacterial blooms.

As always, these test results should be used for general guidance only, as cyanobacteria and their toxins will not be evenly spread through any lake and as in Rotoroa may be concentrated in some areas by wind and water movements.
During blooms, lakes should not be used for any activity which involves skin contact with the water,” said Dr Hood. Scums are a particular risk and contact with scums should be avoided. If contact does occur, skin should be rinsed clean and clothing changed as soon as possible.

“If people still choose to do this when warnings are in place, they should shower and change their clothing as soon as possible afterwards, even if no symptoms are noticeable.”

Swallowing water from lakes affected by blooms should also be avoided.

While not everyone will be affected, for some, the risks include rash, skin and eye irritation, allergy symptoms such as hayfever and asthma and possibly stomach upsets such as diarrhoea and vomiting.

These effects may not appear until some time after contact with the affected water. Long term exposure to cyanobacterial toxins may bring additional health risks. Scums on any lake are likely to contain high levels of toxin.

The Waikato DHB Population Health Service would like to be informed about health problems which develop after exposure to any of the Waikato lakes. Rotoroa (Hamilton Lake) is a particular concern because of the high public use of the area.

“Up-to-date information on cyanobacterial cell counts is available from local councils and Waikato Regional Council. The Waikato Regional Council website: up to date results.

Health advice is available from the Population Health Service (07) 839 8899 in and out of hours.


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