Waikato Shallow Lakes Cyanobacterial

WARNING UPDATE

Monitoring results from late October 2010 have shown that cyanobacterial cells are multiplying, as is expected as the days lengthen and the temperatures rise. The human health threat from cyanobacteria in three of the tested lakes remains acceptably low. Lake Waikare, where a health warning is already in place, has shown a very marked rise in biovolume, and Lake Hakanoa has had a warning reinstated. Lakes Waahi, Whangape and Ngaroto have all shown a rise but remain below the biovolume warning level (1.8 cubic mm/litre).

Environment Waikato is no longer routinely testing Lake Kainui.

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 may be concentrated in some areas by wind and water movements. Even just a few days of warm calm sunny weather could produce rapid algal growth, which would be noticeable to users by discoloration of the water or unusual scums forming near the margins.

New guidelines for measuring and responding to cyanobacteria in fresh water recreation sites were published last summer.[1] Warning levels now take into account not only the number of known toxin producing cells present but also their size. In recent years some warnings may have been overly cautious when the cyanobacterial species present in large numbers was known to have a small cell size. Environment Waikato and Waikato District Health Board Population Health have moved to full use of the new guidelines as information about the cell size of various species found in local lakes is measured. Warnings are now issued when the "biovolume" exceeds 1.8 mm3/L. Warnings are based on the likelihood of cyanobacterial toxin being in the water, not on actual toxin measurements.

The biovolume result for Waikare has shown a nine fold rise since it was last measured two months ago, but the cell count, although high, has risen by a much smaller proportion. Biomass measurement is a better predictor of risk because the rapidly increasing species are much larger than the dominant species present at the last test.

During blooms, lakes should not be used for any activity which involves skin contact with the water," said Dr Hood.

"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.

The Waikato DHB Population Health Service would like to be informed about health problems which develop after exposure to any of the Waikato lakes.

This allows recording of location, time, the activity taking place and length of time the problem lasted. Follow up testing may be done, depending on the situation.

"Up-to-date information on cyanobacterial cell counts is available from local councils and Environment Waikato. The Environment Waikato website: http://www.ew.govt.nz/Environmental-information/Rivers-lakes-and-wetlands/healthyrivers/Waikato-River/Algal-Blooms-in-the-Waikato-region/#Heading4 has 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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