Algal Blooms in the Ashley/Rakahuri River


By Robyn Bristow

A health warning along the length of the Ashley/Rakahuri River from the Okuku River Confluence to Ashley SH1 has been issued by the Canterbury District Health Board’s Community and Public Health unit.

Moderate to high cover of potentially toxic algae (benthic cyanobacteria) has been found at various points along the Ashley/Rakahuri River.

Due to the various access points along the river that may have benthic cyanobacteria present, people are advised to treat every low-flowing river cautiously, check for the presence of the toxic algae and avoid contact.

The Ashley River SH1 site is still in warning, and although the Loburn-Rangiora site has just been lifted, due to the site being dry – there is already new growth.

People should avoid the area of the river and animals, particularly dogs should not be allowed near the water until the health warning has been lifted.

Dr Ramon Pink, Canterbury Medical Officer of Health, says the algae look like dark brown to black mats and can produce toxins harmful to people and animals.

“Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.

“If you experience any of these symptoms, visit your doctor immediately, also let your doctor know if you’ve had contact with dark brown/black algal mats or water in this area,” Dr Pink says.

Reticulated town water supplies are currently safe but no one should drink water from the river at any time

Pets that show signs of illness after coming into contact with algal mats should be taken to a vet immediately.

Environment Canterbury is monitoring the sites and the public will be advised of any changes in water quality.

Facts about cyanobacteria:

  • Appears as dark brown/black mats attached to rocks along the riverbed.
  • A low cover of the algae can occur naturally but can increase rapidly during warmer months. Algal blooms are influenced by a combination of available nutrients in the water and sediments (such as nitrogen and phosphorus), a sustained period of low and stable flows, and favourable weather conditions (e.g. increased temperature, calm days).
  • It often has a strong musty smell and algal toxin concentrations can vary over short periods.
  • Although high river levels will remove the algal bloom, detached mats can accumulate along the shore and increase the risk of exposure to toxins.
  • If a health warning is in place avoid contact with the water.
  • Although district or city councils may place warning signs, these may not be seen at the numerous river access points, hence the need for people/ dog-walkers to treat every low-flowing river cautiously.

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