Canterbury DHB’s Community and Public Health unit has lifted its algal bloom health warnings for the Selwyn/Waikirikiri River at Coes Ford issued on 25 January 2021 and Cust Main Drain at Skewbridge issued on 10 May 2021.
Recent cyanobacteria surveys at the Coes Ford and the Cust Main Drain sites have shown the cover of potentially toxic algae (benthic cyanobacteria) has reduced and is now below levels that are of concern to public health.
Canterbury Medical Officer of Health Dr Ramon Pink says Environment Canterbury’s monitoring of these two sites will resume next summer when there is increased likelihood of cyanobacteria growth.
Facts about cyanobateria:
- 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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