| 12 Apr 2022

Health Warning Removed – Algal Bloom in Waihao River at Bradshaws Road

South Canterbury District Health Board’s Community and Public Health unit has lifted its algal bloom health warning issued along the Waihao River at Bradshaws Bridge.

Recent cyanobacteria surveys of the Waihao River at Bradshaws Bridge has shown the cover of potentially toxic algae (benthic cyanobacteria) in the river has reduced and is now below levels that are of concern to public health.

Canterbury Medical Officer of Health Dr Matt Reid says Environment Canterbury’s monitoring of Waihao River at Bradshaws Bridge has finished for this season will resume next summer when there is increased likelihood of cyanobacteria growth.

Facts about benthic 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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Media enquiries are to be directed to:

Karen Berry
Communications Manager
South Canterbury District Health Board
P: 03 687 2395 | M: 021 139 7442