Algal Bloom in Lake Pegasus

15 February 2021

The health warning for algal bloom at Lake Pegasus has been lifted.

Recent water testing at Lake Pegasus has shown the quantity of potentially toxic blue-green algae in the lake has reduced and concentrations are now below levels that are of concern to public health.

30 November 2020

Lake Pegasus is closed to all recreational users because toxic algae (plankton cyanobacteria) was found in the water and Canterbury DHB’s Community and Public Health unit have now issued a health warning.

People and their dogs should avoid all contact with the water until the health warning has been lifted. 20201203 - Pegasus Lake

Samples of the water have been taken for testing with results to be confirmed later this week and signage will be put in place around the lake warning people of the risk.

Environment Canterbury is monitoring the site, with Canterbury District health Board being responsible for issuing health warnings. The public will be advised of any changes in water quality via our website and Facebook.

Facts about cyanobacteria:

  • In lakes cyanobacteria is generally suspended in the water column and typically forms a thick, bright green colouration to water and can produce scums on the water.
  • In rivers 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 water levels in rivers may 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.

For further information visit:

Learn what to look for in lakes and rivers:
ECan - Potentially toxic cyanobacteria (pdf)

Or contact Community and Public Health on (03) 364 1777:

Or you can read our Frequently Asked Questions factsheet