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Algal bloom

For information about current algal blooms and health warnings please go to health warnings.

Algae are simple rootless plants that grow in sunlit waters in proportion to the amount of available nutrients. The more nutrients in the lake the more algae grows.  When there is too much algae in the lake an algal bloom may occur.  Some types of algae are toxic in high concentrations which means the lake may not be safe for recreational activities until the algal bloom clears.

Algal blooms are more common in the warmer summer months.  Water quality in the lakes are monitored all year round and if an algal bloom occurs a health warning will be issued for the bay or lake that is affected.

To minimise the number of algal blooms in the Rotorua Te Arawa Lakes we need to reduce the nutrient supply to the lakes.

Effects of algal blooms

Algal blooms make the lake unpleasant to look at and unsafe to use.

During an algal bloom the following can occur:

  • Water clarity (or clearness) decreases
  • Water turns a green or brownish colour, forming layers of scum or foam on the lake surface
  • Lake can emit unpleasant smells

It is not safe to come into contact with water when the algal blooms occur as it can cause:

  • severe dermatitis
  • burning or itching of the skin
  • erythematous wheals
  • redness of lips and eyes
  • sore throat
  • asthma symptoms
  • dizziness

If you notice a possible bloom (such as described above) in an area where there is no health warning in place, please report the bloom location during normal working hours to Bay of Plenty Regional Council on 0800 884 880 or after hours on the Pollution Hotline 0800 884 883.

There have been some concerns raised about chronic toxicity effects from eating trout from Rotorua lakes with cyanobacterial blooms. Research so far has indicated that unless trout is eaten every day, there should be no long-term effects from toxins. However the internal organs of the fish absorb much higher toxin levels and should not be eaten.