River swimmability

What are blue-green algae?

Blue-green algae (cyanobacteria) occur naturally in fresh water lakes and rivers throughout the world. They are usually found in small amounts and can only be seen through a microscope. However, when water levels drop and water-bodies heat up, blue-green algae can multiply and form blooms or mats. Free living blue-green algae can also be concentrated and carried onto shore by wind to form harmful scums.

What could happen to me if I'm affected by blue-green algae?

Someone who is affected by blue-green algae could have:

Hay fever, eczema and dermatitis may also get worse and exposure to blue-green algae may cause an asthma attack.

Those at greatest risk of health effects are children and those with pre-existing medical conditions, such as long-term liver or kidney disease.

However, even healthy people or animals may suffer significant harm when exposed to some forms of toxins. Blue-green algae can produce toxins that can damage the liver or poison the nervous system and in high doses can bring about convulsions and respiratory arrest.

The higher the amount of blue-green algae and its toxins and the longer the contact with the water, the more severe the symptoms are likely to be.

Children are at greater risk because they might be attracted to play with scum on the shoreline or, worse still, place in their mouths mats or scum taken from shallow areas of a lake or river.

What do blue-green algae mats look like?

Blue-green algal mats:

• Are dark brown/black or golden brown in colour.
• Can form in rivers and around lake edges where they usually cover rocks, sand, gravel, logs and weed.
• Look like moss but have a slimier texture and in shallow areas may glisten when exposed to air.
• Can be lighter in colour and look golden brown in shallow areas.
• Can easily come away from the riverbed and form floating 'rafts'.

See pictures of blue-green algae mats.

What do free living blue-green algae blooms look like?

When present in high numbers free living blue-green algae form blooms that:

• Will usually make the water a dense blue-green or mustard colour.
• May form floating scums on the water surface or on the shore or at the river edge.
• May produce a musty or earthy smell.

See pictures of free living blue-green algae.

Where am I likely to find blue-green algae?

Blue-green algae are widespread throughout New Zealand.  Toxic algal mats could be found in any lake or river - even those with relatively clean looking waters.

Extensive areas of blue-green algal mats have been identified in Rangitāiki, Waimana, and Whakatāne Rivers in the Eastern Bay of Plenty.

You should keep an eye out for any sign of algal mats in these rivers.

In the past fifteen years, blooms of free living blue-green algae have regularly occurred in several of the Rotorua Lakes, including Lake Rotorua and Rotoiti and occasionally in the Kaituna River which Lakes Rotorua and Rotoiti feed into.

How am I likely to come into contact with blue-green algae?

People can be exposed to blue-green algae by:

• Drinking water containing blue-green algae toxins.
• Swallowing the affected water while swimming.
• Placing mat or scum material in their mouths (as might an infant).
• Breathing in droplets of the affected water while swimming.
• Bathing in the affected water.
• Direct skin contact with the affected water e.g. through paddling, wading, fishing, or launching a boat.

The risk is much higher where there are visible signs of blue-green algae blooms, scum collections, or mats.

What happens when blue-green algae is found?

In summer and autumn the Bay of Plenty Regional Council regularly tests freshwater (rivers as well as lakes) for blue-green algae. If high amounts of blue-green algae are found, the Medical Officer of Health will issue a health warning. This will advise the public not to drink or use affected water and to keep away from areas affected with blue-green algae.

When a Health Warning is issued, the City or District Council puts signs up at places where public access the river or lake.

It's not always possible to put signs all the way along a river or lake that may be affected. You should look out for blue-green algae and avoid using any areas that have visible blue-green algae. Don't swim if you think it's there.

You should not paddle, wade, swim, fish, or do anything that might involve contact with blue-green algae. Keep your pets and livestock out of areas affected by blue-green algae.

Anyone suffering illness after contact with blue-green algae should seek medical assistance. Any illness in pets or livestock after contact with affected water should be reported to the Medical Officer of Health.

Who should I contact?

I think I have found blue-green algae

• Bay of Plenty Regional Council - 0800 884 880
  • Pollution Hotline - 0800 884 883 (after hours)
• Waikato Regional Council 07 378 6539 (Taupo office)
  • Pollution Hotline 0800 800 401 (after hours)

What do I do if I think I'm sick or my child is sick from contact with affected water?

If you or your child become sick and think it may be from contact with affected water, you should contact your family doctor immediately.

If you have any concerns, you can contact a Health Protection Officer at Toi Te Ora - Public Health for queries relating to human health and drinking water. Their contact details are:

• Toll free - 0800 221 555

What do I do if I think my pets/livestock are sick from contact with affected water?

If you are concerned about your animals, you should contact a vet immediately. You or your vet should then contact your local council to report any illness resulting from contact with the blue-green algae.

What do I do if my water supply comes from a stream, river or lake affected by blue-green algae?

Boiling the water is not recommended as boiling will cause the blue-green algae cells to break open and will actually increase the toxic levels.
Check the water upstream of your intake and at your intake for the presence of blue-green algae. 

If your water is affected, close off your intake and find an alternative water supply.

Reticulated town supply water is monitored and adequately treated.

Can I water my garden with water affected with blue-green algae?

Yes. Fruit and vegetables do not appear to absorb the toxins. However, fruit and vegetables should be washed in clean water as the blue-green algae may form a residue on the surface, which can remain toxic even when dry.

What is being done to get rid of blue-green algae?

There is little that can be done immediately to get rid of the blue-green algae. Blue-green algae are naturally occurring microscopic plants and it is only at high numbers that they become a problem, such as when they form blooms or extensive mats. Bay of Plenty Regional Council is looking into the ways the land is being used in affected lake catchments to see if anything can be done to reduce nutrient inputs to streams, rivers and lakes.

Local councils and Toi Te Ora - Public Health are responsible for issuing health warnings and making sure that people are aware of the potential risks of blue-green algae.

Where can I get more information on the effects of blue-green algae?

You can visit the Toi Te Ora - Public Health website (below) or call them on:

• Toll free - 0800 221 555
• Tauranga - 07 571 8975
• Rotorua - 07 349 3520
• Whakatāne - 07 306 0847