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Watch out for algae in lakes and rivers

HEALTH ADVISORY: Watch out for algae in lakes and rivers


Dr Phil Shoemack, Medical Officer of Health.

The recent spell of warm humid weather increases the chance of problems with algae in our lakes and rivers. People should always exercise their own judgement about the quality of the water they are about to swim or paddle in.

The lakes in the Rotorua district, and the rivers of the Eastern Bay of Plenty, are particularly prone to problems caused by blue-green algae (also known as cyanobacteria). Algal blooms have been a summer phenomenon in the Rotorua lakes for the past twenty years.

Significant steps have been taken to reduce the nutrient load in the lakes, but blooms were still recorded in Lakes Okaro, Rotorua, Rotoiti, and Rotoehu over different periods last summer. The only current health warning affects Lake Okaro where a bloom was first identified in November.

Similarly, some of the rivers in the Eastern Bay have been affected by algal mats and scums the past few summers. The Waimana, Whakatane, and Rangitaiki Rivers have been most problematic.

Routine water quality monitoring is conducted by Environment Bay of Plenty on a weekly basis throughout summer. Whenever a significant algal problem is detected the Medical Officer of Health issues a public warning for the relevant lake or river. However, these algal blooms can occur at any time and they can arise and then disappear between sampling events. Hence, the routine monitoring will not always pick them up.



People are advised that contact with the water affected by blue-green algae may be hazardous. Any recreational activity which is likely to involve significant contact with, or swallowing of, the water could result in health problems. The toxins produced by blue-green algae can trigger asthma and hayfever attacks in susceptible individuals, as well as causing skin rashes, tummy upsets, and even neurological effects such as tingling round the mouth, headaches, general breathing difficulties and visual problems.

Cyanobacteria often multiply to excessive levels during periods of warm, dry, sunny weather when lake and rivers levels are low. In rivers they can form extensive black or brownish slime-like mats that cling to rocks and logs. Significant clumps sometimes break off and float free, eventually collecting on vegetation at the water's edge.


People need to make their own visual assessment of the water and avoid diving in if lake water is discoloured or smelly or if black and brown algal mats are found covering large areas of a river bed. People should also avoid swimming in or drinking water that has a strange musty smell.

if in doubt, go somewhere else.

People need to take steps to prevent animals, both farm stock as well as pets, from having direct contact with the mats by keeping the animals away from the affected stretch of river. Highest risk areas tend to be shallow river margins where infants and dogs are most likely to come in contact with the mats.

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