Summer search for blue green algae

Gemma Kerrisk, ensuring lakes are safe for swimming. Photo: Supplied.

Gemma Kerrisk is an algal monitoring technician with the BOP Regional Council, a job description that means she spends the summer months ensuring regional lakes are safe for swimming.

Gemma spends four days a week over the summer before a microscope, checking water samples for algae levels.

Water samples from lakes Okaro, Rotoehu, Rotorua, Rotoiti and Tarawera are tested weekly. Testing is carried out on both the water and the algae, says Gemma. Using a microscope she counts the cells of cyanobacterial (blue-green algae) colonies and then calculates the overall volume.

If there are too many of the potentially toxic cells, an alert is sent to all the relevant authorities and Toi Te Ora Public Health issues a warning to the public.

The results are usually published within 24 hours on BOPRC’s and the Rotorua Te Arawa Lakes Programme’s website and graded according to Ministry for the Environment’s New Zealand Guidelines for Cyanobacteria in Recreational Fresh Waters. Green is safe, with red considered a potential health risk, and the level at which a public health warning is usually issued.

“An amber or orange grading is a warning that elevated levels have been detected, and extra monitoring is being done to check if a health warning is needed,” says Gemma.

In some cases, the testing can give early indication of a potential bloom when results are compared to past results.

“Combine this with forecast weather conditions and you can get an idea of what sites may bloom in the coming weeks.”

Gemma holds a Bachelor of Science with a major in genetics, followed by a postgraduate Diploma in Science with a major in environmental management, has been trained by senior scientists to identify algae as part of the role.

While four days a week are spent behind the microscope using the skills learnt at university to analyse the samples, she manages to get out in the environment once a week to sample the lakes.

Between November and May, she collects samples from 13 sites in the Rotorua area each week. The frequency can change depending on the health status of the lakes – and sites with blooms may be tested in more places and more frequently.

Gemma says she loves using her molecular and microbiology skills to ensure the environment is a safe place that communities can enjoy.

“I am very passionate about the environment and being able to enjoy what nature has to offer,” says Gemma.

To check out the health of the region’s waterways, go to go to, or visit.

Freshwater toxic algae, known scientifically as cyanobacteria, are an ancient group of photosynthetic bacteria that are common in rivers and lakes in New Zealand. They occur in waterways with good water quality and most of the time don’t cause any problems. During summer, with higher temperatures and lower water levels, cyanobacteria can form extensive blooms which can be toxic, particularly to dogs if they eat the algae mats.

The only way of working out if cyanobacteria blooms are toxic is laboratory testing.

In rivers, cyanobacteria generally form brown or black mats that grow on rocks in the river bed. Mats that come loose from the riverbed can wash up on the banks or form floating ‘rafts’ in shallow areas. When exposed, the mats may dry out and turn a light brown or white colour and may also produce a strong musty odour. Dogs love the smell and can eat it. Even a small amount, the size of a 50 cent piece, can kill a dog.

Most people have to eat toxic algae for it to be harmful. A small number of people are very sensitive and may experience skin irritation after being exposed. If you experience a reaction after contact or swimming, contact your doctor.

Dogs are most at risk. Keep an eye on your dog when they are near the river or lake. If there is an alert or you think you have spotted toxic algae mats keep your dog on a lead, out of the water and most importantly, ensure it does not eat any algae mats in the water or at the water’s edge.

If you suspect that your dog has eaten toxic algae, treat it as an emergency and contact your vet immediately. In extreme cases, death can occur within 30 minutes after the first signs of illness appear. Signs a dog has been poisoned by toxic algae include lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsions.


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