Lake Rotoiti algal bloom colours popular spot green

Water at the Ōkere Inlet at Lake Rotoiti is a vivid shade of green. Photo / Laura Smith

Algal blooms have coloured Lake Rotoiti a vivid shade of green.

A Rotorua kayaking company owner described it as being “smacked in the face” after months of halted business.

An environmental scientist says it is the result of windy weather. However, one environmental advocate has questioned if enough is being done to help prevent algal blooms from happening.

Toi Moana Bay of Plenty Regional Council environmental scientist James Dare said the green water was host to potentially toxic cyanobacteria, dominated by a species called Anabaena circinalis.

Normally, the blooms last for about two weeks, but more intense blooms, such as this one, may last longer.

The green colour comes from the pigment used to capture sunlight, called phycocyanin.

“The colour is so vivid in this bloom due to the number of cells that are present in the water.”

Cyanobacteria cells can be moved by the wind, so the past couple of days of southerly winds caused accumulation around the Ōhau Channel outflow of Lake Rotorua.

Lake Rotoiti is patchworked with algal bloom, discolouring the water in parts. Photo / Laura Smith

This, in turn, caused a high concentration of cells to be washed downstream into the Ōkere Arm, he said.

While it was hard to define exactly when blooms occurred, he said the most important factors were nutrient supply and climate.

“This year has seen a number of heavy rainfall events which have supplied a significant load of nutrients to the lake.”

Combined with warm and still weather, there were “perfect” conditions for growth.

”Algal blooms typically form in the summer months due to water temperature. We can expect blooms to reduce as we head into winter.”

Toi Te Ora Public Health medical officer of health Dr Lynne Lane previously said it was important to avoid any activity which results in contact with the lake water when a warning is in place.

A health warning is in place at the Ōkere Arm of Lake Rotoiti. Photo / Laura Smith

Health warnings are in place for lakes Rotorua, Rotoiti, Rotoehu, and the Kaituna River.

“Cyanobacteria can release toxins which can trigger asthma and hay fever symptoms, skin rashes, stomach upsets and, in some cases, neurological effects such as tingling around the mouth, headaches, breathing difficulties and visual problems,” Lane said.

If the water is discoloured or has a strange smell, consider swimming or rafting elsewhere. Current health warnings can be found on Toi Te Ora Public Health’s website.

Rotorua Rafting owner Sam Sutton said it was not operating because of the bloom. This followed 230 days of the business being closed due to rain and high waters, which had only now dropped low enough.

“It sucks ... to have one obstacle behind you and get smacked in the face with another.”

He hoped the wind would change direction and the cooler weather would ease the issue.

Lakes Water Quality Society (LWQS) chairman John Gifford questioned whether damage at the nearby Ōhau Diversion Wall impacted the bloom at Ōkere Falls.

“The loss of integrity of the wall is a very real concern to the [society] if this is the case, and urgent action is required to repair the wall.”

He also believed there was rising concern over how effective measures had been to date in addressing nutrient, nitrogen and phosphorous input into the lakes.

Lakes Water Quality Society chairman John Gifford at Lake Rotorua. Photo / Laura Smith

In his view: “There needs to be a significant lift in action to more effectively manage nutrient inputs into the lakes and a community commitment to head in this direction.”

Bay of Plenty Regional Council Rotorua catchments manager Helen Creagh said there had been actions that led to significant improvements in water quality since the early 2000s when annual long-term blooms were a regular occurrence.

“In reality, even though the water quality has improved, seasonal weather impacts and climate change impacts will continue to contribute to conditions that can create algal blooms from time to time.”

Local Democracy Reporting asked if the damaged wall would impact algal bloom growth and spread. This was not answered, but Creagh said it was aware of corrosion in the wall.

Stabilising work had been undertaken to protect the structure, but she said extensive maintenance needed to be balanced with good options analysis.

“These are not decisions we make without good information, and a good understanding of long-term benefits.”

It was contracting divers to inspect the wall below the surface of the lake.

Water at the Ōkere Inlet at Lake Rotoiti is the colour of the trees surrounding it. Photo / Laura Smith

Once the extent of the corrosion was known, and the University of Waikato had completed preparing information on the benefits of the wall in achieving water quality, it would look at options for repair.

In response to Gifford’s comments on how effective measures to achieve better water quality were, she said a review of progress in achieving a sustainable nutrient load for Lake Rotorua had recently been completed.

This involved independent scientific input and consultation with stakeholders, including LWQS. This work would be repeated in 2027 to see if it was on track and to inform any adaptations that were needed.

“In the meantime, we are asking that everyone with rural land over five hectares in the Lake Rotorua catchment does their bit by making sure they have been in contact with us and understand their obligations under the Nutrient Management rules for Lake Rotorua.”

Public Interest Journalism funded through NZ on Air.