Algal blooms have coloured a popular Rotorua lake a vivid and vibrant shade of green.

A Rotorua kayaking company owner has called the situation at Lake Rotoiti a smack in the face after months of high water 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.

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

While it's hard to define exactly when blooms occur, he says the most important factors are 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, these are 'perfect” conditions for growth.

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

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

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

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

If the water is discoloured or has a strange smell, consider swimming or rafting elsewhere. Current health warnings can be found on its website.

Rotorua Rafting owner Sam Sutton says it's not operating because of the bloom. This follows 230 days of being closed due to rain and high waters which had now dropped low enough.

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

He hopes the wind will change direction and the cooler weather will ease the issue.

Lakes Water Quality Society chairman John Gifford questions whether damage at the nearby Ōhau Diversion Wall impacted on 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 believes there is rising concern over how effective measures have been to date in addressing nutrient, nitrogen and phosphorous input into the lakes.

'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.”

Regional council's Rotorua catchments manager Helen Creagh says there have been actions that have 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 on algal bloom growth and spread. This was not answered but Creagh says the council is aware of corrosion in the wall.

Stabilising work has been undertaken to protect the structure but she says extensive maintenance needs 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's contracting divers to inspect the wall below the surface of the lake.

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

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

This involved independent scientific input and consultation with stakeholders, including the society. This work will be repeated in 2027 to see if it's on track and inform any adaptations that sre needed.

'In the meantime, we are asking that everyone with rural land over 5ha 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.”