With warnings already in place at two Rotorua lakes, a scientist says the current warm weather is "favourable" to algal blooms over the summer.

Following the discovery of toxic algae at six popular swimming spots in Lake Taupo, prompting warnings against swimming, some Facebook commenters were worried Rotorua could be next.

Bay of Plenty Regional Council environmental scientist water quality James Dare said the regional council monitored 13 sites spread across four lakes, for cyanobacteria (blue-green algae).

"These sites have been selected because they have been known to bloom in the past and the area is regularly used by the public for recreational activities."


Currently public health warnings are issued for Lake Okaro and Lake Rotoehu.

"Blue-green algae inhabit all natural waters and usually only become a problem when they increase to high concentrations, forming 'blooms'," Dare said.

"We were alerted to a bloom in Lake Rotoehu and Lake Okaro in October this year. Cyanobacteria bio volumes are yet to drop to levels low enough to lift the public health warning for these lakes."

The last time Lake Rotorua had an algal bloom was in March 2015 at the Ohau channel and Rotoiti last bloomed near the Okere Arm in April 2014.

Dare said cyanobacteria responded favourably to calm, warm conditions, much like the plants in our garden.

"In addition, warm, calm periods can cause nutrients to be released from the sediment of lakes through a process called 'internal loading'. This can increase the prevalence of algal blooms especially when winds mix the water following a long calm period."

He said the nutrients entering the lake came from a variety of sources including sediments at the bottom of each lake, groundwater inputs, surface water inputs and atmospheric deposition.

"It can take different lengths of time - sometimes centuries, between any contributing land use activity and when nutrients end up in the lake.

"This makes it very difficult to control in favourable conditions, such as those we are experiencing now."

Meanwhile, Rotorua Motel Association chairman Martin Althuizen said he hadn't seen any effect yet of disappointed Taupo tourists choosing to visit Rotorua instead.

"We don't get a whole lot of people coming here to swim, if they do we see more people looking at Kerosene Creek or Hot and Cold. I don't think it would be a deal breaker.

"If they can't swim in Taupo, then maybe they will think where is the nearest place and look at Rotorua."

What is being done to prevent future algae blooms
· Proposed Plan Change 10 which requires farmers in the Lake Rotorua catchment to reduce the amount of nitrogen entering the lake.
· Lake Rotorua Incentives Programme which aims to permanently remove 100 tonnes of nitrogen from entering Lake Rotorua.
· Phosphorous locking programme - To help reduce the impacts of phosphorus in certain lakes by treating incoming streams with alum.
· Weed Harvesting
· Ohau Diversion Wall
- Bay of Plenty Regional Council