Poor water quality is a problem in Lake Rotoehu. Photo / NZME
More than $100 million has been spent to protect and improve Rotorua's lakes - but experts fear it isn't enough to stop algal blooms from spreading as temperatures rise.
Health warnings were issued for Lake Okaro, Lake Rotoehu, Lake Rotorua, the western end of Lake Rotoiti and the Ohau channel.
However, the warning was lifted on Thursday for Lake Rotorua, the Ohau Channel and the western end of Lake Rotoiti.
Algal bloom - excessive growth of algae on or near the water surface - was found in Lake Rotorua for the first time in a decade, said Toi Te Ora medical officer of health Dr Phil Shoemack.
Shoemack said algal blooms were not going away.
"Climate change means we are seeing slight increases in temperature on a season-by-season basis and that increases the risk.
"As we have more intensive farming, as more people move into the area it's inevitable."
He recalled the only lake with no recorded algal bloom in the past 30 years was Lake Rotoma.
Toi Moana Bay of Plenty Regional Council lakes operations manager Andy Bruere said about $115m had been spent in the past 15 years on the Rotorua Te Arawa Lakes programme, in terms of direct action on water quality in the lakes' catchments.
John Gifford, chairman of the Lakes Water Quality Society, said despite the millions of dollars spent to protect Rotorua lakes in the past 15 years, algal bloom was still present.
"Is it enough, given things like climate change, ongoing rapid urban development, changing land use?"
Gifford said the shallow lakes have all had issues in recent years and in the last few weeks.
Funding spread over the 12 Rotorua lakes has been used for alum dosing, buying back nitrogen from landowners, farm training programmes to minimise loss of nutrients, weed control, the diversion wall at Lake Rotoiti and weekly water sampling for monitoring.
The society has programmes to improve lake water quality, particularly for Lake Okareka, Lake Rotorua, Lake Okaro and Lake Rotoehu.
Gifford said the safest lakes to swim in were usually the Blue Lake, Lake Okareka and Lake Rotoma as they tended to be deeper.
Gifford said the Rotorua Lakes Council would be implementing a "boat certification scheme" this summer, which would encourage boat owners to clean their boats before they enter the lakes.
"The idea of that is to prevent things like catfish and weeds coming in from other parts of New Zealand."
Gifford said while pests don't necessarily affect water quality, they can affect how enjoyable the lakes are to swim in. The programme would make sure boats didn't carry in invasive pests.
Jim Stanton of the Lake Rotoiti Community Association said some members of the public were known to have contracted medical conditions from an algal bloom in the past.
"It is particularly injurious to animals and a number of dog owners have reported severe reactions such as blistering skin, vomiting, convulsions and seizures after contact and drinking the water."
Stanton said the "aesthetic factors" and water quality issues were saddening for the community.
"It is particularly frustrating for those having to curtail recreational activities, especially given the extensive treatments and expenditure in attempts to eliminate algal bloom."
Locals have expressed their concerns on Facebook.
A local commented, "these algal blooms are getting more widespread. Twenty years ago we hardly or never heard of them, but intensive land use has changed all that."
Other locals said the warning was "sad and embarrassing".
"I literally just took my dog for a walk down by the water out Hannahs Bay and there was no signage ... so hopefully dog won't get affected."
Bruere said the restoration programme wasn't about treating lakes as "swimming pools" and it looked ahead to long-term sustainability.
"This is a problem that has built up over the past 100 years or so. So it will take time to improve as the land-use changes mentioned above take effect.
"There are nine other lakes in the district that do not have algal health warnings and it would be advisable to swim in one of those."
Bruere said, "We recommend checking lawa.org.nz to check any health warnings and whether it is safe to swim."
This week's monitoring showed algae bloom was no longer present in Lake Rotorua, so Toi Te Ora also lifted the health warning for Ohau Channel and the western end of Lake Rotoiti.
Despite the lifting of the warnings, the public should always be on the lookout for signs of algal blooms when using lakes.
"Algal blooms can develop rapidly and unpredictably and can sometimes produce toxins that are harmful for people as well as animals. If the water looks discoloured, has an unusual smell or has green or brown particles suspended in it, it is best to avoid contact with it," said Dr Bruce Duncan, medical officer of health for Toi Te Ora.
• Algae are aquatic plants ranging from large seaweeds to tiny phytoplankton. Phytoplankton are microscopic and feed off the sun.
• When a lot of these are in the water, it's known as an algal bloom.
• Blooms often appear as coloured patches (usually brown or red) and usually occur naturally when wind and water currents are favourable.
• Causes can include slow water circulation or unusually high water temperatures.
Source: Fisheries New Zealand
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