Cautious optimism on Rotorua Lake result

Bay of Plenty Regional Council scientists are cautiously optimistic about recent good results for water quality in Lake Rotorua.

Recent water testing shows that the measurement of water quality - the Trophic Level Index (TLI) - for Lake Rotorua has come down to 4.1, below a 4.2 target set in the Regional Council's Regional Water and Land Plan. The TLI measures four factors for water quality, and the lower the TLI, the healthier the water quality is.

This year Lake Rotorua has had no algal blooms in the lake and water clarity has been exceptional. Regional Council General Manager Natural Resource Operations Warwick Murray said there were reasons for the good result - but also sounded a note of caution about the possible short-term nature of the improvement.

"The unusually good result could be due to a combination of a colder and wetter summer earlier this year, coupled with in-lake actions such as alum dosing of streams which feed into the lake to remove the harmful phosphorus," he said.

"Water quality in Rotorua lakes is affected by the nutrients nitrogen and phosphorus entering the lakes.  These nutrients enter the lakes from a range of natural sources and human activities. The largest contributor to nitrogen in Lake Rotorua is agriculture. Our research has determined that we need to remove 320 tonnes of nitrogen from entering Lake Rotorua to achieve sustainable long term water quality," Mr Murray said.

There was a risk that the exceptional TLI result would be seen by some as "fixing" the lake and that no further action was required, but this was not the case.

"While the TLI result for Lake Rotorua is excellent and very promising there is still much work to be done to achieve sustainable long-term gains in water quality. Up to now we have largely focused on treating the symptoms in the lake and in streams in order to get water quality improvements in the shorter term.

"If we are to achieve sustainable long-term improvements, we will need to turn off the nutrient tap up in the catchment. This in particular means making changes to both the way we manage our land and how we use it," he said.

"To ensure this result is not short-lived it is important to continue actions to achieve a sustainable nutrient load for the Lake Rotorua catchment to avoid returning to poor water quality and algal blooms."

As part of its Ten Year Plan the Regional Council has allocated $45.5 million to work with land owners in the Lake Rotorua catchment to help with land use and land management change.

The Rotorua Lakes Protection and Restoration Programme is a partnership between the Regional Council, Rotorua District Council and Te Arawa Lakes Trust. Deed funding from Ministry for the Environment for $72 million has been allocated for the four priority lakes - Rotorua, Rotoiti, Rotoehu and Ōkāreka.