A freshwater enthusiast in Romania has studied decades of data on Rotorua's lakes and found water quality is on an upward trajectory, with Lake Rotorua most improved.
But the outlook closer to home remains sober with some experts saying there is a long way to go.
Catalin Trif, a project manager based in Cluj-Napoca in Romania's northwest, founded his "passion project" Lakepedia in 2015 and said it was "born out of a fascination for lakes and rivers".
The website, an online encyclopaedia of lakes, is run by Trif and one other volunteer.
Trif's latest analysis focused on Rotorua's 12 lakes, finding 2020 was the best year since 2015 for lakes to hit their trophic level index (TLI) targets, and positive changes in trophic states registered by Lakes Ōkaro, Tikitapu and Rotoiti.
The trophic level index is a value derived from four factors: water clarity, and the amount of chlorophyll, phosphorus and nitrogen in the water. The lower the value, out of zero to five, the higher quality the water.
Trif said the data showed the overall quality of the Rotorua lakes had increased from an average TLI of 3.8 between 2001 and 2010 to an average of 3.6 in the decade between 2011 and 2020.
"Lake Rotorua registered the biggest improvement, with a 0.5 reduction of its TLI, from 4.8 to 4.3. In terms of hitting the TLI targets, last year was the best since 2015, with four of the 12 lakes hitting their TLI targets."
The biggest decrease in water quality was at Lake Rotomahana, with a 0.2 TLI increase.
Seven lakes' water quality improved but five had decreased.
Lakepedia founder Catalin Trif. Photo / Supplied
Te Arawa Lakes Trust Environment Manager Nicki Douglas said water quality of the 14 Rotorua Te Arawa lakes was at the forefront of the trust's mahi and was guided by Te Tūāpapa o ngā Wai o Te Arawa - the Te Arawa Cultural Values framework, which took a te ao Māori and mātauranga Māori approach.
"While the Lakes programme focuses on water quality, the Lakes Strategy takes a more holistic view and when it comes to water quality, our measurement is actually very simple – when the quality of the water is such that you can see the footsteps of the kōura on the lakebed - Te mā o te wai e rite ana kia kita i nga tauwae ā te kōura."
Douglas said while there had been "significant improvements" in some areas, there was still a lot of mahi to do.
"From a central and local government level, through to iwi and individual lake users, we all need to take responsibility for protecting the taonga now and for generations to come."
Bay of Plenty Regional Council lakes operations manager Andy Bruere said the water quality improvement was a reflection of the Rotorua Te Arawa Lakes programme, which focused on actions to improve and protect water quality over the last 30 years.
He said the 1990 upgrade of the Rotorua wastewater treatment plant, which saw the discharge of wastewater into the forest, was "the single biggest reduction in nitrogen and phosphorus reaching Lake Rotorua".
He said the main reason Lake Rotorua had met its TLI target over the last decade was alum dosing - a method which removes phosphorus from water - and was only a short-term solution.
"This is only a short-term solution to water quality on the lake and without it the lake is still at risk of algal bloom, similar to those witnessed prior to 2010 when the second alum plant was commissioned."
He said the regional council had a "large programme of work" to remove nutrient run-off to the lake, which, if successful, would mean alum dosing could be phased out in the future.
Lakes Water Quality Society president John Gifford at Lake Rotorua. Photo / File
"However, these changes will take some time to make - 2032 is the target date - and then may take [about] 60 years to reach the lake itself."
He said climate change - through longer hot, dry periods and more frequent storms - will pose a challenge, as will increasing pressure on land-use intensification.
"There is still much to do and we must collectively maintain momentum."
Te Kahui Kounga Wai Lakes Quality Society president John Gifford said the overall improvement in the lakes was due to a "substantial effort by the community, regulatory agencies, government, Te Arawa Lakes Trust and organisations such as ours".
He said there had also been "substantial investment" from those groups, which he estimated to be around $240 million over the last 15 years.
