Lake water quality

Water quality is falling in some Rotorua lakes. This is because they have too much nitrogen and phosphorus. These nutrients feed the algae which degrade water quality. Nutrient levels have increased markedly over the past few decades, and continue to rise.

Some nitrogen and phosphorus has a natural source. For example, phosphorus is naturally present in rock. Rainfall transports nitrogen from the air. However, the pressures on the Rotorua lakes are mostly from the intensification of agriculture and growth in lakeside communities.

Many communities dispose of waste to septic tanks. The nutrients (particularly nitrogen) from this source eventually flow through groundwater into the lake, boosting algae production. Bacteria from septic tanks can also contaminate the lake edge in the paddle zone where people enter the water.

Much of the farming in the lake catchments is low-intensity sheep and beef grazing, which does not cause large nutrient runoff, provided stock are excluded from the water. If farming becomes more intensive, this will put pressure on lake quality.

The Rotorua Lakes Protection and Restoration Action Programme aims to improve water quality in the Rotorua lakes by reducing levels of nutrients.

Water Quality Measurement

Bay of Plenty Regional Council uses the Trophic Level Index (TLI) system to assess water quality. The lower the TLI the better the water quality.

Water Quality Goals

Each of the Rotorua lakes has a water quality goal set for it in the Regional Water and Land Plan.

Lake	Target TLI in Regional Water and Land Plan
Ōkāreka	3.0
Ōkaro	5.0
Ōkataina	2.6
Rerewhakaaitu	3.6
Rotoehu	3.9
Rotoiti	3.5
Rotokakahi (Green Lake)	3.1
Rotomā	2.3
Rotomahana	3.9
Rotorua	4.2
Tarawera	2.6
Tikitapu (Blue Lake)	2.7

Contributing factors and steps for improvement

Some of the contributing factors and steps to improve water quality are listed below:

Lake	Water Quality Problems	Steps to improve water quality
Ōkāreka	Trophic level index showed a small improvement for the 2009-2010 period. Infrequent cyanobacteria blooms and while the TLI remains stable, the risk of intense and long-lasting blooms remains slight.	Action Plan operational. Sewerage reticulation. Constructed wetlands. Treatment of hypolimnetic water with flocculants to remove phosphorus. Environmental programmes for farmers. Land use change for lower nutrient export.
Ōkaro	Lake Ōkaro is classified as super-eutrophic but is improving towards a eutrophic classification. No cyanobacteria bloom over the 2009-0210 summer season, but blooms returned in summer 2010-2011. Recurrent cyanobacteria blooms will remain a feature of this lake while the TLI remains elevated.	Action Plan operational. Chemical flocculant applications to remove phosphorus, construct wetland and riparian retirement.
Ōkataina	Very occasional short-lived blooms, but none reported in recent years. Generally low risk of cyanobacteria blooms occurring. Possible water quality degradation that could come from lake level changes or other natural fluctuations.	Action Plan process beginning.
Rerewhakaaitu	Not routinely monitored for cyanobacteria blooms. However, in the time cyanobacteria monitoring began in the Rotorua Lakes, no blooms have been reported for this lake. Water quality fluctuations noted in the past and there is a risk of summer blooms increasing as the TLI moves back towards 4.0.	A Sustainable Farming Fund project is underway to assist with reducing nutrients. Landowners are also developing and implementing Nutrient Management Plans.
Rotoehu	Severe and persistent cyanobacteria blooms since an increase in nutrients in 1993. No cyanobacteria bloom in the 2003/04 summer, but a return to perennial blooms since.	Action Plan complete. Riparian retirement programme. Floating wetland complete. Treatment box to remove stream nitrate tested. Hornwort harvesting underway since 2007.
Rotoiti	Before the Ōhau Channel diversion ('the wall') ~72 percent of waterborne nutrients came directly from Lake Rotorua and cyanobacteria blooms in Rotoiti were both perennial and severe. No persistent bloom activity has been recorded in both the western and eastern arms of the lake since the wall was completed in October 2008, other than in Okawa Bay (once in February 2009) and in Ōkere Arm. Water in Ōkere Arm is now isolated by the wall from main body of the lake. Ōkere Arm now receives water directly from Lake Rotorua and blooms here now mirror the timing and intensity of blooms in Lake Rotorua, particularly bloom activity closer to the Rotorua end of Ōhau Channel.	Action Plan process underway. Riparian retirement. Sewage reticulation for lakeside settlements underway. Diversion of inflow from Ōhau Channel down the Kaituna River completed and monitoring taking place.
Rotokakahi (Green Lake)	Moderate risk of cyanobacteria blooms increased to high risk as the TLI increased from 3.4 in 1992 to 4.55 in 2010. Persistent blooms were first recorded in this lake and in its Te Wairoa Stream outlet in May 2011. A large fish kill was recorded in the Te Wairoa the same month. Blooms in Rotokakahi lasted until June 2011.	Scientists are investigating the accelerating decline in the lake. A draft action plan is being developed in conjunction with the lake owners.
Rotomā	None. Low risk of cyanobacterial blooms.	Action Plan process complete.
Rotomahana	Occasional algal blooms. Not routinely monitored for cyanobacteria blooms.	Starting the Action Plan process is currently scheduled for July 2012.
Rotorua	Experienced water quality decline between 1978 and 1983. Sustained bloom activity has been recorded most years since monitoring began in 1997. Since the 2008-2009 season Lake Rotorua has had spring and summers free of cyanobacteria. However, the emergence of a new species with a high winter biomass has seen the emergence of autumn blooms that last well into summer. These highly buoyant blooms are carried by prevailing winds to windward shores in the lake and when blown towards Ōhau Channel are recorded in high numbers in Okere Arm. Not since February/March 2006 have there been lake-wide warnings in Lake Rotorua but there is still a low to moderate risk of cyanobacteria blooms occurring in this lake.	Action Plan process underway. Rotorua District Council diversion of treated sewage to land disposal in 1991, and further improvements in progress. Riparian retirement since 1970 as part of Kaituna Catchment Control Scheme. Diversion of nutrient rich spring flows investigated. Flocculant dosing of nutrient-rich streams. Stormwater upgrades. Environmental Programmes. Two alum dosing plants are in place and a Tikitere denitification pilot plant is being tested.
Tarawera	Last sustained cyanobacteria activity in this lake in 2003 but transient bloom activity since. Bacterial issues from septic tanks around some lakeshore areas. Lake Tarawera's TLI is presently stable (as of 2010). However Lake Rotokakahi's TLI, a lake that flows directly into Tarawera, has risen steadily to a high point of 4.55 in 2010.	Action Plan process beginning. Land use change to lower nitrogen and phosphorus export encouraged. Planning for future sewage reticulation.
Tikitapu (Blue Lake)	Gradual lake water decline and Lake Tikitapu now on the cusp of becoming meso-trophic. Low risk of cyanobacterial blooms.	Sewerage reticulation as part of the Lake Ōkareka scheme complete. Action Plan complete.