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.

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

Environment Bay of Plenty 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. The figures below are based on the three-yearly weighted average to June 2006.

 

Lake	Current State (TLI) 3 yearly average to 2009	Target TLI in Regional Water and Land Plan
Okareka	3.3	3.0
Okaro	5.3	5.0
Okataina	2.8	2.6
Rerewhakaaitu	3.7	3.6
Rotoehu	4.5	3.9
Rotoiti	3.9	3.5
Rotokakahi (Green Lake)	4.0	3.1
Rotoma	2.6	2.3
Rotomahana	4.0	3.9
Rotorua	4.7	4.2
Tarawera	2.9	2.6
Tikitapu (Blue Lake)	3.0	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
Okareka	Moderate risk of cyanobacterial blooms.	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.
Okaro	Severe cyanobacterial blooms (alternates between blue-green and green algal). Previous scientific reports dating back to 1966 describe Lake Okaro a having 'clear water' with a pH of 7. Today the lake has a very low clarity and a surface water pH in excess of 8.	Action Plan operational. Chemical flocculant trials, construct wetland and riparian retirement.
Okataina	None - low risk of cyanobacterial blooms. Possible water quality degradation that could come from lake level changes or other natural fluctuations.	Action Plan process beginning.
Rerewhakaaitu	Risk of cyanobacterial blooms during summer where lake is shallow and calm. Water quality fluctuations noted in the past.	Local projects undertaken in catchment with landowners. Trials of treatment boxes to remove nitrogen from streams. Phosphorus-absorbing socks in the Mangakino Stream.
Rotoehu	Severe cyanobacterial blooms (have been more persistent since an increase in nutrients in 1993). 2003/04 summer had no cyanobacterial blooms, but a large bloom returned in the 2004/05 summer along with a large hornwort infestation.	Action Plan process underway. Riparian retirement programme. Constructed wetlands. Treatment box to remove stream nitrate. Hornwort harvesting trial to remove nutrients
Rotoiti	Severe cyanobacterial blooms in Okawa Bay (have become more persistent in past few years). Isolated blooms in eastern lake area. Receives nutrient rich water from Lake Rotorua (~72% nutrients are from Rotorua). Massive algal bloom in early 2003.	Action Plan process underway. Riparian retirement. Sewage reticulation for lakeside settlements. Diversion of inflow from Ohau Channel down the Kaituna River.
Rotokakahi (Green Lake)	Moderate risk of cyanobacterial blooms. Water quality is lower than in the 1950s, but there has been no discernible change since 1970/71.	Actions to improve lake water quality to be negotiated with lake owners.
Rotoma	None. Low risk of cyanobacterial blooms.	Action Plan process beginning.
Rotomahana	Occasional cyanobacterial blooms.	No action at this time, until July 2008.
Rotorua	Experience water quality decline between 1978 and 1983. Foam (associated with kirchneriella algae species). Isolated blooms of nuisance algae. Moderate risk of cyanobacterial blooms.	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.
Tarawera	Occasional cyanobacterial blooms. Bacterial issues from septic tanks around some lakeshore areas.	Action Plan process beginning. Land use change to lower nitrogen and phosphorus export encouraged. Planning for future sewage reticulation.
Tikitapu (Blue Lake)	Probable lake water decline from septic tanks from camping ground and public amenities. Low risk of cyanobacterial blooms.	Sewerage reticulation as part of the Lake Okareka scheme. Action Plan process beginning.