Lake water quality of lakes within the Lakes A Zone

Trophic level index (TLI)
Purpose of monitoring

Lake health is rated the highest environmental concern by Rotorua residents. Lake water quality is measured by Trophic Level Index (TLI) which measures four parameters: water clarity, chlorophyll content, total phosphorus and total nitrogen. From these parameters a TLI value is calculated. The higher the value, the greater the nutrients and fertility of the water which encourages growth, including algal blooms (see figure 1). Monitoring lakes TLI gives an indication of whether water quality is improving, showing no change or declining.

Current trends

Water quality of 12 of the 15 Rotorua district’s lakes has been monitored by the Bay of Plenty Regional Council since 1990. The lakes reported on here are those within the Lakes A Zone. Each of these lakes have a target TLI which if exceeded by 0.2 TLI units for 2 concurrent years then an action plan must be created for that lake. Action plans are intended to halt degradation and, where needed, enhance water quality. They are partnership action plans to encompass community and both regional and district councils involvement.

Lakes can have action plans without exceeding the TLI target; however, this is not a requirement. Lakes Okaro, Okareka and Tikitapu have lakes action plans that were created together with their respective communities. Other lakes in the Lakes A Zone will have action plans in the future. For more information on lakes action plans see Bay of Plenty Regional Council website.

Table 1. Lake TLI unit values and long term trend.
* Long term trends starting point is year 1900
Source: Bay of Plenty Regional Council, 2011
Figure 1. Lake types according to TLI values.

Table 1 shows short term trends from 2007 to 2011. These are shown as 3 year rolling average TLI scores to smooth out fluctuations in the data. A description of the long term trend since 1900 is also included. Short term trends (2007- 2011) are discussed below.

Lake Okaro showed improvement to 2011 with its 3 year rolling average dropping from 5.5 in 2007 to 5.1 in 2010 and 2011.  

Lake Okareka shows no change, with stable scores of 3.3 and 3.2. Similarly Tarawera shows no change with scores between 3.2 and 3.3. Tikitapu (Blue Lake) has stable scores of 3.0 and 3.1. However lower levels of both nitrogen and phosphorus, and better water clarity, were observed in 2010-11. Lake Okataina also remains stable, despite an increase in phosphorus in 2010-11.

The TLI of Lake Rotokakahi (Green Lake) shows some degradation since 2007. Lake Rotomahana’s trend shows possible degradation of water quality. The 2010/11 summer saw the highest phosphorus levels since records for this lake began.

Seepage of sewage from septic tanks located close to lakes has been identified as a contributing factor affecting lake water quality. The Rotorua District Council project for lakeside settlement wastewater reticulation schemes has seen Lake Okareka and Blue Lake communities connected to reticulated wastewater services in 2010. This is expected to result in reducing nutrients input to lakes by approximately 4 tonnes per year. Lake Tarawera settlement is due to be connected to reticulated wastewater services by 2015, further reducing nutrient inputs to lakes by another 4.5 tonnes approximately.

The Rotorua Lakes Strategy 2000 is an overarching document with a vision for the future, and practical steps to achieve that vision. In 2011 preparation of a new strategy began for the Rotorua lakes as many of its recommendations have been achieved, and for some lakes issues have changed. While restoration of lakes is a long term project it is important that strategy and policy documents identify and address the most current and important issues.

The Freshwater National Policy Statement was issued in September 2011. Its objectives and policies address freshwater quality and quantity (eg water takes, damming, diversion and water flows). Regional and district plans must give effect to the national policy statement.

In Summary
  • Lake Okaro shows improvement
  • Lakes Okareka and Tarawera show no change
  • Lakes Tikitapu and Okataina show possible degradation
  • Lake Rotokakahi shows some degradation
Further information sources
Page reviewed: 03 Jul 2019 10:12am