Lake water quality

Trophic level index (TLI) of Rotorua lakes
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Purpose of indicator

Lake health is rated the highest environmental concern by Rotorua residents. Lake water quality is measured by Trophic Level Index (TLI) which consists of measuring 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 information and trend

Water quality of 12 of the Rotorua district’s 15 lakes has been monitored by the Bay of Plenty Regional Council since 1990. Each of these lakes have a target TLI which if exceeded by 0.2 TLI units for 2 concurrent years requires an Action Plan to 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 regional and district councils involvement. A further two lakes are monitored by the Waikato Regional Council and will be reported in future State of the Environment monitoring.

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
Lake type referred to in Table 1 is based on the TLI value. Figure 1 below explains different lake types as they relate to water quality (TLI).

 Figure 1. Lake types according to TLI values.

Table 1 shows short term trends from 2007 to 2011 as TLI 3 scores (3 year rolling average), and a description of the long term trend since 1900. Short term trends are discussed below, and in general shows that the trend for lake water quality since 2007 is getting better.

Lake Okaro showed improvement from 2007 to 2011, with TLI score rolling average decreasing from 5.5 to 5.1. Lake Rotorua has improved in TLI score to levels not seen since 2000, and which Bay of Plenty Regional Council scientists partly attribute to a reduction in phosphorus due to alum dosing of the Puarenga and Utuhina streams. The lake has also seen a change in dominant cyanobacteria shifting from Anabaena spp. to Microcytis wesenbergeii. The later species bloom in autumn rather than in summer when sampling occurred and when the lake is most popular with recreational users.

Lake Rotoehu, the shallowest of Rotorua lakes had a TLI of 4.4 in 2010-11, the best TLI score recorded for this lake in two decades. Harvesting hornwart (an aquatic plant) and removing it from Lake Rotoehu removes substantial amounts of nitrogen and phosphorus, which is considered to have resulted in less cyanobacteria algal blooms in both occurrence and bio-volume.

Lake Rotomahana’s trend shows possible degradation of water quality. The 2010-11 summer saw the highest chlorophyll-a concentration and phosphorus levels since records for this lake began. Lake Rotoiti’s TLI has remained stable over the 2009-2011 period, and its long term trend shows some improvement. Lake Rerewhakaaitu’s three yearly average score shows possible degradation since 2007.

Lake Okareka shows no change, with stable scores of 3.3 and 3.2. This is expected to improve following installation of wastewater reticulation services and as a result of the actions detailed in the Lake Okareka Action Plan. Lake Tikitapu also shows 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 Rotorua Lakes Strategy 2000 is an overarching document that has a vision for the future, and practical steps to achieve that vision. In 2011 a new strategy began being written for the Rotorua lakes as many of the recommendations have been achieved, and for some lakes issues have changed. While restoration of the 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

Since 2007
  • Eight of the 12 lakes show at least possible improvement
  • Two lakes show no change
  • Two lakes show possible degradation
Long term trends (since 1900)
  • Four of the 12 lakes show at least possible improvement
  • Three lakes show no change
  • Five lakes show possible or some degradation 

Page reviewed: 18 Dec 2013 3:34pm