Latest TLI Results for Lake Rotorua
Lake Rotorua water quality has been monitored by Bay of Plenty Regional Council since 1990. The TLI or Trophic Level Index is a number used to indicate the health of lakes in New Zealand. As a general rule of thumb, the lower the TLI, the better the water quality. The TLI target of 4.2 has been set for Lake Rotorua. Over the years good progress has been made in meeting this target as a result of alum dosing into two streams which flow into the lake.
The latest TLI results show that the three-year average TLI for Lake Rotorua remains above the target TLI at 4.4. A stratification event (caused by warm calm weather) in February 2016 may have triggered an increase in algal concentrations, peaking in April 2016.
How is TLI calculated? The TLI number is calculated using four separate water quality measurements:
- total nitrogen
- total phosphorous
- water clarity
- chlorophyll-a
Total nitrogen and total phosphorous are nutrients that plants thrive on. Large amounts of these nutrients in the lakes encourage the growth of algae which can lead to poor water quality. Water clarity is a measurement of how clear the water in the lake is. In general, the clearer the water, the better the water quality. Chlorophyll-a is the green colour in plants. Knowing how much chlorophyll there is in a lake gives us a good idea of how much algae the lake has. The more algae present, the poorer the water quality. The Trophic Level Index combines these four measurements into one number.
The table summarises the latest TLI results for the 12 Rotorua Lakes.
| Lake | Water Quality Target | 2015/ 2016 Result | 2014/ 2015 Result | 2013/ 2014 Result | 2012/ 2013 Result | Stability |
|---|---|---|---|---|---|---|
| Rotoma | 2.3 | 2.4 | 2.5 | 2.38 | 2.36 | To be investigated |
| Tikitapu (Blue Lake) | 2.7 | 2.9 | 2.9 | 2.82 | 2.75 | Stable |
| Okataina | 2.6 | 2.8 | 2.9 | 2.69 | 2.77 | Stable |
| Tarawera | 2.6 | 3.0 | 3.1 | 3.0 | 2.94 | Declining |
| Okareka | 3.0 | 3.2 | 3.3 | 3.30 | 3.15 | Stable |
| Rerewhakaaitu | 3.6 | 3.4 | 3.3 | 3.43 | 3.5 | Stable |
| Rotoiti | 3.5 | 3.8 | 3.8 | 3.40 | 3.44 | To be investigated |
| Rotokakahi | 3.1 | 3.7 | 4.0 | 3.63 | 3.69 | To be investigated |
| Rotomahana | 3.9 | 4.0 | 4.0 | 3.8 | 3.93 | Stable |
| Rotorua | 4.2 | 4.4 | 4.4 | 4.2 | 4.24 | Stable |
| Rotoehu | 3.9 | 4.6 | 4.5 | 3.95 | 3.95 | Declining |
| Okaro | 5.0 | 4.6 | 4.5 | 4.49 | 5.42 | Improving |
