The health of the beautiful lakes in Rotorua is considered one of the most important environmental concerns in the region. This incredible area of New Zealand, with otherworldly geothermal parks, rejuvenating spas, and fascinating Māori culture, also boasts 18 lakes that its residents take a lot of pride in.
The water quality of the most important 12 lakes in Rotorua Lakes District has been monitored by the Bay of Plenty Regional Council since 1990. This is done by measuring the Trophic Level Index (TLI), which is a value derived from four different parameters: water clarity, chlorophyll content, total phosphorus, and total nitrogen. If the TLI is high, that means the lake has greater amounts of nutrients in the water, which encourages algal blooms. If the TLI is low, it means that the water is clear and the water quality is high.
Highlights
- Overall water quality for the monitored lakes has gone up, with Lake Rotorua registering the biggest improvement;
- Last year (2020) was the best year since 2015 in terms of hitting the TLI targets;
- Positive changes in trophic states were registered by Lake Okaro and Lake Tikitapu.
What Do Different TLI Values Represent?
Before going into the actual numbers, it’s best to get an idea of what actually the different TLI values represent. Perhaps counter-intuitively, the lower the TLI of a lake, the better the quality of its water.
The TLI numbers used in this analysis represent the 3-year averages for the lakes, as averages smooth out trends, making them more relevant.
| TLI Score | Description | Water Quality |
|---|---|---|
| 0 - 2 | Microtrophic: The lake is very clean with very low levels of nutrients and algae. The lake can have snow or glacial sources. | Very good |
| 2.1 - 3 | Oligotrophic: The lake is clear and blue, with low levels of nutrients and algae | Good |
| 3.1 - 4 | Mesotrophic: The lake has moderate levels of nutrients and algae. | Fair |
| 4.1 - 5 | Eutrophic: The lake is murky, with high amounts of nutrients and algae. | Poor |
| > 5 | Supertrophic: The lake has very high amounts of phosphorus and nitrogen, and can be overly fertile and often associated with poor water clarity. Excessive algae growth can occur. Suitability for recreational purposes is often poor. | Very Poor |
Lake Rotorua Has Improved the Most in the Last 2 Decades, while Overall Water Quality Has Gone Up as Well
The overall water quality for the 12 monitored lakes has improved from an average TLI of 3.8 in the decade between 2001 and 2010 to an average of 3.6 in the decade between 2011 and 2020.
Lake Rotorua registered the biggest improvement, a massive 0.5 reduction of its TLI, from 4.8 to 4.3. It was followed by Lake Okaro with a 0.4 improvement from 5.4 to 5.0, and Lake Rotoiti, also with a 0.4 improvement, from 4.1 to 3.7. The biggest decrease in water quality happened at Lake Rotomahana, which registered a 0.2 TLI increase, from 3.8 to 4.0. A total of 7 lakes have improved in water quality, while in 5 lakes the water quality has decreased.
Last Year Was the Best Year Since 2015 in Terms of Hitting the TLI Targets
Only 4 of the 12 lakes hit their TLI targets in 2020, but last year was actually one of the best in the last 2 decades in that regard, holding the first position with 2014 and 2015. The worst years were 2009 and 2010, when none of the lakes hit their targets.
How Lakes Have Hit Their TLI Targets by Year
For Lake Rotorua, 2020 was the third year in a row when it hit its TLI target, while Lake Okaro has been doing so every year since 2015 except in 2019. Lake Rotomahana hit its target in 2020 for the first time since 2016, while Lake Rotoma also hit its target in 2020 for the first time since 2012.
How Did the Trophic States of Rotorua Lakes Evolve in the Last 2 Decades?
The biggest change in the trophic states of Rotorua lakes happened in Lake Okaro, which changed from supertrophic to eutrophic in 2015, and managed to stay eutrophic in all the years since, except in 2019. Lake Tikitapu also changed for the better, from mesotrophic to oligotrophic in 2007, and remained in that state for all the years since, except in 2010.
On the other side of transformation is Lake Tarawera, which was oligotrophic until 2017, when it changed to mesotrophic for 3 out of the 4 last years. However, there’s also a silver lining for Lake Tarawera, as it changed back to oligotrophic in 2020.
Overall, the biggest improvement is the fact that there were no supertrophic lakes in 5 of the last 6 years, the exception being 2019, when Lake Okaro was in that trophic state.
Sources
Catalin