The Trophic Level Index (TLI) is an indicator of lake water quality. Four parameters are combined to construct the TLI: total nitrogen, total phosphorus, clarity and chlorophyll a. The parameters reflect the dynamics of the annual lake cycle.
Nitrogen and phosphorus are essential plant nutrients. In large quantities they can encourage the growth of nuisance aquatic plants such as algal blooms. High levels of water-bound nitrogen and phosphorus most often come from agricultural runoff and urban wastewater, but can also come from geothermal inputs and deep springs that leach phosphorus from the rock geology.
Clarity is measured using a Secchi disc attached to a tape measure. The depth at which the disc disappears from sight is recorded by the tape measure.
Chlorophyll a is the green pigment in plants used for photosynthesis. It is a good indicator of the total quantity of algae in a lake. Algae are a natural part of any lake system, but large amounts of algae decrease water clarity, make the water look green, can form surface scums, reduce dissolved oxygen levels, can alter pH levels, and can produce unpleasant tastes and smells.
Calculations for the TLI:
- TLn = -3.61 + 3.01log (TN)
- TLp = 0.218 + 2.92log(TP)
- TLs = 5.10 + 2.27log(1/SD – 1/40)
- TLc = 2.22 + 2.54log(Chla)
- TLI = Ó (TLn + TLp + TLs + TLc)/4
The higher the TLI, the lower the water quality. Trophic level bands are grouped into trophic states for quantitative description, microtrophic to hypertrophic as shown below.
| Trophic state |
Nutrient enrichment category |
Trophic Level |
Chla (mg/m3) |
Secchi depth (m) |
TP (mg/m3) |
TN(mg/m3) |
|
Ultramicrotrophic |
Practically Pure |
0.0 to 1.0 |
< 0.33 |
>25 |
< 1.8 |
<34 |
|
Microtrophic |
Very Low |
1.0 to 2.0 |
0.33 - 0.82 |
25 - 15 |
1.8 - 4.1 |
34 - 73 |
|
Oligotrophic |
Low |
2.0 to 3.0 |
0.82 - 2.0 |
15 - 7 |
4.1 - 9.0 |
73 - 157 |
|
Mesotrophic |
Medium |
3.0 to 4.0 |
2 - 5 |
7.0 - 2.8 |
9 - 20 |
157 - 337 |
|
Eutrophic |
High |
4.0 to 5.0 |
5 - 12 |
2.8 - 1.1 |
20 - 43 |
337 - 725 |
|
Surpertrophic |
Very High |
5.0 to 6.0 |
12 - 31 |
1.1 - 0.4 |
43 - 96 |
725 - 1558 |
|
Hypertrophic |
Saturated |
> 6.0 |
> 31 |
< 0.4 |
> 96 |
> 1558 |
Trophic states, as determined by the four key variables:
Microtrophic lakes are very clean, and often have snow or glacial sources. Lake Sumner in North Canterbury is a microtrophic lake.
Oligotrophic lakes are clear and blue, with low levels of nutrients and algae. Lake Rotoma is an oligotrophic lake.
Mesotrophic lakes have moderate levels of nutrients and algae. Lake Rerewhakaaitu is a mesotrophic lake.
Eutrophic lakes are green and murky, with higher amounts of nutrients and algae. Lakes Rotorua and Rotoiti are now both eutrophic lakes.
Supertrophic lakes are fertile and saturated in phosphorus and nitrogen, and have very high algae growth and blooms during calm sunny periods. Lake Okaro is a supertrophic lake.
Hypertropic lakes are highly fertile and supersaturated in phosphorus and nitrogen. They are rarely suitable for recreation and habitat for desirable aquatic species is limited. Many lakes in the Waikato are hypertrophic, like Lakes Hakanoa, Ngaroto, Mangahia, Waahi and Waikare.
Blue-green algal blooms occur more frequently above a certain TLI level. The worst quality waters experience the blue-green algae/cyanobacteria blooms e.g. Okawa Bay (TLI 5.3), Lake Okaro (TLI 5.7). Lake Rotoehu began to experience algal blooms in 1994 when the quality of the lake deteriorated and the TLI increased from 3.7 to 4.8. Lake Rotoehu’s TLI has fallen slowly since then and in 2004 no blue-green blooms occurred. The 2004 TLI will likely be closer to 3.7 than 4.8.
Blue-green algae blooms can form in lakes with good water quality, like Lake Tarawera. Here a large inflow of water with a low nitrogen to phosphorus ratio enters the lake along the shoreline adjacent to Rotomahana. This favours blue-green algae and when conditions are calm they can assume bloom proportions.
Dissolved oxygen
Dissolved oxygen is important for fish and other aquatic life to breathe. Water should be more than 80 percent saturated with dissolved oxygen for aquatic plants and animals to live in it.
In deep lakes, where the waters don’t mix for several months over the summer, reduced dissolved oxygen in the stagnant bottom waters is of concern, especially for fish and aquatic animals. Decaying algal material that falls out of the surface water uses up dissolved oxygen in the bottom waters. When this happens, nitrogen and phosphorus are released from the lake bed sediments. When the lake waters re-mix in winter these nutrients are available for plants and algae in the surface waters.
E-coli
Escherichia coli (E. coli) is an indicator organism for disease-causing agents in the water. E-coli come from human and animal faeces, so if they are present in water there are likely to be other nasties as well that make the water unsafe for drinking or swimming. Drinking water should have no detectable E-coli bacteria in it at all. Water used for recreation should have less than 126 E-coli colonies per 100 ml of water.
Water temperature
Water temperature can affect lake dynamics. The temperature of the Rotorua lakes increases at a rate of around 0.01 - 0.07°C per year.
Algal species
There are a variety of blue-green algae and phytoplankton that are present in the Rotorua lakes. The presence, ratio and concentration of these species help understand how a lake’s trophic state affects water quality and algal composition.
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