What is the trophic state?
To help understand eutrophication, scientists have devised a system in which lakes are classified according to their "trophic status" or "trophic state". A lake's trophic state is a measure of its "biological productivity", which is a measure of the mass of plants and animals in a lake.
How do we measure the trophic state?
To determine the biological productivity i.e., the water quality of a lake, four parameters need to be measured: total nitrogen, total phosphorus, clarity and chlorophyll a. These parameters reflect the dynamics of the annual lake cycle. Combined they form the Trophic Level Index (TLI). The higher the TLI, the lower the water quality.
Nitrogen and phosphorus are nutrients that often limit the concentrations of algae in a lake; the higher the concentration of these nutrients, the more eutrophic a lake. In large quantities they can encourage the growth of nuisance aquatic plants such as algal blooms. High levels of nitrogen and phosphorus in the water most often come from agricultural runoff and urban wastewater, but can also come from geothermal inputs and deep springs where leaching of phosphorus has occured as a result if the the rock geology.
Water clarity is measured with a Secchi disk, a circular white or black-and-white disk attached to a tape measure that is lowered through the water to a depth where it disappears from view. In clear, oligotrophic lakes such as Rotoma, the Secchi depth may be greater than 10 metres while in eutrophic lakes with low visibility, the Secchi depth can be less than 1 metre.
Concentrations of algae are the best measure of lake trophic status though it is difficult to measure algae directly. Chlorophyll a concentration is used to give an indirect measure of algal concentrations. Chlorophyll a is the pigment present in plants that makes them appear green and allows them to obtain energy from the sun (photosynthesis).
Levels of oxygen provide an indirect measure of lake trophic status. Eutrophic lakes often have little or no oxygen in the bottom waters through summer when there is little mixing between surface and bottom waters.
The TLI has been adopted for the New Zealand Lakes Water Quality Monitoring Programme, and is recommended for trophic level assessments by the Ministry for the Environment.
Trophic states as determined by the four key variables are grouped from microtrophic to hypertrophic as shown below:
- 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.
- Hypertrophic lakes (sometimes referred to as supertrophic) are fertile and have extremely high levels of phosphorus and nitrogen. They are rarely suitable for recreation and habitat for desirable aquatic species is limited. Many lakes in the Waikato are hypertrophic, including Lakes Hakanoa, Ngaroto, Mangahia, Waahi and Waikare, while Lake Okaro one of the Rotorua lakes, is hypertrophic.