From the above presented parameters, trophic state of the water body can be determined. Trophic state is another indicator of water quality. Lakes can be divided into three basic categories based on trophic state—oligotrophic, mesotrophic, and eutrophic. These categories reflect a lake’s nutrient and clarity levels.
Oligotrophic lakes are generally clear, deep and free of weeds or large algae blooms. They are low in nutrients and do not support large fish populations. However, they can develop food chain with salmonidae fish.
Eutrophic lakes are high in nutrients and support a large biomass (all the plants and animals living in a lake). They are very productive, usually either weedy or subject to frequent algae blooms, or both. Eutrophic lakes often support large fish populations. They are also susceptible to oxygen depletion. Small, shallow, eutrophic lakes are especially vulnerable to winterkill which can reduce the number and variety of fish. Cyprinidae fish are commonly found in eutrophic lakes.
Mesotrophic lakes lie between the oligotrophic and eutrophic stages. Primary production is increased and occasional algal blooms can emerge.
Various methods are used to calculate the trophic state of lakes. Common characteristics used to make the determination are:
• total phosphorus concentration (important for algae growth)
• chlorophyll a concentration (a measure of the amount of algae present)
• water clarity
Low levels of phosphorus are associated with low levels of algae (chlorophyll a), which are associated with high Secchi disc readings.
Trophic state of the lake can be determined also on the basis of the calculated average biovolume of the phytoplankton (mm3/L).