A lake is usually classified as being in one of three possible classes:
oligotrophic,
mesotrophic or
eutrophic. Lakes with extreme trophic indices may also be considered
hyperoligotrophic or
hypereutrophic (also "hypertrophic"). The table below demonstrates how the index values translate into trophic classes. Oligotrophic lakes generally host very little or no
aquatic vegetation and are relatively clear, while eutrophic lakes tend to host large quantities of organisms, including algal blooms. Each trophic class supports different types of fish and other organisms, as well. If the algal biomass in a lake or other water body reaches too high a concentration (say >80 TSI), massive
fish die-offs may occur as decomposing biomass deoxygenates the water.
Oligotrophic , an oligotrophic lake in the
Tatra Mountains of southern
Poland Limnologists use the term "
oligotrophic" or "hipotrophic" to describe lakes that have low
primary productivity due to
nutrient deficiency. (This contrasts against eutrophic lakes, which are highly productive due to an ample supply of nutrients, as can arise from human activities such as agriculture in the watershed.) Oligotrophic lakes are most common in cold, sparsely developed regions that are underlain by crystalline
igneous,
granitic bedrock. Due to their low
algal production, these lakes consequently have very clear waters, with high
drinking-water quality. Lakes that have intermixing of their layers are classified into the category of
holomictic, whereas lakes that do not have interlayer mixing are permanently stratified and thus are termed
meromictic. Generally, in a holomictic lake, during the fall, the cooling of the epilimnion reduces lake stratification, thereby allowing for mixing to occur. Winds aid in this process. Thus it is the deep mixing of lakes (which occurs most often during the fall and early winter, in holomictic lakes of the monomictic subtype) that allows oxygen to be transported from the
epilimnion to the hypolimnion. In this way, oligotrophic lakes can have significant
oxygen down to the depth to which the aforementioned seasonal mixing occurs, but they will be oxygen deficient below this depth. Therefore, oligotrophic lakes often support
fish species such as
lake trout, which require cold, well-
oxygenated waters. The oxygen content of these lakes is a function of their seasonally mixed
hypolimnetic volume. Hypolimnetic volumes that are anoxic will result in fish congregating in areas where oxygen is sufficient for their needs.
Mesotrophic Mesotrophic lakes are lakes with an intermediate level of productivity. These lakes are commonly clear water lakes and ponds with beds of submerged aquatic plants and medium levels of nutrients. The term mesotrophic is also applied to terrestrial habitats. Mesotrophic soils have moderate nutrient levels.
Eutrophic and hypertrophic Eutrophic ,
Sichuan, China A eutrophic water body, commonly a lake or pond, has high biological productivity. Due to excessive nutrients, especially nitrogen and phosphorus, these water bodies are able to support an abundance of aquatic plants. Usually, the water body will be dominated either by aquatic plants or algae. When aquatic plants dominate, the water tends to be clear. When algae dominate, the water tends to be darker. The algae engage in photosynthesis which supplies oxygen to the fish and biota which inhabit these waters. Occasionally, an excessive algal bloom will occur and can ultimately result in fish death, due to respiration by algae and bottom-living bacteria. The process of
eutrophication can occur naturally and by
human impact on the environment. Eutrophic comes from the
Greek eutrophos meaning "well-nourished", from
eu meaning good and
trephein meaning "to nourish".
Hypertrophic Hypertrophic or hypereutrophic lakes are very nutrient-rich lakes characterized by frequent and severe nuisance
algal blooms and low transparency. Hypereutrophic lakes have a visibility depth of less than , they have greater than 40 micrograms/litre total
chlorophyll and greater than 100 micrograms/litre
phosphorus. The excessive algal blooms can also significantly reduce oxygen levels and prevent life from functioning at lower depths creating dead zones beneath the surface. Likewise, large algal blooms can cause
biodilution to occur, which is a decrease in the concentration of a pollutant with an increase in
trophic level. This is opposed to
biomagnification and is due to a decreased concentration from increased algal uptake. ==Trophic index drivers==