Trophic state index
Encyclopedia
The quantities of nitrogen
, phosphorus
, and other biologically useful nutrients are the
primary determinants of a body of water's trophic state index (TSI). Nutrients such as nitrogen and phosphorus
tend to be limiting resources in standing water bodies, so increased concentrations tend to
result in increased plant growth, followed by corollary increases in subsequent trophic level
s.
Consequently,
a body of water's trophic index may sometimes be used to make a rough estimate of its biological
condition. Although the term "trophic index" is commonly applied to
lakes, any surface water body may be indexed.
by the United States Environmental Protection Agency
. The
trophic state is defined as the total weight of biomass
in a given water body at the time of measurement. Because they are of public concern, the Carlson index uses the algal biomass as
an objective classifier of a lake or other water body's trophic status.
Oceanography. 22:2 361--369. According to the US EPA, the Carlson Index should only be
used with lakes that have relatively few rooted plants and non-algal turbidity sources.
Index: chlorophyll pigments, total phosphorus and Secchi depth. Of these three,
chlorophyll will probably yield the most accurate measures, as it is the most accurate predictor
of biomass. Phosphorus may be a more accurate of a water body's summer trophic status than
chlorophyll if the measurements are made during the winter. Finally, the Secchi depth is probably
the least accurate measure, but also the most affordable and expedient one. Consequently, citizen
monitoring programs and other volunteer or large-scale surveys will often use the Secchi depth.
By translating the Secchi transparency values to a log base 2 scale, each successive doubling of
biomass is represented as a whole integer index number. The Secchi depth, which measures water
transparency, indicates the concentration of dissolved and particulate material in the water,
which in turn can be used to derive the biomass. This relationship is expressed in the following
equation:
mesotrophic or eutrophic. Lakes with extreme trophic indices may also be considered
hyperoligotrophic or hypereutrophic. 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 TI),
massive fish die-offs may occur as decomposing biomass deoxygenates the water.
An oligotrophic lake is a lake with low primary productivity, the result of low nutrient
content. These lakes have low algal production, and consequently, often have very clear waters, with high drinking-water
quality. The bottom waters of such lakes typically have ample oxygen
; thus, such lakes often support many fish
species, like lake trout
, which require cold, well-oxygenated
waters. The oxygen content is likely to be higher in deep lakes, owing to their larger hypolimnetic
volume.
Ecologists use the term oligotroph
ic to distinguish unproductive lakes, characterised by nutrient deficiency, from productive, eutrophic lakes, with an ample or excessive nutrient supply. Oligotrophic lakes are most common in cold regions underlain by resistant igneous rocks (especially granitic bedrock).
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
, and other biologically useful nutrients are the
primary determinants of a body of water's trophic state index (TSI). Nutrients such as nitrogen and phosphorus
tend to be limiting resources in standing water bodies, so increased concentrations tend to
result in increased plant growth, followed by corollary increases in subsequent trophic level
Trophic level
The trophic level of an organism is the position it occupies in a food chain. The word trophic derives from the Greek τροφή referring to food or feeding. A food chain represents a succession of organisms that eat another organism and are, in turn, eaten themselves. The number of steps an organism...
s.
Consequently,
a body of water's trophic index may sometimes be used to make a rough estimate of its biological
condition. Although the term "trophic index" is commonly applied to
lakes, any surface water body may be indexed.
Carlson's Trophic State Index
Carlson's index is one of the more commonly used trophic indices, and is the trophic index usedby the United States Environmental Protection Agency
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...
. The
trophic state is defined as the total weight of biomass
Biomass (ecology)
Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms,...
in a given water body at the time of measurement. Because they are of public concern, the Carlson index uses the algal biomass as
an objective classifier of a lake or other water body's trophic status.
Oceanography. 22:2 361--369. According to the US EPA, the Carlson Index should only be
used with lakes that have relatively few rooted plants and non-algal turbidity sources.
Index variables
Because they tend to correlate, three independent variables can be used to calculate the CarlsonIndex: chlorophyll pigments, total phosphorus and Secchi depth. Of these three,
chlorophyll will probably yield the most accurate measures, as it is the most accurate predictor
of biomass. Phosphorus may be a more accurate of a water body's summer trophic status than
chlorophyll if the measurements are made during the winter. Finally, the Secchi depth is probably
the least accurate measure, but also the most affordable and expedient one. Consequently, citizen
monitoring programs and other volunteer or large-scale surveys will often use the Secchi depth.
By translating the Secchi transparency values to a log base 2 scale, each successive doubling of
biomass is represented as a whole integer index number. The Secchi depth, which measures water
transparency, indicates the concentration of dissolved and particulate material in the water,
which in turn can be used to derive the biomass. This relationship is expressed in the following
equation:
- where z = the depth at which the disk disappears,
- I0 is the intensity of light striking the water's surface,
- Iz is about 10% of I0 and is considered a constant,
- kw is a coefficient for the attenuation of light by water and dissolved substances,
- α is treated as a constant with the units of square meters per milligram and
- C is the concentration of particulate matter in units for milligrams per cubic meter.
Trophic classifications
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. The table below demonstrates how the index values
translate into trophic classes.
| TI | Chl | P | SD | Trophic Class |
|---|---|---|---|---|
| <30—40 | 0—2.6 | 0—12 | >8—4 | Oligotrophic |
| 40—50 | 2.6—20 | 12—24 | 4—2 | Mesotrophic |
| 50—70 | 20—56 | 24—96 | 2—0.5 | Eutrophic |
| 70—100+ | 56—155+ | 96—384+ | 0.5—<0.25 | Hypereutrophic |
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 TI),
massive fish die-offs may occur as decomposing biomass deoxygenates the water.
Oligotrophic
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...
content. These lakes have low algal production, and consequently, often have very clear waters, with high drinking-water
Drinking water
Drinking water or potable water is water pure enough to be consumed or used with low risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually...
quality. The bottom waters of such lakes typically have ample oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
; thus, such lakes often support many fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
species, like lake trout
Trout
Trout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water...
, which require cold, well-oxygenated
Oxygenation (environmental)
Environmental oxygenation can be important to the sustainability of a particular ecosystem. Insufficient oxygen may occur in bodies of water such as ponds and rivers, tending to suppress the presence of aerobic organisms such as fish...
waters. The oxygen content is likely to be higher in deep lakes, owing to their larger hypolimnetic
Hypolimnion
The hypolimnion is the dense, bottom layer of water in a thermally-stratified lake. It is the layer that lies below the thermocline.Typically the hypolimnion is the coldest layer of a lake in summer, and the warmest layer during winter...
volume.
Ecologists use the term oligotroph
Oligotroph
An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments...
ic to distinguish unproductive lakes, characterised by nutrient deficiency, from productive, eutrophic lakes, with an ample or excessive nutrient supply. Oligotrophic lakes are most common in cold regions underlain by resistant igneous rocks (especially granitic bedrock).