Trophic state index

Lake George, New York,an oligotrophic lake

TheTrophic State Index(TSI) is a classification system designed to rate water bodies based on the amount ofbiological productivitythey sustain.Although the term "trophic index" is commonly applied to lakes, any surface water body may be indexed.

The TSI of a water body is rated on a scale from zero to one hundred.Under the TSI scale, water bodies may be defined as:

• oligotrophic(TSI 0–40, having the least amount of biological productivity, "good" water quality);
• mesotrophic(TSI 40–60, having a moderate level of biological productivity, "fair" water quality); or
• eutrophictohypereutrophic(TSI 60–100, having the highest amount of biological productivity, "poor" water quality).

The quantities ofnitrogen,phosphorus,and other biologically useful nutrients are the primary determinants of a water body's TSI. Nutrients such as nitrogen and phosphorus tend to belimiting resourcesin standing water bodies, so increased concentrations tend to result in increased plant growth, followed by corollary increases in subsequenttrophic levels.Consequently, trophic index may sometimes be used to make a rough estimate of biological condition of water bodies.

Carlson's Trophic State Index

Carlson's index was proposed by Robert Carlson in his 1977 seminal paper, "A trophic state index for lakes".It is one of the more commonly used trophic indices and is the trophic index used by theUnited States Environmental Protection Agency.Thetrophic stateis defined as the total weight ofbiomassin 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.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 variable

Because they tend to correlate, three independent variables can be used to calculate the Carlson Index:chlorophyll pigments,total phosphorusandSecchi 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 estimation 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:

$\left({\frac {1}{z}}\right)\left(\ln {\frac {I_{0}}{I_{z}}}\right)=k_{w}+\alpha C$

wherez= the depth at which the disk disappears,

I₀is the intensity of light striking the water's surface,

I_zis about 10% ofI₀and is considered a constant,

k_wis a coefficient for the attenuation of light by water and dissolved substances,

αis treated as a constant with the units of square meters per milligram and

Cis 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,mesotrophicoreutrophic.Lakes with extreme trophic indices may also be consideredhyperoligotrophicorhypereutrophic(also "hypertrophic" ). The table below demonstrates how the index values translate into trophic classes.

Relationships between Trophic State Index, chlorophyll, phosphorus, Secchi depth, and trophic class (after Carlson 1996)

Trophic State Index	Chlorophyll (μg/L)	Phosphorus (μg/L)	Secchi depth (m)	Trophic Class
< 30—40	0—2.6	0—12	> 8—4	Oligotrophic or hipotrophic
40—50	2.6—7.3	12—24	4—2	Mesotrophic
50—70	7.3—56	24—96	2—0.5	Eutrophic
70—100+	56—155+	96—384+	0.5— < 0.25	Hypertrophic

Oligotrophic lakes generally host very little or noaquatic vegetationand 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), massivefish die-offsmay occur as decomposing biomass deoxygenates the water.

Oligotrophic

Kurtkowiec Lake,an oligotrophic lake in theTatra Mountainsof southernPoland

Limnologistsuse the term "oligotrophic"or" hipotrophic "to describe lakes that have lowprimary productivitydue tonutrientdeficiency. (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 crystallineigneous,graniticbedrock. Due to their lowalgalproduction, these lakes consequently have very clear waters, with highdrinking-waterquality.

Lakes that have intermixing of their layers are classified into the category ofholomictic,whereas lakes that do not have interlayer mixing are permanently stratified and thus are termedmeromictic.

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 theepilimnionto the hypolimnion.

In this way, oligotrophic lakes can have significantoxygendown to the depth to which the aforementioned seasonal mixing occurs, but they will be oxygen deficient below this depth. Therefore, oligotrophic lakes often supportfishspecies such aslake trout,which require cold, well-oxygenatedwaters. The oxygen content of these lakes is a function of their seasonally mixedhypolimneticvolume. Hypolimnetic volumes that are anoxic will result in fish congregating in areas where oxygen is sufficient for their needs.

Anoxia is more common in the hypolimnion during the summer when mixing does not occur.In the absence of oxygen from the epilimnion,decompositioncan cause hypoxia in the hypolimnion.

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

Main article:Eutrophication

Algal bloom in a village river in the mountains nearChengdu,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 ofeutrophicationcan occur naturally and byhuman impact on the environment.

Eutrophic comes from theGreekeutrophosmeaning "well-nourished", fromeumeaning good andtrepheinmeaning "to nourish".

Hypertrophic

Hypertrophic or hypereutrophic lakes are very nutrient-rich lakes characterized by frequent and severe nuisancealgal bloomsand low transparency. Hypereutrophic lakes have a visibility depth of less than 3 feet (90 cm), they have greater than 40 micrograms/litre totalchlorophylland greater than 100 micrograms/litrephosphorus.

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 causebiodilutionto occur, which is a decrease in the concentration of a pollutant with an increase introphic level.This is opposed tobiomagnificationand is due to a decreased concentration from increased algal uptake.

Trophic index drivers

Both natural andanthropogenicfactors can influence a lake or other water body's trophic index. A water body situated in a nutrient-rich region with highnet primary productivitymay be naturally eutrophic. Nutrients carried into water bodies fromnon-point sourcessuch as agricultural runoff, residential fertilisers, and sewage will all increase the algalbiomass,and can easily cause an oligotrophic lake to become hypereutrophic.

Freshwater lakes

Although there is no absolute consensus as to which nutrients contribute the most to increasing primary productivity, phosphorus concentration is thought to be the main limiting factor in freshwater lakes.This is likely due to the prevalence of nitrogen-fixing microorganisms in these systems, which can compensate for a lack of readily available fixed nitrogen.

Marine ecosystems

In some coastal marine ecosystems, research has found nitrogen to be the key limiting nutrient, driving primary production independently of phosphorus.Nitrogen fixation cannot adequately supply these marine ecosystems, because the nitrogen fixing microbes are themselves limited by the availability of variousabiotic factorslike sunlight and dissolved oxygen.However, marine ecosystems are too broad a range of environments for one nutrient to limit all marine primary productivity. The limiting nutrient may vary in different marine environments according to a variety of factors like depth, distance from shore, or availability of organic matter.

Management targets

Often, the desired trophic index differs between stakeholders. Water-fowl enthusiasts (e.g. duck hunters) may want a lake to be eutrophic so that it will support a large population of waterfowl. Residents, though, may want the same lake to be oligotrophic, as this is more pleasant for swimming and boating. Natural resource agencies are generally responsible for reconciling these conflicting uses and determining what a water body's trophic index should be.

See also

• Biomass (ecology)
• Eutrophication
• Nonpoint source pollution
• Secchi disk
• Surface runoff
• Trophic level
• Trophic level index,a similar measure used in New Zealand
• Water quality
• List of biological development disorders