Trophic state index
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 levels.  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 used 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. 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. 
Because they tend to correlate, three independent variables can be used to calculate the Carlson 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:
- 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.
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.
Relationships between Trophic Index (TI), chlorophyll (Chl), phosphorus (P, both micrograms per litre), Secchi depth (SD, metres), and Trophic Class (after Carlson 1996)
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.
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 oligotrophic 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).
Mesotrophic lakes are lakes with an intermediate level of productivity, greater than oligotrophic lakes, but less than eutrophic lakes. 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.
An eutrophic body of water, commonly a lake or pond, has high primary productivity due to excessive nutrients and is subject to algal blooms resulting in poor water quality. The bottom waters of such bodies are commonly deficient in oxygen, ranging from hypoxic to anoxic. Eutrophic waters commonly lack fish species like trout, which require cold, well-oxygenated water. This oxygen deficiency is most apparent in shallow lakes, owing to the smaller hypolimnetic volume. The process of eutrophication may occur naturally or be the result of anthropogenic influences. 
Hypereutrophic lakes are very nutrient-rich lakes characterized by frequent and severe nuisance algal blooms and low transparency. Hypereutrophic lakes are the most biologically productive lakes, and support large amounts of plants, fish and other animals. Hypereutrophic lakes have a visibility depth of less than 3 feet, they have greater than 40 micrograms/litre total chlorophyll and greater than 100 micrograms/liter 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
Both natural and anthropogenic factors can influence a lake or other water body's Trophic Index. A water body situated in a nutrient-rich region with high Net Primary Productivity may be naturally eutrophic. Nutrients carried into water bodies from non-point sources such as agricultural runoff, residential fertilisers, and sewage will all increase the algal biomass, and can easily cause an oligotrophic lake to become hypereutrophic.
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, on the other hand, 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.
- Biomass (ecology)
- Nonpoint source pollution
- Secchi disk
- Surface runoff
- Trophic level
- Trophic level index, a similar measure used in New Zealand
- Water quality
- ^ Note that this use of trophic levels refers to feeding dynamics, and has a much different meaning than a body of water's trophic index.
- ^ a b c United States Environmental Protection Agency (2007) Carlson's Trophic State Index. Aquatic Biodiversity. http://www.epa.gov/bioindicators/aquatic/carlson.html accessed 17 February, 2008.
- ^ a b Carlson, R.E. (1977) A trophic state index for lakes. Limnology and Oceanography. 22:2 361--369.
- ^ a b Carlson R.E. and J. Simpson (1996) A Coordinator's Guide to Volunteer Lake Monitoring Methods. North American Lake Management Society. 96 pp.
- ^ Campbell, Neil A.; Reece, Jane B. (2005). Biology. Benjamin Cummings. p. 1230 p. ISBN 0-8053-7146-X.
- ^ Definition of eutrophic at dictionary.com.
Modelling ecosystems – trophic components General Producers Consumers Decomposers Microorganisms Food webs Example webs Processes
- Cascade effect
- Climax community
- Competitive exclusion principle
- Consumer-resource systems
- Ecological network
- Ecological succession
- Energy quality
- Energy Systems Language
- Feed conversion ratio
- Feeding frenzy
- Mesotrophic soil
- Nutrient cycle
- Paradox of the plankton
- Trophic cascade
- Trophic mutualism
- Trophic state index
Defense/counter Modelling ecosystems – other components Population ecology
- Allee effect
- Ecological yield
- Effective population size
- Intraspecific competition
- Logistic function
- Malthusian growth model
- Maximum sustainable yield
- Overpopulation in wild animals
- Population cycle
- Population dynamics
- Population modeling
- Population size
- Predator–prey equations
- Small population size
- Density-dependent inhibition
- Ecological effects of biodiversity
- Ecological extinction
- Endemic species
- Flagship species
- Gradient analysis
- Indicator species
- Introduced species
- Invasive species
- Latitudinal gradients in species diversity
- Minimum viable population
- Neutral theory
- Occupancy-abundance relationship
- Population viability analysis
- Priority effect
- Rapoport's rule
- Relative abundance distribution
- Relative species abundance
- Species diversity
- Species homogeneity
- Species richness
- Species distribution
- Species-area curve
- Umbrella species
Species interaction Spatial ecology
- Cross-boundary subsidy
- Edge effect
- Foster's rule
- Habitat fragmentation
- Ideal free distribution
- Intermediate Disturbance Hypothesis
- Island biogeography
- Landscape ecology
- Landscape epidemiology
- Landscape limnology
- Patch dynamics
- r/K selection theory
- Source–sink dynamics
Niche Other networks
- Assembly rules
- Bateman's principle
- Ecological collapse
- Ecological debt
- Ecological deficit
- Ecological energetics
- Ecological indicator
- Ecological threshold
- Ecosystem diversity
- Extinction debt
- Kleiber's law
- Liebig's law of the minimum
- Marginal value theorem
- Thorson's rule
- Alternative stable state
- Balance of nature
- Biological data visualization
- Constructal theory
- Ecological economics
- Ecological footprint
- Ecological forecasting
- Ecological humanities
- Ecological stoichiometry
- Ecosystem based fisheries
- Evolutionary ecology
- Functional ecology
- Industrial ecology
- Natural environment
- Systems ecology
- Theoretical ecology
List of ecology topics
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