Trophic state index

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The quantities of nitrogen, phosphorus, and other biologically useful nutrients are the primary determinants of a lake'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. [1] Consequently, a lakes trophic index may sometimes be used to make a rough estimate of its biological condition.[2] Although the term "trophic index" is commonly applied to lakes, any surface water body may be indexed.

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[edit] 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 Unites States Environmental Protection Agency.[2] 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.[3] 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. [2]

[edit] Index Variables

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.[4] 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

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.[3]

[edit] 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.

Relationships between Trophic Index (TI), chlorphyll (Chl), phosphorus (P, both micrograms per litre), Secchi depth (SD, metres), and Trophic Class (after Carlson 1996[4])

TI Chl P SD Trophic Class
<30--40 0--2.6 0--12 >8--4 Oligotrophic
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 Hypereutrophic

Oligotrophic lakes generally host very little or no aquatic vegetation and is 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.

[edit] 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.

[edit] 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, 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.

[edit] See also

[edit] References and Footnotes

  1. ^ Note that this use of trophic levels refers to feeding dynamics, and has a much different meaning than a lake's trophic index.
  2. ^ 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.
  3. ^ a b Carlson, R.E. (1977) A trophic state index for lakes. Limnology and Oceanography. 22:2 361--369.
  4. ^ 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.