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Carlson's Trophic State Index
The cloudiness of lake water and how far down you can see is often related to the amount of nutrients in the water. Nutrients promote growth of microscopic plant cells (phytoplankton) that are fed upon by microscopic animals (zooplankton). The more the nutrients, the more the plants and animals and the cloudier the water is. This is a common, but indirect, way to roughly estimate the condition of the lake. This condition, called eutrophication, is a natural aging process of lakes, but which is unnaturally accelerated by too many nutrients.
A Secchi disk is commonly used to measure the depth to which you can easily see through the water, also called its transparency. Secchi disk transparency, chlorophyll a (an indirect measure of phytoplankton), and total phosphorus (an important nutrient and potential pollutant) are often used to define the degree of eutrophication, or trophic status of a lake.
The concept of trophic status is based on the fact that changes in nutrient levels (measured by total phosphorus) causes changes in algal biomass (measured by chlorophyll a) which in turn causes changes in lake clarity (measured by Secchi disk transparency). A trophic state index is a convenient way to quantify this relationship. One popular index was developed by Dr. Robert Carlson of Kent State University.
Trophic State Index
Carlson's index uses a log transformation of Secchi disk values as a measure of algal biomass on a scale from 0 - 110. Each increase of ten units on the scale represents a doubling of algal biomass. Because chlorophyll a and total phosphorus are usually closely correlated to Secchi disk measurements, these parameters can also be assigned trophic state index values. The Carlson trophic state index is useful for comparing lakes within a region and for assessing changes in trophic status over time. Thus it is often valuable to include an analysis of trophic state index values in summary reports of a volunteer monitoring program. The program manager must be aware, however, that the Carlson trophic state index was developed for use with lakes that have few rooted aquatic plants and little non-algal turbidity. Use of the index with lakes that do not have these characteristics is not appropriate.
TSI = 60 - 14.41 ln Secchi disk (meters)
TSI = 9.81 ln Chlorophyll a (ug/L) + 30.6
TSI = 14.42 ln Total phosphorus (ug/L) + 4.15
where:
TSI = Carlson trophic state index
ln = natural logarithm
The formulas for calculating the Carlson Trophic State Index values for Secchi disk, chlorophyll a, and total phosphorus are presented below. Also presented is a table that lists the trophic state values and the corresponding measurements of the three parameters. Ranges of trophic state index values are often grouped into trophic state classifications. The range between 40 and 50 is usually associated with mesotrophy (moderate productivity). Index values greater than 50 are associated with eutrophy (high productivity). Values less than 40 are associated with oligotrophy (low productivity).
Presented below are Carlson trophic state index values for "Volunteer"
Lake. Summer averages (June 15 - September 1) are used in the calculations.
As seen from the TSI values, Volunteer Lake can be classified somewhere
near the border of mesotrophy and eutrophy.
Secchi Disk Calculations
Average Summer Secchi disk = 5.9 feet = 1.8 meters
TSI = 60 - 14.41 (ln Secchi disk (meters))
TSI = 60 - (14.41) (0.59)
TSI = 51.5
Total Phosphorus
Average Summer Surface Total Phosphorus = 19.6 ug/L
TSI = 14.42 (ln Total phosphorus (ug/L)) + 4.15
TSI = (14.42) (2.98) + 4.15
TSI = 47.1
Chlorophyll a
Average Summer Chlorophyll a = 17.2 ug/L
TSI = (9.81) (ln Chlorophyll a (ug/L)) + 30.6
TSI = (9.81) (2.84) + 30.6
TSI = 58.5
More Resources:
- The Great North American Secchi
Dip In by Dr. Carlson with detailed explanation of the TSI (a must
see!)
- Please also see the Lake
Assessment Process and Methods for Minnesota Lakes and their "indicator
for the month" - water transparency.