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The Trophic State Index

Updated July 21, 2022
David Kringen

David Kringen

SDSU Extension Water Resources Field Specialist

Secchi disk being used to measure lake transparency.
Figure 1. Measuring lake transparency using a secchi disk. Courtesy: U.S. Geological Survey

The Trophic State Index (TSI) is a means for lake professionals and volunteers alike to numerically measure the productivity of a natural lake or man-made reservoir. The trophic state is defined as the total biomass in a waterbody at a specific location and time.

The most-common index used today is the Carlson TSI (Carlson 1977). The primary indicators used in Carlson’s TSI are phosphorus, secchi disk transparency, and chlorophyll-a as they are related. For example, when phosphorus increases, algal concentrations increase, as there is more food available for the algae. When algal concentrations increase, water transparency decreases (RMB Environmental Laboratories, Inc.).

Through the use of equations, these separate measurements are standardized and converted to a trophic state index number. The overall TSI of a lake is the average of the TSI for phosphorus, the TSI for chlor-a, and the TSI for secchi depth (RMB Environmental Laboratories, Inc.). TSI values range from 0 to 100.

Trophic Classification

A trophic classification is also used in association with the TSI number. Trophic states are based on lake fertility and are typically classified as one of the following:

  • Oligotrophic: Few nutrients (nitrogen and phosphorus), high levels of dissolved oxygen, low algal concentrations, deep, clear water.
  • Mesotrophic: Medium amount of nutrients, moderately clear water, some algal blooms in late summer, oxygen depletion at bottom of lake as biomass decomposes.
  • Eutrophic: Nutrient rich, highly productive plant growth, shallow lakes, seasonally deficient in oxygen, may lead to fish kills.
  • Hypereutrophic: Extremely rich in nutrients, highest amount of plant and algal productivity.

South Dakota Surface Water Quality Assessment

While South Dakota Department of Agriculture and Natural Resources (SDDANR) officials used all 3 parameters in their TSI calculations at one time, the chlor-a TSI is used exclusively now, as it tends to be the best descriptor of lake productivity and trophic status.

Of the 122 lakes that were assessed for the 2022 South Dakota Integrated Report for Surface Water Quality Assessment, <1% were considered oligotrophic, 21% were classified as mesotrophic and 78% were classified as eutrophic or hypereutrophic. Lakes can naturally become eutrophic over long periods of time, or they can become eutrophic much faster as a result of human activity.

Table 1. Trophic Status of Assessed Lakes*

TSI
(chl-a)
Chl-a
(ug/L)
Classification
Number
of Lakes
Lake
Acreage
Percent of
Assessed Acres
<30
0.95
Oligotrophic (O)
0
0
0%
30-40
0.95 – 2.6
Oligotrophic (O)
5
939
<1%
40-50
2.6 – 7.3
Mesotrophic (M)
14
25,363
21.30%
50-60
7.3 - 20
Eutrophic (E)
41
34,753
29.20%
60-70
20 - 56
Eutrophic (E)
46
42,028
35.30%
70+
>56
Hypereutrophic (H)
16
15,914
13.30%

*122 lakes assessed based on two years of data over a 15 year period (9/30/2006 to 9/30/2021) during growing season (May through September). Source: 2022 S.D. Integrated Report for Surface Water Quality Assessment, SD DANR

Targets for South Dakota Ecoregions

Color-coded map depicting South Dakota’s four ecoregions: Black Hills, Western Lakes, Ecoregion 42, and Eastern Lakes
Figure 2. Ecoregion Map for Nutrient Criteria-Lakes. Courtesy: South Dakota Department of Agriculture and Natural Resources

SDDANR has also developed numeric chlorophyll-a targets to address nutrient-related narrative standards designed to protect waters from nutrient related impacts. Numeric chlorophyll-a targets were developed for each ecoregion (Figure 2) and are listed below. These targets are intended to protect the waterbodies in their respective ecoregion from experiencing frequent algae blooms dominated by blue-green algae with a low to moderate risk of cyanotoxin production.

  • Black Hills Ecoregion 17
    • Large waterbodies: Average growing season chlorophyll-a ≤ 7 ug/L
    • Small waterbodies: Average growing season chlorophyll-a ≤ 8 ug/L
  • Western Lakes Ecoregions 25, 43, 44
    • Average growing season chlorophyll-a ≤ 14 ug/L
  • Eastern Lakes Ecoregions 42, 46, 47
    • Deep lakes (>15’ max depth): Average growing season chlorophyll-a ≤ 20 µg/L
    • Shallow lakes (≤ 15’ max depth): Average growing season chlorophyll-a ≤ 25 µg/L.

Additional Information

Related Topics

Conservation