In recent years FDEP staff have encountered problems interpreting Secchi depth data in many tannic (tea or coffee-colored) waterbodies where transparency is often reduced due to naturally-occurring dissolved organic matter in the water. As a result, Secchi depth has been dropped as an indicator in FDEP's recent TSI calculations (1996 Water Quality Assessment for The State of Florida Section 305(b) Main Report). This modification for black water TSI calculation has also been adopted by the Water Atlas.
Also, according to Florida LAKEWATCH, use of the TSI is often misinterpreted and/or misused from its original purpose, which is simply to describe the level of biological productivity. It is not meant to rate a lake's water quality. For example, higher TSI values represent lakes that support an abundance of algae, plants and wildlife. If you love to fish, this type of lake would not be considered to have "poor" water quality. However, if you are a swimmer or water skier, you might prefer a lake with lower TSI values.
The Trophic State Index is one of several methods used to describe the biological productivity of a waterbody. Two scientists, Forsberg and Ryding, 1980, developed another method that is widely used. It is known as the Trophic State Classification System. Using this method, waterbodies can be grouped into one of four categories, called trophic states:
Oligotrophic (oh-lig-oh-TROH-fik) where waterbodies have the lowest level of productivity;
Mesotrophic (mees-oh-TROH-fik) where waterbodies have a moderate level of biological productivity;
Eutrophic (you-TROH-fik) where waterbodies have a high level of biological productivity;
Hypereutrophic (HI-per-you-TROH-fik) where waterbodies have the highest level of biological productivity. The trophic state of a waterbody can also affect its use or perceived utility. Figure 1 illustrates this concept.
Figure 1: Trophic states, concentrations of constituents, and typical waterbody uses.
Water Atlas Lake Water Quality Pages
The Trophic State Index is used by the Water Atlas to provide the public with an estimate of their lake resource quality. A "Good" quality lake is one that meets all lake use criteria (swimmable, fishable and supports healthy habitat). Based on the discussion above, lakes that are in the oligotrophic through low eutrophic range, for the most part, meet these criteria. A trophic state below 60 indicates lakes in this range and these lakes are given the "Good" descriptor. A trophic state above 60 but below 70 can be considered highly productive and a reasonable lake for fishing and most water sports. This lake is considered "Fair", while a lake in the Hypereutrophic range with a TSI greater than 70 will probably not meet the lake use criteria and these lakes are considered to be poor. Please see table below.
Table 2. Comparison of Classification Schemes.
Trophic State Index
Trophic State Classification
Also see LAKEWATCH publication, "Trophic State: A Waterbody's Ability to Support Plants Fish and Wildlife" at https://lakewatch.ifas.ufl.edu/extension/information-circulars/.