Learn More: Trophic State Index (TSI)

Trophic (pronounced TROH-fik) means “of or relating to nutrition.” The Trophic State Index (TSI) is a classification system designed to “rate” individual lakes, ponds and reservoirs based on the amount of biological productivity occurring in the water. Using the index, one can gain a quick idea about how productive a lake is. Classifications range from 1 to 100 and are generally described as follows:

• Lakes (water bodies classified as lakes, ponds or reservoirs) with TSI values below 30 are considered to have scant nutrients and be minimally productive; they would be good areas for water sports and good sources for drinking water. These lakes exhibit clear water with good visibility but may not provide the necessary nutrients and algae to maintain a healthy environment for fish and wildlife.
• Lakes with TSI values ranging between 30 and 45 are considered to have a mid-range of nutrients and are reasonably productive; they have an adequate amount of nutrients and generally support a fair amount of algae, aquatic plants, birds, fish, insects and other wildlife.
• Lakes with TSI values ranging between 46 and 70 are considered to be waterbodies with good or sufficient nutrients and have fairly high productivity; they have a greater amount of nutrients and are able to support an abundance of algae, aquatic plants, birds, fish, insects and other wildlife.
• Lakes with TSI values ranging between 71 and 100 are considered to be waterbodies with an overabundance of nutrients and are the most productive trophic class of lakes. Because they have the highest nutrient concentrations, these waterbodies have the potential to support the highest level of biological productivity (e.g., an abundance of algae, aquatic plants, birds, fish, insects, and other wildlife.) These waterbodies have the greatest potential for widely ranging dissolved oxygen conditions, which can have a detrimental effect on native plants and animals.

An excellent source of information about trophic states of Florida Lakes is the Florida LAKEWATCH Circular–Trophic State: A Waterbody's Ability to Support Plants, Fish and Wildlife.

Trophic State Index (TSI) values can be used in many different ways:

• TSI values can be used to rank lakes within a region and between regions. This ranking enables water managers to target lakes that may require restoration or preservation activities.
• An increasing trend in TSI values over a period of several years may indicate a degradation of the health of a lake. At one time, the FDEP used a change in TSI over time as one of the evaluation criteria in determining whether a lake was “impaired” under its interpretation of the Impaired Waters Rule (IWR). Other methods of assessment are now used, but the TSI remains a valuable measure of water quality.

The Water Atlas shows a scatter-plot graph with TSI values for the past 10 years, and a boxplot graph which shows how TSI values from month to month. In addition, the “limiting nutrient” in the lake is shown. The limiting nutrient is the one with the lowest concentration, and that therefore controls the ability of plants to grow. See the calculations section below for a discussion of how the limiting nutrient is determined.

TSI is determined from water chemistry samples and the Secchi depth measurement, a measure of water clarity (see Caveats and Limitations below for Florida). The water chemistry samples must be analyzed for one (or more) of the following water chemistry parameters: total nitrogen, total phosphorus, and/or chlorophyll a. If the lake is being assessed in accordance with the Impaired Waters Rule, then the sample water color must also be determined and used in the calculation. Analysis is done using the appropriate (Standard Operational Procedure (SOP). Ideally, monitoring should occur monthly for a period of at least two years prior to determining a TSI.

TSI values can be calculated using data from any (or all) of the four parameters described above.

When it was used as a measure of water quality, the FDEP calculated TSI values using data from the Water Quality Assessment for the State of Florida 305(b) Report. As defined by the Florida Administrative Code (FAC) 62-303.200 “Trophic State Index” or “TSI” was based on chlorophyll a, Total Nitrogen, and Total Phosphorus levels, and was calculated following the procedures outlined on pages 86 and 87 of the State's 1996 305(b) report, which are incorporated by reference. The Water Atlas relies on the same calculations. The equations below show how to use the Florida Trophic State Index calculation method found in the “1996 Water Quality Assessment for the State of Florida, Section 305 (B) Main Report”. This method of determining TSI is the official “Florida TSI Method”.

To apply this method, the waterbody’s limiting nutrient must be determined. The limiting nutrient is the one with the lowest concentration, and that therefore controls plant growth. This nutrient is normally phosphorus or nitrogen and in lakes it is most often phosphorus (TSI is also used to rank and evaluate estuaries; the primary limiting nutrient for estuaries is nitrogen). When a lake has a “balanced” TSI, the relative concentrations of nitrogen and phosphorus are the same.

The method calculates a separate component TSI for nitrogen, phosphorus and chlorophyll a. These components are then combined, as indicated in equations A-C below, to determine the overall TSI. This procedure is the basis for all Water Atlas TSI calculations. As previously stated, the procedure first calculates separate TSI values (via empirical equations that use the natural logarithm [ln], an exponential function in which the base is 2.71828+) for chlorophyll (a) [chl(a)], total nitrogen [TN] and total phosphorus [TP] sample concentrations, and then combines the values through addition. The calculations are shown in the empirical equations one through five below. These equations calculate the TSI for various nutrient relationships. The result of equation one is used for all calculations. The result of equations two and three are used for nutrient balanced lakes (those where the TN to TP ratio is greater or equal to 10 and less or equal to 30). The result of equation four is used for phosphorus limited lakes (those where the TN to TP ratio is greater 30) and the result of equation five is used for nitrogen limited lakes (those with a TN to TP ratio of less than 10).

The final TSI is then determined by averaging the above values based on the limiting nutrient determined for the lake using final equations A-C below.

Figure 1. Florida Trophic State Index Calculation Method found on page 87 of the 1996 FDEP 305 (B) Report.

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.

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/.

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• Fish Communities and Trophic Status in Florida Lakes
• Lake Ecology Primer
• Lake level and trophic state variables among a population of shallow Florida lakes and within indivi
• Nutrients in the Nation's Waters: Too Much of a Good Thing?
• Relations between trophic state indicators and fish in Florida lakes
• The Role of Eutrophication in the Global Proliferation of Harmful Algae Blooms
• Trophic State: A Waterbody's Ability To Support Plants, Fish, and Wildlife