An Introduction to Trophic States

Southwest Region

Using Water Chemistry and Aquatic Plants to
Estimate a Lake’s Biological Productivity

Example of oligotrophic lake. Example of hypereutrophic lake Example of mesotrophic lake. Example of eutrophic lake.


Have you ever noticed how different lakes are, throughout Florida? Some have crystal clear water and sandy bottom sediments while others are pea-soup green in color with lots of mucky sediments. Some lakes have reddish water with a few plants floating on the surface and others are so chalk full of plants, it seems we could walk across them. All of these characteristics provide clues about the waterbody's biological productivity – its ability to support life.

The Trophic State Classification System is useful for a number of reasons: It helps us understand why lakes differ so much and also provides a handy way to describe a lake or waterbody. For example, if a lake scientist (limnologist) in Spain uses the term “oligotrophic” while describing a lake to his colleague in Canada, the Canadian will be able to gain a general idea of the waterbody’s productivity without even seeing it.

When faced with the challenge of trying to describe the various levels of biological productivity, scientists developed the Trophic State Classification System. Using this system, a waterbody can be grouped into one of four categories called trophic states. The adjectives used to denote each of these four trophic states, from the lowest level of biological productivity to the highest, are as follows:



 Estimating the Biological Productivity of a Waterbody

There are several Trophic State Classification Systems used today. In this lesson, we’ll be using a system developed by two scientists, Forsberg and Ryding. Their criteria for classifying lakes are based on the following four water chemistry parameters (see definitions below). Note: These four parameters are used to estimate biological productivity, as it’s simply not possible to measure every living thing in a lake or pond at any given time.

Example of secchi disk.

Data for these same four water chemistry parameters are collected every month from hundreds of lakes throughout the state by Florida LAKEWATCH volunteers. See page 2 for a data summary of lakes in your part of the state.

Chlorophyll – is the dominant green pigment found in most algae. Since algae are a basic food source for many aquatic animals, we often measure the total chlorophyll (algal) concentrations found in collected water samples to help us estimate the biological productivity of a waterbody. Chlorophyll is the main parameter used to assess productivity in this trophic state classification system.

Total phosphorus – is a measure of all the forms of phosphorus found in a collected water sample. Phosphorus is an element (nutrient) necessary for the growth of all plants, including algae and aquatic plants. When this nutrient is in low supply, low biological productivity can be generally expected whereas an abundance of phosphorus generally results in an abundance of algae and/or plants.

Total nitrogen – is a measure of all the forms of nitrogen found in a collected water sample. Nitrogen is also a necessary nutrient for the growth of plants, including algae and aquatic plants. When total nitrogen is in low supply, low biological productivity can generally be expected, along with clearer water .

Secchi depth is a measurement that indicates water clarity, which is influenced by several factors including free-floating algae, dissolved organic compounds (tannins) and suspended particles.

There are several other variables that impact this classification system – especially in Florida, where lakes often have an abundance of plants. For example:

The purpose of this activity is to gain a better understanding of the different levels of biological productivity found in lakes throughout Florida and the influence that aquatic plants may have on the overall “equation.”

Click here to view the PDF File of Trophic State: A Waterbody's Abilitiy to Support Plants, Fish, and Wildlife, by LAKEWATCH.

Directions: Use the water chemistry data (below) and the attached Trophic State Classification handout to identify the trophic state of each lake.



Lake Name / County


Total Chlorophyll Chl (ug/L)


Total Phosphorus TP (ug/L)


Total Nitrogen TN (ug/L)


Secchi Depth (feet)


Percent Area Coverage (% PAC)*


Trophic State

Little Lake Jackson / Highlands







Lake Alice / Hillsborough







Mango / Hillsborough





No data


Lake Dinner / Highlands





No data


Lake Istokpoga / Highlands







Lake Jackson / Highlands





No data


Lake Crews / Pasco





No data


Lake Clear / Lake









1. Which characteristics were most helpful in determining the trophic state of each lake? Explain your answer.




2. Explain the relationship between each of the following:


a. Secchi depth and trophic state–




b. total phosphorus and trophic state –




c. chlorophyll and trophic state –




3. If you were fishing for bass, which lake might you prefer and why?




4. If you wanted to go swimming, which trophic state category would you prefer for a lake? Why?





5. Using data in the table above, what relationship do you find between % PAC and trophic state of the lake? Explain.



Center for Aquatic and Invasive Plants, UF/IFAS

A collaboration of the UF/IFAS Center for Aquatic and Invasive Plants
and the Department of Environmental Protection, Bureau of Invasive Plant Management.