What is Trophic State Index (TSI)?
On April 25, there was an article published in the Becker County Record about county septic inspections for systems over 10 years old. "The lakes to be targeted are those that have been monitored for transparency, chlorophyll-a and total phosphorus and score over 50 on the trophic index."
What does this mean? First let's start with the components: transparency, chlorophyll-a and total phosphorus.
Water transparency is how deep sunlight penetrates through the water and is dependent upon the amount of particles in the water. These particles can be algae or sediment from erosion, the more particles - the less water transparency. Water transparency is measured with a Secchi disk and is also referred to as the Secchi depth.
Chlorophyll-a is the pigment that makes plants and algae green. Chlorophyll-a is measured in lakes to determine algal concentration. A high measurement of chlorophyll-a means that there is a large amount of algae in the lake.
Phosphorus is food for plants and algae, so the more phosphorus there is in the lake, the more plants and algae can grow. Phosphorus can enter the lake through runoff from agriculture, fertilized lawns, erosion, manure, improperly maintained septic systems, leaf and yard litter, and many other sources.
These three water quality measurements are related. When phosphorus increases, that means there is more food available for algae, so algal concentrations increase. When algal concentrations increase, the water becomes less transparent and the Secchi depth decreases.
The resulting numbers from these three measurements cover different units and ranges and thus cannot be directly compared to each other or averaged. In order to standardize these three measurements to make them directly comparable, we convert them to a trophic state index using an equation. For those that are interested, you can find the equations online at: http://dipin.kent.edu/tsi.htm.
The overall trophic state index (TSI) of a lake is the average of the TSI for phosphorus, the TSI for chlorophyll-a and the TSI for secchi depth; therefore, it can be thought of as the lake condition taking into account phosphorus, chlorophyll-a and secchi depth.
It is important to understand that Trophic States are defined divisions of a continuum in phosphorus and algal concentration. The TSI ranges from 0-100. 0-30 is Oligotrophic, where water is very clear, phosphorus is low, and algae is sparce. 30-50 is an in-between stage where the number of aquatic plants algae increase due to more available phosphorus.
A TSI of over 50 describes a lake that is eutrophic, with a high density of plants and algae that could be unpleasant for swimming at certain times in the summer. Some lakes may be naturally eutrophic, having a TSI of 50 or greater for the last 100 years. Other lakes have gradually increased in TSI as a result of human activities. The Minnesota Pollution Control Agency recommends 8-10 years of quality long term data on a lake for the determination of increasing or decreasing TSI trends.
Trophic State Index is not necessarily interchangeable with water quality. Water quality is subjective and depends on how you intend to use the water body. A lake that is good for duck hunting is not necessarily good for water skiing. In turn, a lake that is great for swimming may not be great for bass fishing.
Next week, I will describe the trophic states (Oligotrophic, Mesotrophic, Eutrophic) as well as natural and cultural eutrophication (the process of increasing TSI) in more detail.
Until next week, enjoy the lakes!
Moriya Rufer is the Lakes Monitoring Program Coordinator for RMB Environmental Laboratories in Detroit Lakes, 218-846-1465, lakes.rmbel@eot.com.