Why Does TSI Matter
Welcome to CHAPTER 3 of our educational series,
“Ten Things You Should Know About Eagle Lake’s Environmental Challenges."
 
"The Trophic State Index (TSI) serves as a report card that the WDNR and other decision makers use to monitor and compare the environmental health of lakes.  Given the significant competition for grant funding, driven by the numerous impaired lakes in our state, it is crucial for us to educate ourselves and garner more attention from decision makers to address Eagle Lake’s environmental challenges effectively."
- ELIA Board of Directors
 
Effectively evaluating lake water quality requires a comprehensive approach, which typically includes measuring key parameters such as dissolved oxygen, pH, turbidity (using a Secchi disk), total phosphorus, chlorophyll-a, temperature, and potentially specific nutrients and bacteria levels.  This process often involves both field measurements and laboratory analysis of water samples to gain a thorough understanding of the lake’s health and identify potential concerns like algal blooms or nutrient overload.  With this in mind, the first three years of our Eagle Lake Protection Program (ELPP) have been heavily data driven.  Before establishing a target for lake health, we must first understand its current state.  Among the various methods for assessing our lake's health, the Trophic State Index (TSI) offers the most comprehensive perspective.
 
What Is The 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.  TSI is determined from water chemistry samples and the Secchi depth measurement, a measure of water clarity.  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.  Biological activity is to be expected to some degree, but moderation is essential.
 
The most commonly used TSI is Carlson's index, proposed by Robert Carlson in 1977.  It is utilized by the United States Environmental Protection Agency and calculates the index using three independent variables:
  • Chlorophyll pigments
  • Total phosphorus
  • Secchi depth (water clarity)
The TSI provides a quick and standardized method for assessing the biological productivity and overall health of water bodies, making it a valuable tool for lake management and water quality monitoring.  Fortunately for our concerned community, volunteers have been monitoring and collecting data for the lake since 1990, giving us a revealing look at the current health of Eagle Lake.
 
Box-and-Whiskers
Box-and-whisker plots are useful to show the distribution of the equivalent averages from other Wisconsin lakes in the same natural community.  The midline of the box is the median value, the top and bottom of the box are the 25th and 75th percentiles, and the whiskers extend to the 10th and 90th percentiles.
 
In the chart below, the late summer trophic indicator averages from the last 10 years at Eagle Lake are compared to other SHALLOW LOWLAND lakes.  Late summer results from at least three of the last ten years must be found in order for the average to be calculated.  Points plotted in red are the recent Eagle Lake summer averages for the specified parameter.  As you can see, trends of increasing nutrient levels and biological activity over the past three decades have placed Eagle Lake well outside of the desired range in all three variables of the TSI.
TSI Classifications
Values range from 1 to 100 and the classifications are generally described as follows:
 
Oligotrophic: Oligotrophic lakes are deep, clear, and nutrient-poor bodies of water with Trophic State Index (TSI) values under 40.  These lakes have low biological productivity, meaning they support fewer aquatic plants and animals compared to more nutrient-rich lakes. Their water quality is usually high, with low levels of algae.  Many lakes in northern Wisconsin are oligotrophic.
 
Mesotrophic:  Mesotrophic lakes have a moderate level of nutrients and biological productivity. These lakes, with a TSI between 40 and 60, strike a balance between oligotrophic lakes (low nutrient levels) and eutrophic lakes (high nutrient levels).  This classification applies to lakes with clear water and only occasional algal blooms, supporting a variety of aquatic plants and animals, and generally maintaining good water quality (drops to "fair" with TSI above 45).
 
Eutrophic:  Lakes with a TSI between 60 to 70 have high nutrient levels and high biological productivity.  These lakes often have murky water due to dense algal blooms and a high concentration of aquatic plants.  As a result, they might experience oxygen depletion which can affect aquatic life negatively.  Eutrophic lakes are highly productive ecosystems, but they require careful management to prevent further degradation and maintain ecological balance.
 
Hypereutrophic:  The hypereutrophic classification, when TSI is above 70, represents lakes with extremely high nutrient levels and excessive biological productivity.  These lakes are characterized by very murky water, dense and frequent algal blooms, and an overabundance of aquatic plants.  The high nutrient concentration often leads to severe oxygen depletion, which can negatively impact fish and other aquatic life.  Managing hypereutrophic lakes requires significant intervention to reduce nutrient inputs and restore ecological balance.
Eagle Lake Trophic Classification by WDNR standards:
"Hypereutrophic"
 
The Trophic State Index classification functions as a report card for the Wisconsin Department of Natural Resources (WDNR) and other decision-makers to monitor and compare the environmental health of lakes.  This index guides decision-making and water resource management, helping to identify conditions that limit algal biomass in a lake and determine its suitability for activities like swimming, fishing, and other water sports.
 
Due to the large number of impaired lakes in our state, there is significant competition for grant funding.  Over the past five summers, Eagle Lake's average trophic state was 74, categorizing it as hypereutrophic.  This is considered poor for a shallow lowland lake.  Using the report card analogy, Eagle Lake has essentially received a "D" grade for the past several years.
How do we compare with other area lakes?
 
Lake Acres Max Depth Index Classification
Silver Lake 516 44 43 Mesotrophic (Good)
Powers Lake 451 33 44 Mesotrophic (Good)
Browns Lake 397 44 46 Mesotrophic (Fair)
Wind Lake 936 52 49 Mesotrophic (Fair)
Delavan Lake 1,906 52 49 Mesotrophic (Fair)
Tichigan Lake 279 63 51 Mesotrophic (Fair)
Eagle Lake 529 11 74 Hypereutrophic (Poor)
 
In 2016, Eagle Lake was designated as an Impaired Water by the State of Wisconsin due to excessive nutrients and poor runoff quality from agricultural areas.  We are not alone in this struggle, as over 1,200 lakes and rivers in Wisconsin share this designation.  Water quality and nutrient pollution are not only pressing issues here in Wisconsin but also on a global scale.  While these challenges are complex, there are strategies and practices that can have a positive impact.  With fierce competition for the limited grants and conservation funds, it is crucial that our community maintains its high level of engagement and financial support.
So, where do we go from here?
With Eagle Lake’s Trophic State Index (TSI) currently at a hypereutrophic level, we must navigate a number of obstacles in our quest to achieve a mesotrophic state.  Should this be our goal?  Absolutely.  However, we must acknowledge that it will require significant time and financial investment.  A more immediate and attainable goal is to reverse the trend of declining lake health and foster conditions that align with the characteristics of healthy shallow lakes.
 
We encourage you to explore the upcoming topics in our Education Series.  In these chapters, you will learn about the ongoing volunteer activities and initiatives currently underway.  We have numerous reasons for optimism, and we believe that by working together, we can make fundamental changes to enhance the enjoyment of Eagle Lake for future generations.
 
Thank you for visiting CHAPTER 3 of our educational series,
“Ten Things You Should Know About Eagle Lake’s Environmental Challenges."
 
 
Eagle Lake Improvement Association
PO Box 655
Kansasville, WI 53139
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The Eagle Lake Improvement Association is a volunteer based 501(c)(3) nonprofit organization. We are generously supported by nearly 200 households and businesses within the watershed who share our vision for Eagle Lake.