TSI – What Does It All Mean?
Sandra Holm, Citizen Monitoring Coordinator SI is the acronym for the Carlson’s Trophic State Index used by professionals and lay lake advocates to describe how productive (trophic) a lake is. Prior to its development, measurements of lake productivity were based on a variety of parameters, most of which were expensive and complicated to collect, calculate and interpret. In addition they often provided conflicting information. For example, one index would define a lake as oligotrophic (clear, nutrient poor) while another might define it as eutrophic (cloudy, nutrient rich). What was needed was a consistent and clearly understandable system to describe a lake’s trophic state. In the late 1970’s Robert E. Carlson developed just such an index at the University of Minnesota’s Limnological Research Center. He developed his TSI based on three interrelated but distinct measures of productivity: water transparency as measured by the Secchi disk (SD), chlorophyll ‘a’ (Chl ‘a’) and total phosphorus (TP). Simply stated, the basic relationship assumption is that the more total phosphorus present, the more algae (Chl ‘a’), and the lower the water clarity as measured by the Secchi disk. The indices are based on measurements of each parameter collected during the growing season (May- September). Data not collected during this time period should not be used to define TSI and draw conclusions based on them. The Secchi disk value is the easiest and most commonly measured parameter. It is considered an indirect measure of algal populations. Chlorophyll ‘a’ is a pigment found in algae and is a direct measure of algal biomass (quantity). Total phosphorus is relatively stable throughout the year and usually the major factor limiting algae growth. It is the predictor of potential algal biomass. Easily calculated mathematical equations for each of the three parameters are used to transform their numeric values into an index ranging from 1-100 trophic units. These categories are grouped into broader classifications on a continuum of oligotrophic to hypereutrophic (See chart). An increase of ten units indicates a doubling of the algal population. Because each TSI classification represents a wide range of numeric values, it is important not just to use the classification name, but also the value used in its calculation. Each index indicates something different about a lake’s dynamic. If the three TSIs fall within a range of 5 TSI units they can be averaged into one TSI value. If not, it is important to evaluate the data to determine the reason for, and relevance of, the differences and which index is the most accurate predictor of trophic state. For example, one index may be based on numerous measurements (many sampling events), while the other may be based on one sampling measurement. In this case the former would most likely be the best indicator. TSI helps us understand where a lake is in terms of its aging or productivity, however it is not a “water quality” index, which is a subjective judgment depending on the water use and local attitudes of the people using it. What’s good water quality for a largemouth bass isn’t good for trout. Or, what’s considered good water quality in southwestern Minnesota may not be considered good water quality in northern Minnesota. For this reason, the Minnesota Pollution Control Agency (MPCA) has established water quality standards based on water use (Chapter 7050 of Minnesota Rules) and ecoregions, which are based on relatively large expanses of land and the geographically distinct collection of plants, animals, natural communities and environmental conditions found there. The MPCA uses the indices to assess a lake’s water quality in terms of its being able to support its designated use of aquatic recreation. The Secchi disk is the commonly used tool for a preliminary assessment. To fully assess a lake, 12 paired sets of SD, TP and Chl ‘a’ parameters are collected consecutively during 2-3 growing seasons. A paired set means the three measurements are taken at the same time. Only the June-September data is used in their assessment. May data is an important diagnostic tool, as it provides a sense of how the lake starts out and a basis for understanding the seasonal changes taking place within the lake. There is commonly poor agreement within the three TSI parameters in May because of factors that may keep algae concentrations lower than predicted by TP. Carlson’s TSIs have become an intricate part of the water quality vocabulary and an important tool used to assess water quality and understand a lake’s dynamics. Therefore it is important to understand their significance. For additional information on TSI and ecoregions refer to: http://www.pca.state.mn.us/water/lakeacro.html
Minnesota Waters Newsletter Spring 2007