Measurement

Trophic State Index

The Carlson Trophic State Index (TSI) is a numerical scale from 0 to 100 that classifies lakes by their nutrient and productivity levels.

What It Means for Your Lake

The Carlson Trophic State Index (TSI), developed by Robert Carlson in 1977, is the standard classification system for lake productivity in North America. The index runs from 0 to 100 and is calculated from three independent measurements: Secchi depth, total phosphorus, and chlorophyll-a. Each metric produces its own TSI value using logarithmic formulas, and the values are averaged to produce an overall TSI score. Lakes are classified into four trophic states based on TSI ranges: oligotrophic (TSI below 40) lakes are clear, nutrient-poor, and support coldwater fisheries like lake trout. Mesotrophic (40 to 50) lakes have moderate nutrients and support diverse warm and coolwater fish communities. Eutrophic (50 to 70) lakes are nutrient-rich with frequent algae blooms and warm water fisheries dominated by bass, panfish, and rough fish. Hypereutrophic (above 70) lakes have excessive nutrients, persistent algae blooms, and severely impaired water quality. In Minnesota and Wisconsin, approximately 20% of monitored lakes are oligotrophic, 30% mesotrophic, 40% eutrophic, and 10% hypereutrophic. The TSI is valuable because it provides a standardized comparison framework, but it should be interpreted alongside other factors. Some naturally eutrophic lakes support excellent fisheries, while some oligotrophic lakes may have limited fish populations due to low productivity.

Frequently Asked Questions