Trophic State Index: Lake Eutrophication Assessment
The trophic state index (TSI) provides a numerical scale (0–100) describing lake productivity and nutrient enrichment. Developed by Robert Carlson in 1977, TSI condenses measurements of water clarity, algal biomass, and nutrient concentrations into a single classification ranging from oligotrophic (clear, low nutrients) to hypereutrophic (dense algae, poor clarity). Resource managers use TSI to communicate lake status, set restoration targets, and evaluate policy outcomes. This article defines the index, reviews its history, elaborates supporting concepts, and outlines management applications.
Pair this explainer with the turbidity article to link Secchi depth measurements with TSI, and explore the rainwater harvesting calculator for catchment interventions that curb nutrient inflows.
Definition and Indicator Relationships
Carlson’s equations
Carlson defined TSI using logarithmic relationships for Secchi depth (SD in metres), chlorophyll-a (Chl in micrograms per litre), and total phosphorus (TP in micrograms per litre): TSI(SD) = 60 − 14.41 ln(SD), TSI(Chl) = 9.81 ln(Chl) + 30.6, and TSI(TP) = 14.42 ln(TP) + 4.15. Comparable values across indicators suggest equilibrium conditions, while divergence signals unusual factors such as non-algal turbidity or nitrogen limitation.
Classification bands
TSI scores below 40 denote oligotrophic waters with high clarity and low productivity. Scores between 40 and 60 represent mesotrophic to eutrophic conditions with moderate algae. Values above 70 indicate hypereutrophic states prone to algal blooms, fish kills, and odour issues. Agencies often publish maps with colour-coded TSI ranges for quick public reference.
Supplementary metrics
Managers supplement TSI with dissolved oxygen profiles, cyanotoxin assays, and zooplankton counts to capture ecological impacts. Integrating TSI with nutrient loading models supports adaptive management plans.
Historical Development
Origins in Carlson’s research
Robert Carlson introduced the TSI while studying Midwestern lakes, seeking a simple yet informative scale for public communication. His 1977 paper in Limnology and Oceanography provided equations and classification guidance that remain widely used.
Adoption in monitoring programs
State and provincial agencies quickly integrated TSI into routine monitoring, pairing it with volunteer Secchi disk networks. The North American Secchi Dip-In, launched in 1994, leverages citizen science to map transparency and TSI trends annually.
Contemporary refinements
Modern adaptations incorporate satellite-derived chlorophyll, differentiate between nitrogen- and phosphorus-driven blooms, and extend the index to reservoirs and estuaries. Decision support systems now automate TSI calculation from real-time sensors.
Applications and Importance
Nutrient management
Watershed planners use TSI to set nutrient reduction targets and track best management practices. Coupling TSI observations with the water efficiency savings calculator helps justify conservation programs that reduce runoff loads.
Recreation and public health
Beach managers post advisories when TSI indicates harmful algal bloom risk. Camps and marinas monitor TSI to schedule closures or aeration treatments, protecting swimmers and infrastructure.
Long-term climate indicators
Climate change alters stratification and nutrient cycling, affecting TSI trends. Integrating TSI with watershed footprints using the household water footprint calculator engages communities in mitigation efforts.