Lake Trophic State Explained
The Carlson Trophic State Index (TSI) classifies lakes on a spectrum from ultra-clean to heavily enriched with nutrients. It is one of the most widely used frameworks in lake science. LakeGrade calculates TSI for every lake using the same three measurements that determine grades: Secchi depth, phosphorus, and chlorophyll-a.
The Four Trophic States
Low nutrients, clear water, excellent for swimming
- TSI:
- Under 40
- Clarity:
- Over 13 ft
- Phosphorus:
- Under 12 µg/L
- Fish:
- Cold-water species (trout, cisco, whitefish)
Moderate nutrients, good water quality
- TSI:
- 40 - 50
- Clarity:
- 6.5 - 13 ft
- Phosphorus:
- 12 - 24 µg/L
- Fish:
- Diverse species (walleye, bass, panfish)
High nutrients, frequent algae, reduced clarity
- TSI:
- 50 - 70
- Clarity:
- 3 - 6.5 ft
- Phosphorus:
- 24 - 96 µg/L
- Fish:
- Warm-water species (catfish, carp, bullhead)
Very high nutrients, dense algae, poor clarity
- TSI:
- Over 70
- Clarity:
- Under 3 ft
- Phosphorus:
- Over 96 µg/L
- Fish:
- Tolerant species only, frequent fish kills
How TSI Is Calculated
LakeGrade computes TSI from three independent formulas developed by Robert Carlson in 1977:
- TSI from Secchi depth = 60 - 14.41 x ln(Secchi depth in meters)
- TSI from phosphorus = 14.42 x ln(phosphorus in µg/L) + 4.15
- TSI from chlorophyll-a = 9.81 x ln(chlorophyll-a in µg/L) + 30.6
The overall TSI is the average of available index values. When all three metrics agree, the lake is in a "balanced" state. When they diverge, it can indicate specific conditions — for example, high phosphorus TSI with low chlorophyll TSI may indicate a lake where nutrients are present but algae growth is limited by some other factor (light, mixing, grazing).
Frequently Asked Questions
What is the Trophic State Index?
The Carlson Trophic State Index (TSI) is a number from 0 to 100+ that classifies lakes by nutrient levels. It is calculated from three measurements: Secchi depth (water clarity), total phosphorus, and chlorophyll-a (algae). Lower TSI means clearer, less productive water. Higher TSI means more nutrients and algae.
What is an oligotrophic lake?
An oligotrophic lake (TSI below 40) has low nutrients, excellent water clarity (often 15+ feet), minimal algae, and high dissolved oxygen. These are typically deep, clear lakes ideal for swimming and often support cold-water fish like trout and cisco.
What is a eutrophic lake?
A eutrophic lake (TSI 50-70) has high nutrient levels, frequent algae blooms, reduced water clarity (usually under 6 feet), and may experience low oxygen levels in deep water during summer. Swimming may not be safe during bloom events.
Can a eutrophic lake be restored?
Yes, but it requires reducing nutrient inputs — controlling agricultural runoff, upgrading septic systems, managing stormwater, and sometimes treating internal phosphorus loading from lake sediments. Restoration is expensive and slow but has succeeded in lakes across Minnesota and Wisconsin.