Phosphorus, Chlorophyll-a (algae concentration) and Secchi depth are related. When phosphorus increases, that means there is more food available for algae, so algal concentrations increase. When algal concentrations increase, the water becomes less transparent and the Secchi depth decreases.
The resulting numbers from these three measurements cover different units and ranges and thus cannot be directly compared to each other or averaged. In order to standardize these three measurements to make them directly comparable, we convert them to a trophic state index using an equation. You can find the equations online at: http://www.secchidipin.org/tsi.htm.
The overall trophic state index (TSI) of a lake is the average of the TSI for phosphorus, the TSI for chlorophyll-a and the TSI for secchi depth; therefore, it can be thought of as the lake condition taking into account phosphorus, chlorophyll-a and secchi depth.
It is important to understand that Trophic States are defined divisions of a continuum in phosphorus and algal concentration. The TSI ranges from 0-100. 0-30 is Oligotrophic, where water is very clear, phosphorus is low, and algae is sparce. 30-50 is an in-between stage where the number of aquatic plants algae increase due to more available phosphorus.
A TSI of over 50 describes a lake that is eutrophic, with a high density of plants and algae that could be unpleasant for swimming at certain times in the summer. Some lakes may be naturally eutrophic, having a TSI of 50 or greater for the last 100 years. Other lakes have gradually increased in TSI as a result of human activities. The Minnesota Pollution Control Agency recommends 8-10 years of quality long term data on a lake for the determination of increasing or decreasing TSI trends.
Trophic State Index is not necessarily interchangeable with water quality. Water quality is subjective and depends on how you intend to use the water body. A lake that is good for duck hunting is not necessarily good for water skiing. In turn, a lake that is great for swimming may not be great for bass fishing.
Explanation of trophic state and a list of possible changes that might be expected in a north temperate lake along the trophic state gradient.
| TSI | Chl-a(ug/L) | SD (ft) | TP (ug/L) | Attributes | Fisheries & Recreation |
| <30 | <0.95 | >26.2 | <6 | Oligotrophy: Clear water, oxygen throughout the year at the bottom of the lake, very deep cold water. | Trout fisheries dominate |
| 30-40 | 0.95-2.6 | 13.1-26.2 | 6-12 | Bottom of shallower lakes may become anoxic (no oxygen). | Trout fisheries in deep lakes only. Walleye, Tullibee present. |
| 40-50 | 2.6-7.3 | 6.6-13.1 | 12-24 | Mesotrophy: Water moderately clear most of the summer. May be “greener” in late summer. | No oxygen at the bottom of the lake results in loss of trout. Walleye may predominate. |
| 50-60 | 7.3-20 | 3.3-6.6 | 24-48 | Eutrophy: Algae and aquatic plant problems possible. “Green” water most of the year. | Warm-water fisheries only. Bass may dominate. |
| 60-70 | 20-56 | 1.6-3.3 | 48-96 | Blue-green algae dominate, algal scums and aquatic plant problems. | Dense algae and aquatic plants. Low water clarity may discourage swimming and boating. |
| 70-80 | 56-155 | 0.8-1.6 | 96-192 | Hypereutrophy: (light limited productivity). Dense algae and macrophytes. | Water is not suitable for recreation. |
| >80 | >155 | <0.8 | 192-384 | Algal scums, few aquatic plants | Rough fish (carp) dominate; summer fish kills possible |