Trophic State Index (TSI)
Source: Wisconsin DNR
|TSI < 30||Classical oligotrophy: clear water, many algal species, oxygen throughout the year in bottom water, cold water, oxygen-sensitive fish species in deep lakes. Excellent water quality.|
|TSI 30-40||Deeper lakes still oligotrophic, but bottom water of some shallower lakes will become oxygen-depleted during the summer.|
|TSI 40-50||Water moderately clear, but increasing chance of low dissolved oxygen in deep water during the summer.|
|TSI 50-60||Lakes becoming eutrophic: decreased clarity, fewer algal species, oxygen-depleted bottom waters during the summer, plant overgrowth evident, warm-water fisheries (pike, perch, bass, etc.) only.|
|TSI 60-70||Blue-green algae become dominant and algal scums are possible, extensive plant overgrowth problems possible.|
|TSI 70-80||Becoming very eutrophic. Heavy algal blooms possible throughout summer, dense plant beds, but extent limited by light penetration (blue-green algae block sunlight).|
|TSI > 80||Algal scums, summer fishkills, few plants, rough fish dominant. Very poor water quality.|
What Does This Mean?
The trophic state index (TSI) score places a lake into a category of oligotrophic, mesotrophic, eutrophic, or hyper-eutrophic. Lakes naturally occur in each of the first three categories, but hyper-eutrophic lakes are within that category because of human-caused nutrient enrichment.
Oligotrophic lakes are generally very clear, deep, and cold. The lake substrate is typically firm and sandy. Nutrient levels are low, so the lake generally does not support large populations of aquatic plants, animals, or algae. The fish that occur in oligotrophic lakes are often low in abundance, but large in size. Many oligotrophic lakes divide into two layers in the summer, a condition known as stratification. The lower layer, called the hypolimnion, is cold and supports cold-water specialist fishes, like lake trout and cisco. These species require cold temperatures and high oxygen levels, so they remain in the lake’s lower level throughout the summer.
Mesotrophic lakes contain moderate amounts of nutrients, and contain healthy, diverse populations of aquatic plants, algae, and fish. Occasional algae blooms may occur. If the lake is deep enough to stratify, the hypolimnion often becomes low in oxygen by the end of summer, and may result in some phosphorus release from the sediments.
Eutrophic lakes are high in nutrients and contain large populations of aquatic plants, algae, and fish. The lake substrate is typically soft and mucky. The aquatic plants and algae often grow to nuisance levels, and the fish species are generally tolerant of warm temperatures and low oxygen conditions. Common fish species include carp, bullheads, and bluegills. If the lake is deep enough to stratify, the hypolimnion is usually very low in oxygen by mid-summer. This results in a release of phosphorus from the sediments, which can fuel algae blooms.
Hyper-eutrophic lakes are very high in nutrients, and often exhibit large algae blooms, which may include dangerous levels of blue-green algae. Fish communities in hyper-eutrophic lakes are dominated by carp and other species that can tolerate warm temperatures and low oxygen conditions. Most hypereutrophic lakes are small impoundments of streams, and fed by large watersheds composed of urban and/or agricultural land uses.