Trophic state - Understanding Lake Data

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FIGURE 3. Lake aging process

Three cross sections of lakes: young, medium and old lake ages

Trophic state is another indicator of water quality. Lakes can be divided into three categories based on trophic state-oligotrophic, mesotrophic, and eutrophic. These categories reflect a lake's nutrient and clarity levels.

Oligotrophic lakes are generally clear, deep and free of weeds or large algae blooms. Though beautiful, they are low in nutrients and do not support large fish populations. However, oligotrophic lakes often develop a food chain capable of sustaining a very desirable fishery of large game fish.

Eutrophic lakes are high in nutrients and support a large biomass (all the plants and animals living in a lake). They are usually either weedy or subject to frequent algae blooms, or both. Eutrophic lakes often support large fish populations, but are also susceptible to oxygen depletion. Small, shallow, eutrophic lakes are especially vulnerable to winterkill which can reduce the number and variety of fish. Rough fish are commonly found in eutrophic lakes.

Mesotrophic lakes lie between the oligotrophic and eutrophic stages. Devoid of oxygen in late summer, their hypolimnions limit cold water fish and cause phosphorus cycling from sediments.

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A natural aging process occurs in all lakes, causing them to change from oligotrophic to eutrophic over time, and eventually to fill in (Figure 3). People can accelerate the eutrophication process by allowing nutrients from agriculture, lawn fertilizers, streets, septic systems, and urban storm drains to enter lakes.

In nutrient-poor areas, the aging process may lead instead to dystrophic and bog lakes which are highly colored, acid, and not as productive as eutrophic lakes.

Researchers use various methods to calculate the trophic state of lakes. Common characteristics used to make the determination are:

The trophic states associated with these three measures are shown in Table 4. Clearly, low levels of phosphorus are associated with low levels of algae (chlorophyll a), which are associated with high Secchi disc readings.

TABLE 4. Trophic classification of Wisconsin Lakes based on chlorophyll a, water clarity measurements, and total phosphorus values. (Adapted from Lillie and Mason, 1983.)


Trophic class Total phosphorus ug/l Chlorophyll a ug/l Secchi Disc feet
Oligotrophic            3                 2                  12
                       10                 5                   8
Mesotrophic            18                 8                   6
                       27                10                   6
Eutrophic              30                11                   5
                       50                15                   4

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For more information on this topic, contact:

James Vennie
Watershed Management
(608) 266-2212


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Last Revised: Thursday January 25 2007