Trophy system

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The trophy system is a classification and evaluation system that characterizes the condition of stagnant water in terms of nutrients . The classification of the trophic system is based on the content of nutrient elements ( P , N , C , S ), whereby (with the exception of intentionally fertilized waters such as carp ponds) the phosphorus content has turned out to be the decisive and limiting factor ( minimum factor ). It does not matter that these elements are mostly bound in organic material. The ideal nutritional status of the aquatic plants is called eutrophy (from the Greek εύτροφος , “well nourishing”).

The division comes from Einar Naumann .


In limnology , the waters are divided into four trophic levels based on the annual average values ​​of chlorophyll and total phosphorus content. In addition, secondary oxygen saturation , water color , depth of view and other parameters are used for assessment which result as a consequence of the intensity of organic production caused by the nutrient elements: e.g. B. bio and chemical oxygen demand ( BOD and COD ) or the number of bacteria .

In general, the less oxygen is available, the more plant nutrients the water contains. With over-fertilization (hypertrophic, see also eutrophication ), an extremely low-oxygen soil layer forms after the summer .

The trophic system of stagnant waters is not to be confused with the saprobic system developed for flowing waters , which also defines four levels, the water quality classes. These are based on exposure to organic, biodegradable material. In the last few decades, due to the advances in sewage technology, the problem of pollution with inorganic nutrients in rivers has increased compared to saprobic pollution with organic material. Therefore an attempt is being made to develop a trophy system for rivers as well. Furthermore, when the trophic problem was recognized, the treatment technology was supplemented by methods of phosphorus retention.


Oligotroph (trophy level I)

Oligotroph ("nutrient poor") are bodies of water with few nutrients and therefore low organic production. The low phosphate intake limits the growth of plants and algae. The plankton is rich in species, but poor in individuals. The water body feeds only a small amount of fish. Oligotrophic waters often have coarse-grained bank structures with little vegetation. Your water is very clear. It appears blue to dark green. The depth of view is usually greater than 6 m, but at least 3 m. The oxygen saturation at the end of summer stagnation is more than 70%. In the oxygen-rich trivalent given deepwater iron ions precipitate released phosphate and withdraw it as the material cycle ( phosphate case ).

Mesotroph (trophy level II)

Mesotrophic waters are called which are in a transition stage from oligotrophy to eutrophy. The nutrient content is higher and light can penetrate deeper water layers. With the increasing density of the phytoplankton , the depth of penetration of the light changes. The visibility depth is still more than two meters and the oxygen saturation at the end of summer stagnation is between 30 and 70%. The phosphate trap remains effective.

Eutrophic ( Trophy level III)

Eutrophs are waters with a high phosphate content and therefore high biomass production. The hypolimnion (cold soil layer) of eutrophic water becomes very poor in oxygen in summer, while the epilimnion, on the other hand, is oversaturated with oxygen through photosynthesis. The plankton is very rich in species and individuals. The bottom of the water is covered with an anaerobic digested sludge layer ( mudde , sapropel ), which is populated en masse with mud tube worms and mosquito larvae . Iron (II) phosphate diffuses out of this layer during the water circulation in spring and autumn and contributes to the rapid re-fertilization of the water. After the full spring circulation, algae bloom often occurs . The water is cloudy and mostly colored green to yellow-brown by different algae. The visibility depth is usually less than two meters and the oxygen saturation at the end of summer stagnation is less than 30%.

Hypertrophic (also polytrophic ; Trophy level IV)

Hypertrophic is the name given to waters in which the nutrient content and thus the biomass production is so high that the oxygen in the layers close to the ground (hypolimnion) is largely used up by the end of summer stagnation. Only the uppermost water layers of the epilimnion still show tolerable growth conditions for specialized organisms. Fish kills often occur at night and in the morning . The viewing depth is less than one meter. Colloquially, such waters are called "overturned".

Artificially fertilized small bodies of water, such as carp ponds , can also be hypertrophic . They are intentionally laid out very flat in order to prevent the formation of an oxygen-depleted bottom layer and to be able to use the increased production for fishing.

Parameter table of the four trophy levels

parameter oligotrophic mesotrophic eutrophic hypertrophic
Viewing depth (in meters) 5–10, at most 15–20 1–2, at most 5–10 less than 1, at most 2–3 less than 1
Depth limit of the submerged vegetation 12-30 m 5-10 m less than 2 m less than 1 m
Phosphate content (mg / m³) 4–10 (0–4 = ultra oligotrophic) 10-35 35-100 more than 100
Nitrate and ammonium content in autumn
(mg N per l)
at most 1 at most 1 more than 2 more than 2
Annual mean chlorophyll content (mg / m³) less than 3.5 less than 7.0 less than 11 more than 11
O 2 content (in mg / l) more than 8 6-8 2-4  
BOD 2 (in mg / l) 0.5 1.1-2.2 4-7  
BOD 5 (in mg / l) at most 3 3-5.5 5.5-14 more than 14
COD (in mg / l) 1-2 8-9 20-65  
Number of bacteria (number per ml) less than 100 about 10,000 up to 100,000 more than 100,000

BOD x : biological oxygen demand in x days
COD : chemical oxygen demand (determination with potassium dichromate K 2 Cr 2 O 7 )

See also