What is an aquatic ecosystem



70 percent of the earth's surface is covered by water. For this reason alone, it is necessary to deal intensively with aquatic ecosystems. A distinction is made between fresh and salt water and between standing and flowing water. Although the standing freshwater reservoirs (lakes) only make up the smallest percentage of area, they have been processed far better than any of the other systems. On the one hand, this is due to their manageability and, on the other hand, because they can so to say- can consider closed systems, the development of which can be followed over longer periods of time, while rivers are flow systems.

The ecosystem research of inland waters is limnology, that of the seas is oceanography. As in every scientific sub-discipline, an independent terminology has been established here, so first some definitions (for comparison see also disturbance of an equilibrium): Based on their way of life, a distinction is made between planktonic organisms (plankton) that live freely in the water and those that are fixed, benthic living (benthos). To characterize the habitats, a distinction is made between the open water zone (pelagial), the bottom zone (benthal) and the bank zone (littoral). Nutrient-poor waters are called oligotrophic, nutrient-rich eutrophic.

The relatively warm and well-ventilated surface zone illuminated by daylight is the epilimnion, the oxygen-limited area of ​​cold water underneath is the hypolimnion. Because of the large differences in density between cold and warm water, there is hardly any water exchange between above and below in summer (!) (No vertical exchange). The boundary between the two zones is therefore marked by a drastic drop in temperature (thermocline). The stratification is typical for eutrophic lakes, in oligotrophic lakes it is only weak or nonexistent. In between, the weakly eutrophic, partially stratified lakes would have to be located.

Algae, including blue-green algae, make up the majority of the primary producers of aquatic ecosystems. Higher plants are mostly restricted to the littoral, whereby a distinctive zoning that depends on the water depth can be recognized. A distinction is usually made between the reed belt, floating leaf belt and belt of submerged aquatic plants.


The algae of the pelagic (phytoplankton) are mostly unicellular or united to form few-celled colonies. In order to counteract any subsidence, they are often surrounded by voluminous jellies; they often contain vacuoles containing oil or gas or have protruding floating appendages. It is noticeable that these are more pronounced in marine algae than in freshwater algae, although the buoyancy in salt water is higher than in fresh water. Numerous phytoplankton are flagellated and, due to their phototactic behavior, can collect just below the surface of the water. The life of freshwater organisms is mainly influenced by the following parameters:

The analysis of aquatic ecosystems began like that of terrestrial ecosystems with inventories. The first lists of species were drawn up at the end of the last century, and it was seen very early that the species composition changes in the annual cycle and that certain species dominate in certain periods of time and can thus cause striking water blooms.

The importance of the inorganic and physical parameters was increasingly recognized, the relevant quantities were continuously recorded, so that today, at least for a few (few) lakes, mathematical models can be created that simulate the course of the year. In Germany, especially the Plusssee, located north of Plön, and some of the other lakes in the East Holstein Lake District and Lake Constance are under constant observation; in the USA it is Lake Mendota, on the banks of which is the campus of the University of Wisconsin, Madison. To illustrate the problem and the method of operation, some of the results obtained by working on the Plusssee are presented below.