Defensive reactions of freshwater ecosystems against external influences

Eutrophication and toxic loading of freshwater occurred even in early geological epochs as a result of natural factors (e.g., large animals, volcanism), and nutrients and xenobiotics are more quickly integrated in material cycling in aquatic than in terrestrial systems. Therefore, aquatic ecosystems show many defensive mechanisms against organic and toxic loading. Many other defensive reactions can be described in addition to the well-known example of microbial self-purification.Freshwater ecosystems possess compartments which cooperate towards the function and protection of the whole system but, in opposition to these “euoecisms”, there are also “dysoecisms”. The defensive reactions of an ecosystem are founded largely on species-egoistic adaptations that have an (accidental) system-altruistic effect. The whole ecosystem reacts only seldom, and it is not clear whether there are selection processes which favour water bodies with a slow eutrophication and therefore slow silting-up, because the freshwaters are important for the global water balance.It is possible to compare organismic with ecosystemic defensive reactions but the origin of both reactions is very different.