In a healthy river environment, communities of animals and plants are born and grow, feed, breed and die in close link with one another (producers, consumers, decomposers). The whole of living organisms (bacteria, algae, larvae, shellfishes, fish, etc.…) within the water, in the substrate and in the banks forms the ecosystem of the river. The river is not an isolated, self-sufficient ecosystem however, and in fact, it receives inputs from the surrounding environment: for example the mineral salts in the water and the vegetal and animal wastes are the first sources of nourishment for invertebrates, or the small insects on which fish, birds and terrestrial mammals depend (questions 13-14).
Each stretch of the river houses a well-organized biological community. In particular, the macro-benthonic communities (“macro” = visible to the naked eye and “benthonic” = living on the ground of the river) are formed by small organisms, which attach themselves to the riverbed and draw nourishment from the water that flows over them. When water quality worsens, sensitive species are the first to disappear (Plecoptera) and are then followed by the others (Efemenoptera and Tricoptera). However, during poor water quality conditions, the strongest species survive (Diptera or Oligochaeta) and even proliferate in this non-competitive environment. The composition of these communities varies according to water quality: it varies from the optimum condition, where all of the locally-found species thrive, to the most damaged condition, where only a the strongest species can survive.
Organisms, that indicate the quality of the water by their presence or absence, are called bio indicators.
It should be noted that not every single one of the macro-benthonic species lives in every water body. It should also be stressed that a varied macro-benthonic community often utilizes all the nourishments in the river, is more adaptable to changes, and therefore can perform a better self-purifying function (to deepen this aspect go to Biotic Index).
Ecological zoning describes the different communities of organisms that live in different stretches of the river, and adapt to the different environmental conditions.
In the mountains or in sloping stretches the current is fast, the waters are turbulent, and rocks, pebbles,sand and mud form the substrate of the river. The water is well-oxygenated and cold. The organisms here are well adapted to life in cold and well-oxygenated waters. The stones on the riverbed are often covered with algae and give refuge to numerous larvae (Diptera, Efemenoptera, Plecoptera and Tricoptera), which use their suckers and legs with hooks to cling to the substrate. The dominant fishes are trout (Salmonoid) and greyling (Thymallidae). This stretch of the river is called Rhitron or salmonoid region (question 14).
In the low-lands or plains with less slope, the flow of the water slows down and the river-bed is wider, the lighter sediments settle out, and the ground is formed by sand drifts and mud deposits. The typical organisms found here easily tolerate changes in temperature and survive with low concentrations of oxygen. The water is often turbid because of the suspended material that is transported; the large quantities of dissolved mineral salts and the slow current allow the growth of plankton and immersed vegetation to grow (Myriophyllus, Muckweeds, Crowfoot). The presence of this vegetation gives shelter and food to many other organisms (shellfish, annelids). The dominant fish are the Cyprinids. This stretch of the river is called Potamon or barbell region (named after the dominant fish species). Other very common fish species include carp, tinch, perch, pike, eel and the char in high altitudes.
Wetlands, or moist environments, are essential for the survival of amphibians such as green frog, salamander, tritons and the yellow-bellied toads as well as many reptiles. Here many bird species build their nest (wild ducks, water hens, herons, etc.), spend their winter and stop during seasonal migrations (the tufted duck, the grey heron, the spoonbill duck, the cormorant). The presence of so many living creatures attracts predators such as the brown kite, the duck hawk and the hen harrier. There are also rare reptiles and mammals found here such as the otter, which has only survived in very few wild rivers.