Aquatic plants depend on the riverine environment and their existence is influenced by many factors. For example, periods of submersion, the aquifer level, the force of the current, the composition of the riverbed and the transparency of the water all play a role. Some plants are completely submerged, while others root in the riverbed and emerge from the water at various heights, and yet others are without roots and float on the water altogether.
In stretches of the river, where the water flows with force and speed, the plants are sparse. The riverbed is instable and moves continuously, and only a small community of algae is able to colonize (question 10). Some of these algae are microscopic, the Diatomeae and the Cianoficeae, and form a coloured film on the substrate (e.g. the rocks). Where the current reduces its speed and the water is calm, different species of algae, again floating types, as well as higher plants such as Crowfoots proliferate.
Riparian vegetation includes shrubs and arboreal species (reed beds, willows, alders, poplars), which are found in areas between aquatic plants and other plant species, farther away from the river. They are hydrophilic species, whose roots are connected with the ground-water table (aquifer).
In the mountains, riparian vegetation principally consists of humid meadows which are gradually replaced by riparian woods of alders as the slope of the ground decreases. Various species of willows (White Willow, Riparian Willow) and poplars, which prevail on the valley floor or plain, slowly replace the alders. A rich undergrowth, which is characterized by grasses, bushes, reed beds, and sometimes orchids, grows besides the riparian woods (question 2).
Aquatic plants almost totally depend on the river water for their nourishment and survival, and so they effectively reveal its chemical, physical and biological alterations. For this reason they are bio indicators, together with the macro invertebrates.
The role played by this kind of vegetation is fundamental for the balance of the river: the zone of riparian vegetation works as a filter and therefore plays a key role in the purification of the water. Through their roots, aquatic plants are able to absorb the inorganic pollutants such as phosphates and nitrates which are found in detergents (soap, laundry detergent, etc.) and fertilizers that are increasingly used in agriculture. Because of this reasons a “buffer” zone of natural vegetation between the rivers and the farmlands is extremely important.
Roots from these plants play another important function: they preserve the river banks from erosion. Alders and willows, for example, have large and deep root systems, which consolidate and make banks resistant to the force of the current.
Vegetation found along the course of the river, often called the “green corridor”, provides the shade necessary to keep the water temperature stable. Water temperature influences the amount of available oxygen dissolved in the water on which aquatic life forms depend (questions 3-4).
Thus, these bordering areas between water and land have great ecological value. The riparian zones, when intact, cross and link the land with the river, which serves as a biological corridor and is vital for the vertebrate and invertebrate fauna. Unfortunately, these important areas have been severely reduced and fragmented for reasons of development for housing, roads, bridges and buildings, flood regulations and the desire for new and better farmland.