Description
Lake Costa originated by 'alluvial damming', i.e. due to the formation of a sort of natural dam that prevented water from flowing into the valley, causing it to stagnate.
The pond is fed by underground springs that flow mainly in the eastern part of the basin, preventing the formation of ice in winter in this area. There is a temporary tributary, coming from the Canzolino Lake above, and a perennial effluent which, after a short journey through cultivated land, flows into the Fersina stream.
It hosts some very uncommon types of aquatic and marsh vegetation.
The water mirror vegetation includes plants that live completely submerged, such as the water milfoil (Myriophyllum sp.) and the ceratophyllum (Ceratophyllum demersum), providing a habitat for various species of fish that find food and shelter here. Other plants root in the mud of the seabed but develop leaves and flowers above the water surface, such as Polygonum amphibium. The marsh vegetation, in addition to the water reed (Phragmites australis) that predominates and also reaches into the water, includes some showy species, such as the beautiful yellow iris (Iris pseudacorus).
On the banks, in addition to a few rare tufts of sedges (Carex elata and Carex pseudocyperus), there are black alder (Alnus glutinosa), white alder (Alnus incana) and white willow (Salix alba), forming a patch of hygrophilous woodland.
The fauna of the biotope consists of species typically associated with water.
Numerous invertebrates live in the pond and in the tributary: insects such as dragonflies; molluscs such as water snails (Limnaea stagnalis) and the filter-feeding bivalve anodonta (Anodonta cygnea); crustaceans such as the crayfish (Austropotamobius pallipes).
The fish fauna is very rich and includes mainly stillwater fish but also a rare running water species, the Po river goby (Padogobius martensi), which lives in the outfall. The biotope, used by the green frog (Rana 'esculenta') and the toad (Bufo bufo) for laying their eggs, is also a nesting place for some valuable water birds such as the mallard (Anas platyrhynchos), moorhen (Gallinula chloropus), coot (Fulica atra) and water rail (Rallus aquaticus).
