Lago di Cadagno
Lake Cadagno

A special and mysterious lake

Lake Cadagno displays a unique feature; two chemically very different water layers are permanently stratified over the seasons. A small transition zone separating the two layers presents steep physical and chemical gradients, really an unparalleled ecosystem.


The upper layer is in contact with the atmosphere and is similar to a common alpine lake which is fed by surface water from the snowmelt. In Lake Cadagno the main tributary originates from the Lago dello Stabbio flowing over granite rocks in the North; this water is low in minerals.


The water in the lower layer of the lake lacks oxygen; it is fed by subaquatic springs from the dolomite.


A band of limestone and dolomite follows along the Piora Valley between granite and gneiss in the North and South of it; Lake Cadagno touches in its South part (direction towards the river Murinascia) this geological formation. The water is enriched in minerals and contains ions of the elements calcium, magnesium, sulfur, phosphorus and nitrogen. The high salinity causes a higher density of the water which stabilizes the lower part and slows down the mixing with the upper layer. This phenomenon is called meromixis, meaning only partial mixing during the seasons, mixing only in the upper part of the lake.

The zone where the two layers touch each other is called transition zone or chemocline. Here steep chemical gradients are found of oxygen (concentration decreasing with depth) and of reduced compounds (concentration increasing with depth)  such as hydrogen sulphide or ammonia.  Small amounts of light, about 1 to 5% of the surface sunlight, still reach this depth. Here at about 12 m the environmental conditions allow a massive growth of anaerobic phototrophic bacteria (e.g. Chromatium okenii), strongly increasing the primary production in the lake.


Oxygen-rich upper layer: this zone is fed by streams running through the granite rocks. Low in mineral salts, its "lightness" allows it to remain on the surface and recharge with oxygen. This water provides an ideal habitat for numerous organisms such as plankton and fish.


Intermediate layer with purple sulphur bacteria: the water in this layer, which is about one metre thick, is purple. This colour comes from photosynthetic purple sulphur bacteria such as Chromatium okenii, which contains red pigments and filters out toxic water from the deep layers; this layer therefore allows life in the upper layer.


Toxic lower layer, without oxygen: here sulphur and some salts accumulate, brought by sub-lacustrine springs. These salts make the water denser and therefore heavier; consequently it never comes into contact with the air at the surface and always remains oxygen-free. Only a few bacteria can develop in this layer (e.g. sulphate-reducing bacteria); these use sulphates as nutrients.

Centro Biologia Alpina, Piora

Via Mirasole 22a
CH-6500 Bellinzona


Repubblica e Cantone Ticino Università di Zurigo Università di Ginevra Università della Svizzera Italiana



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