The meromictic alpine Lake Cadagno: Orographical and biogeochemical description

Lake Cadagno is a 21 m deep alpine meromictic lake situated at an altitude of 1921 m in the Piora valley in the southern part of central Switzerland. The bedrock of the valley containing dolomite and gypsum determines the chemistry of the water. The lake basin was created by glacial erosion and originally dammed by a glacial moraine. The water body is structured in 3 distinct layers, the oxic mixolimnion, the anoxic monimolimnion and a narrow chemocline in between. The water masses of the lake are stabilized by density differences of salt-rich water which is constantly supplied by subaquatic springs to the monimolimnion. In contrast the mixolimnion is fed by electrolyte-poor surface water. Sulfate, hydrogen carbonate, calcium and magnesium are the dominant ionic species. In the monimolimnion sulfide concentrations of more than 1 mM are found. The chemocline at a depth of 10 to 13 m is characterized by steep chemical and physical gradients. It contains dense populations of up to 10(5) cells/mL of phototrophic sulfur bacteria consisting of predominantly Chromatium okenii, C. minus and Amoebobacter purpureus. The lake has proven to be an excellent model system for studies of the role of planktonic bacteria which dominate the sulfur cycle.