Lago di Cadagno is a well studied lake in the south of Switzerland. It's a textbook example of a meromictic lake. These lakes lack complete circulation during spring and fall. The geochemistry of the catchment basin reveals sulfate containing rocks.

Weathering processes transport significant amounts of sulfate into lake Cadagno. As you will see in this section, sulfur and light are key players in this lake ecosystem and we will focus on sulfur cycling mechanisms used by indigenous organisms.

1 - Lago di Cadagno

Stable stratification

Lake Cadagno shows a stable stratification all over the year. Neither wind nor water inflow disturbs these strata. As a result, the hypolimnion gets anaerobic. The catchment basin of lake Cadagno consists of bedrock containing sulfate.
 

2 - Stratification in lake Cadagno

a underwater spring
b anaerobic bacteria
c sulfide
d purple bacteria
e green light
f red light
g blue light
h algae

Underwater springs (a) feed substantial amounts of leached sulfate into the hypolimnion of lake Cadagno. This sulfate rich, heavier water rests at the bottom of the lake and prevents mixing during spring and fall.
Anaerobic bacteria (b) reduce this sulfate to sulfide, which is toxic for aerobic organisms.
A hydrogen sulfide containing layer (c) develops and increases, so the oxygen containing upper tier diminishes continuously. The propagation of the poison is stopped only by purple bacteria (Chromatium spp.) (d). They are able to detoxify hydrogen sulfide by reoxidizing it to sulfate. This reaction depends on green light (e). These bacteria have to confine on green light, because longer wavelengths (red) are absorbed in the first few meters by water itself (f) and blue light (g) is consumed by the algae layer (h) above. (Also see Transmission of light in water)

Night

Winter

The darkness phase lasts much longer in winter, when the lake lies under an ice and snow cover (i). The basic living conditions are withdrawn from the bacterium stratum during several months. The organisms die gradually and both the algae (h) and the purple bacteria (d) strata thin out. The room for the oxygen dependant organisms gets narrower and narrower.

7 - Winter conditions

a underwater spring
b anaerobic bacteria
c sulfide
d purple bacteria
h algae
i ice and snow cover
j reflected light

29 August 2011
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