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How much does the lake warm or cool over the year? How does its oxygen level change? And how does its transparency and the development of algae fluctuate? To better understand its functioning, follow the evolution of the lake with the seasons.

Lake Geneva in winter

Winter mixing and end of algal development

In winter, the campaigns of measurement of the quality of the water of the lake become monthly again because the development of the algae in suspension in water slows down. In fact, it is at this time of the year that the surface waters are the most transparent.

Winter mixing adds oxygen to the lake by mixing the oxygen-rich surface water with the deep water, which contains less oxygen because of decomposing algae. In the case of a complete winter mixing, the oxygen deficit is completely filled. In the case of a repeat of incomplete winter stirring, as has been the case since the winter of 2012-2013, the oxygen deficit at the bottom of the lake increases. Currently, the lake lacks oxygen at the bottom, resulting in adverse consequences for aquatic life and the release (salting out) of phosphorus or certain metals, which were trapped in the sediments.

Winter mixing also leads to the remobilization towards the surface of nutrients necessary for the development of algae (mainly nitrogen and phosphorus). But the cold weather, combined with the low solar radiation, does not immediately allow the algae suspended in the water to develop.


The temperature of the waters of Lake Geneva in winter

Cold and windy episodes mix surface water with deep water. Winter mixing is complete if the lake waters manage to mix completely to the bottom. Otherwise, mixing is said to be incomplete. The more severe the winter, the greater the mixing depth.


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