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THE 4 SEASONS OF LEMAN

To what extent does the lake warm up or cool down over the year? How does its oxygen content change? And how does its transparency and algae growth fluctuate? To better understand how it works, follow the lake's evolution through the seasons.

Lake Geneva in winter

Winter mixing and end of algal development

In winter, lake water quality measurement campaigns return to a monthly schedule, as the development of algae in suspension in the water slows down. In fact, it's at this time of year that the surface waters are the most transparent.

Winter circulation adds oxygen to the lake by mixing oxygen-rich surface water with deep water, which contains less oxygen due to the decomposition of algae. In the case of a complete winter circulation, the oxygen deficit is fully compensated. In the case of repeated incomplete winter mixing, as has been the case since winter 2012-2013, the oxygen deficit at the bottom of the lake widens. At present, the lake lacks oxygen at depth, with adverse consequences for aquatic life and the release of phosphorus and certain metals trapped in the sediments.

Winter mixing also brings the nutrients needed for algae growth (mainly nitrogen and phosphorus) to the surface. But the cold, combined with low solar radiation, means that algae suspended in the water cannot develop immediately.

 

Lake Geneva water temperature in winter

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

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