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THE 4 SEASONS OF LEMAN

To what extent does the lake warm up or cool down during the year? How does its oxygen level change? And how does its transparency and the development of algae fluctuate? To better understand how it works, follow the evolution of the lake through the seasons.

Lake Geneva in winter

Winter mixing and end of algal growth

In winter, the water quality measurement campaigns in the lake become monthly again because the development of algae in suspension in the water slows down. In fact, it is at this time of 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 due to the decomposition of algae. In the case of a complete winter mixing, the oxygen deficit is fully compensated. In the case of repeated incomplete winter mixing, as has been the case since the winter of 2012-2013, the oxygen deficit at the bottom of the lake increases. At present, the lake lacks oxygen at the bottom, with negative consequences for aquatic life and the release of phosphorus or certain metals, which were trapped in the sediments.

The winter mixing also leads to the remobilisation towards the surface of the nutrients necessary for the development of the algae (mainly nitrogen and phosphorus). However, the cold weather and low solar radiation do not immediately allow the algae suspended in the water to develop.

The temperature of the waters of Lake Geneva in winter

Cold and windy periods mix the surface water with the 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.

Results of the winter 2021-2022 surveys

Station SHL2

Thursday 20 January 2022

  • Temperature of the brewing bed: 7.5 °C
  • Brewing depth: 80 m
  • Temperature at the bottom of the lake: 6.06°C
  • Oxygen content at the bottom of the lake: to come
  • Surface water transparency: 13.4 m
  • Amount of algae in surface waters (mean 0-30 m): to be announced

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