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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 autumn

Cooling and further algal growth

In autumn, the lake remains divided into three superimposed layers of water. A surface layer that is almost homogeneous in temperature (epilimnion), below this a layer where the temperature decreases rapidly as the depth increases (metalimnion), and deeper down a layer that is relatively homogeneous in temperature and cold (hypolimnion). As temperatures drop and daylight hours shorten, the epilimnion cools and mixes with the metalimnion. The autumn cooling is characterised by a thickening of the epilimnion to a depth of about 50 m.

The summer development of the algae in suspension in the water has led to a decrease in the content of nutrients (mainly nitrogen and phosphorus) necessary for their development in the epilimnion. As the metalimnion is richer in nutrients than the epilimnion, the autumn cooling remobilises nutrients to the surface, and this remobilisation allows the algae to continue to grow. The more algae there are, the less transparent the surface water becomes.

Algae are important for the functioning of the lake as they form the basis of the food chain. Most of them are not a problem, but some species are problematic: those that clog fishing nets, and toxic cyanobacteria.

When the algae die, they sediment towards the bottom. Their decomposition during sedimentation consumes oxygen and lowers the oxygen content in the hypolimnion.

Results of the latest surveys

Station SHL2

Tuesday 16 November 2021

  • Surface water temperature: 11.8 °C (homogeneous between the surface and 23 m depth)
  • Surface water transparency: 8.6 m
  • Amount of algae in surface water (average 0-30 m): 3.4 µg chlorophyll/L
  • Temperature at the bottom of the lake: 6.04°C
  • Oxygen content at the bottom of the lake: 1.4 mg/L

Station GE3

Wednesday 10 November 2021

  • Surface water temperature: 11.4 °C (homogeneous between the surface and 50 m depth)
  • Surface water transparency: 9 m

Measurement stations

Lake Geneva is monitored by two measuring stations: station SHL2, located at the deepest point of the lake (309 m), is monitored by CARRTEL of INRAE in Thonon-les-Bains and station GE3, located in the lesser lake where the depth is 70 m, is monitored by the Geneva Water Ecology Department. The previous data recorded at station SHL2 can be downloaded from the OLA website.

Results of previous surveys

Station SHL2

JThursday 4 November 2021 (measurement campaign incomplete due to wind)

  • Surface water temperature: 13.1°C (homogeneous between the surface and 31 m depth)
  • Temperature at the bottom of the lake: 6.04°C

Tuesday 19 October 2021

  • Surface water temperature: 15.4°C
  • Surface water transparency: 7.6 m
  • Amount of algae in surface water (average 0-30 m): 2.9 µg chlorophyll/L
  • Temperature at the bottom of the lake: 6.05°C
  • Oxygen content at the bottom of the lake: 2.1 mg/L

Monday 27 September 2021

  • Surface water temperature: 19.6°C
  • Surface water transparency: 7.4 m
  • Amount of algae in surface water (average 0-30 m): 3.1 µg chlorophyll/L
  • Temperature at the bottom of the lake: 6.03°C
  • Oxygen content at the bottom of the lake: 1.8 mg/L

Station GE3

Mard 19 October 2021

  • Surface water temperature: 14.6 °C (homogeneous between the surface and 23 m depth)
  • Surface water transparency: 5 m

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