What does the spring of 2026 tell us about the state of Lake Geneva?

Each season provides new insights into the evolution of Lake Geneva. Thanks to the limnological monitoring conducted by CIPEL, it is possible to observe the changes taking place, view them from a long-term perspective, and better understand the mechanisms that influence the lake’s functioning.

📍 The data presented below were measured at point SHL2, located directly above the deepest point of the lake (309 m). Five measurement campaigns were conducted between March and May 2026 and compared to the 1991–2020 reference period.

🔎 KEY FINDINGS:

🌡️ Water temperature

This spring, the average surface water temperature (0–10 m) was 10.90 °C, which is +1.91 °C higher than the reference period. The average temperatures for March, April, and May were all higher than those of the reference period, at +1.76 °C, +1.82 °C, and +2.16 °C, respectively.

The average deep-water temperature was 6.64 °C, which is 0.98 °C higher than the reference period, confirming the ongoing warming of the lake bottom.

💧 Dissolved oxygen concentration

During the spring, the concentration of dissolved oxygen in deep waters (250–309 m) averaged 2.86 mg/L. This observation confirms that hypoxia in deep waters persisted throughout the spring.

🌿 Chlorophyll a concentration

This spring, average chlorophyll a concentrations remained close to normal in the 0–10 m layer (average deviation: −0.04 µg/L). This result stems from a more productive March than the reference period (+1.96 µg/L), followed by a slightly below-average April (-0.40 µg/L) and a significantly less productive May (-1.67 µg/L).

In the 0–30 m layer, concentrations were higher than the reference period average throughout the spring (average deviation: +1.09 µg/L), particularly in March (+1.70 µg/L).

👁️ Water clarity

This spring, water transparency followed the trend in chlorophyll a concentrations, with the exception of March.

💬 These observations are taken from the CIPEL limnological bulletin, which is available online.