Winter Limnological Bulletin for Lake Geneva: Significant Developments for the Lake

Observations made between December 2025 and February 2026 at point SHL2, located above the deepest part of the lake, reveal several trends for this winter.

The average surface water temperature (0–10 m) was 8.29 °C, which is 1.05 °C higher than the 1991–2020 reference period. This increase was observed across all winter months, with positive anomalies in December (+1.27 °C), January (+0.79 °C), and February (+1.00 °C).

Deep waters also confirm this warming trend, with an average temperature of 6.60 °C, which is 0.98 °C above the reference value.

Chlorophyll a concentrations remained generally below seasonal reference averages. At the same time, water transparency was particularly high, reaching 16 meters in January—a value close to the winter record.

However, the level of dissolved oxygen in deep waters remains a cause for concern: hypoxia persisted throughout the winter, with an average of 2.69 mg/L—below the critical minimum threshold of 4 mg/L for many species of fish and macroinvertebrates.

These findings underscore the importance of regularly and closely monitoring the lake’s physical and chemical changes, given the challenges posed by climate change and pressures on aquatic environments.

View the full limnological report here