Will it mix?
Cool the surface of a lake and watch how deep the water turns over. Somewhere in the range there is a point where the answer changes. Find it.
Depth in metres. 309 m at its deepest.
Winters CIPEL has measured. 2011/12 reached the bed at 309 m. Since then, 2021/22 130 m, 2024/25 110 m, 2025/26 140 m.
As the surface cools its density rises, the line straightens, and the column can turn over. The whole width of this chart is only 0.47 kg/m³: very small density differences decide it.
110 m of 309 m
2.4 mg/L (low)
Sensitive species need at least 4 mg/L.
Deep water stays near 6 °C all year, so the surface has to cool towards it before the column can turn over.
Surface temperature 7.8 degrees. Partial mixing. Mixing reaches 110 m of 309 m. Oxygen at the bed is 2.4 milligrams per litre, which is low.
Illustrative model. Real mixing also depends on wind strength, basin shape and lateral currents.
Mixing depth data: CIPEL.
Drag slowly from the warm end. For a while, cooling the surface buys you a little more depth each time. Then it stops behaving like that. Note the temperature where the change happens, and look at the density line either side of it.
Lake Geneva has been doing this
The lake last turned over completely in the winter of 2011/12, after a harsh winter. The winters since have reached partway and stopped. Oxygen at the bed has fallen from 8.4 mg/L in 2012 to 2.4 mg/L in 2025, and the deep water has warmed by 1.2 °C over the same period.
Weather still decides any given year
Winter 2025/26 mixed to about 140 m, deeper than the 110 m of 2024/25, because it was colder. The lake also went through a long run of winters without complete mixing between 1986 and 1999. What has changed is the starting temperature, not whether this can happen at all.
Then press Ocean
The same slider, the same behaviour, and something different is lost. Carbon dioxide dissolves into the surface layer. If that water stops sinking, the surface approaches its saturation point and can take up no more, so the carbon sink stalls even though the water below is nowhere near full.