For a lake to remain healthy the water needs to mix regularly. Oxygen rich surface water and nutrient dense bottom water need to mix to maintain a healthy ecosystem. In Lake Geneva mixing, which normally occurs annually, has not happened for 11 years.
Without mixing surface water becomes nutrient-poor and there is too little oxygen in the water below. In addition, when mixing occurs after a hiatus excess nutrients are brought to the surface causing environmental problems such as algal bloom.
A key driver of mixing is a cold surface temperature. Cold water is heavier than warm water and so cold surface water sinks, starting a cycle of mixing. However, a cold winter is a prerequisite for cold surface water.
The hotter the surface water gets the less likely mixing becomes. This is the biggest fear now, according to Cipel President Nicole Gallina, who spoke to SRF – Cipel is an organisation monitoring and advocating for the health of Lake Geneva.
A lack of mixing is a problem for many Swiss lakes. Lake Zurich has not mixed for 3 years, Lake Constance has not done so for 6 years and Lake Maggiore, which tip extends into Switzerland, has not mixed for 18 years.