Lake pavin, a crater lake

Nestled at an altitude of 1,200 meters, near Besse in the Puy-de-Dôme, Lac Pavin attracts many visitors. But do you know the many mysteries that surround it?

Located halfway between the towns of Besse and Super Besse in Puy-de-Dôme, Lac Pavin continues to attract visitors. It is one of the favorite destinations for Clermont Ferrand residents looking for a bit of coolness in summer.

This natural setting in its raw state retains a great deal of mystery. It has fueled the imagination of the inhabitants of the region. Some say it intrigued first before it interested researchers. It is a lake on which there have been hundreds of scientific publications in international journals, dozens of theses defended, numerous research programs.


A unique lake in mainland France

But if it is so intriguing, it is firstly because Lake Pavin is a meromictic lake, that is to say made up of two different layers of water that are superimposed on one another. Christian Amblard is Honorary Research Director at the CNRS and Vice-President of the CSRPN Auvergne-Rhône-Alpes (Regional Scientific Council for Natural Heritage). He knows the lake by heart.
He explains: “It is the only lake of this type in mainland France. Everything comes from his training. It is a lake that has a volcanic origin. It is said to be an explosion crater lake. These lakes of this type are generally circular, deep, Lake Pavin is 92 meters deep, but these lakes have a small area. The Pavin is 750 meters in diameter. When a lake is very deep and has a small area, it has a very steep slope. It is this shape that makes this lake unique. In the lakes there is what is called the stirring of the waters. Every year, in the lakes, depending on the temperature and the wind, the waters are mixed. Surface water plunges deep and, conversely, deep water will rise. While in most lakes this mixing of water goes to the bottom, in Pavin Lake this mixing does not go to the bottom.
It is divided into two large areas. There is a zone of the surface at 60 meters, the mixolimnion, a mixed, stirred and oxygenated zone. The area between 60 and 92 meters is never going to be oxygenated. This is roughly the same layer of water since the lake was created, just over 6,000 years ago. Since there is no light, no oxygen, since the temperature is stable, there is a completely original environment. This is what characterizes this lake, this unstirred bottom layer which is called monimolimnion
“.

A rich microbial life

It is this deep layer that makes it so special. “This layer has no light, no oxygen, lots of chemicals like hydrogen sulfide, methane. The temperature there is more or less stable, between 4 and 5 degrees. These are extreme living conditions. And yet there is extremely rich and abundant microbial life. In this medium one finds on average 1 million microorganisms per milliliter. They are very diverse communities. Three-quarters of the species described by our laboratory had never been described in the world before, “explains the scientist. This very rich life fascinates researchers. His study takes them back to the origins of life.
Chrisitian Amblard specifies: “We can say that microorganisms must have developed original metabolisms and life strategies. It got us going in two directions. A direction to seek to have applications in biotechnology. An innovative company has come up to use these assemblages of microorganisms to do work in cosmetics, in the degradation of non-food waste. The second way is to think that the characteristics of the lake bottom resemble the characteristics of the primitive soup of the ocean from which life sprang. There are studies to understand the first stages in the evolution of living things “confides Christian Amblard. Primitive soup is a physicochemical mixture described and studied by scientists who seek to understand, model or reproduce the origins of life on Earth.

Lac Pavin, Auvergne in French from Pascal Vincent on Vimeo.