We're not talking here exclusively about marine cephalopods, but about a mysterious lake phenomenon.
"They look like gigantic waves, prodigiously weak and prodigiously slow; they look like tides in miniature with singularly rapid periods". (F. A. Forel).
This phenomenon can cause water levels to rise from a few centimetres to more than a metre in the space of a few minutes to more than half an hour. These cuttlefish have always intrigued scientists. As far back as the 18th century, the Geneva engineer Jean-Christophe Fatio de Duillier reported that local residents called this strange phenomenon seiches. The word seiche comes from the patois word for "coming and going". The seiche is thought to be caused by local winds.
This hypothesis is refuted because cuttlefish can occur in calm, fine, dry weather.
But where do these changes in water level come from?
Other hypotheses have been put forward. The seiches could be the result of a rise in water due to melting snow, or electric clouds (storm clouds) attracting and lifting a mass of water at one point, then the water falls back, creating ripples. Horace Bénédict de Saussure, in 1779, came closer to the current explanation. The seiches would be caused by atmospheric pressures unevenly distributed across the lake, acting like pistons that would push the water in certain places. This hypothesis was supported and confirmed in 1803 by Jean-Pierre-Etienne Vaucher. Variations in atmospheric pressure create winds that push the water from one part of the lake to another. In the low-pressure region, the water level is higher than in the high-pressure region, causing a pendulum movement. When the wind stops, the water returns to equilibrium.
Seiches are therefore phenomena of water mass movement caused by winds, leading to changes in water level, like a pendulum.
The scientist François-Alphonse Forel was able to establish that the largest cuttlefish were found in shallow "funnel" areas such as Geneva. The record amplitude measured was 2 m 15 in 1841 in Geneva.
Thanks to this information, he was able to solve the problem of Euripi (Greece), which had remained unsolved for 23 centuries. In Greece, water mills were installed in the Strait of Euripi. Powered by the tides of the Aegean Sea, the locals were amazed to see them spinning backwards 7 times a day. Many scientists studied the phenomenon, but none were able to solve it. Legend has it that Aristotle, in despair over the problem, threw himself into the turbulent waters of the Euripi. F.-A. Forel was able to shed light on why these mills were so capricious. There are seiches on Lake Talanti, located in the Euripi strait, which move in the opposite direction to the tidal movements. As these phenomena are amplified by the narrowness of the strait, they take over from the tides.
Let's return to Lake Geneva. F.-A. Forel's research has shown that there are two types of cuttlefish: longitudinal cuttlefish, which oscillate from Chillon to Geneva, and transverse cuttlefish, which oscillate from Morges to Evian.
In addition to these surface seiches, Lake Geneva is also affected by internal seiches.
These seiches form most often in summer because of the presence of the thermocline layer (see the 60s of 11.09.2019: Un Léman à trois étages). The thermocline is a layer of rapid thermal transition between the surface waters (warm) and the deep waters (cold). The wind blowing over the surface tilts this layer, causing the water mass to move at great depth and with great amplitude. These movements can return to the surface in the form of small oscillations without there being any wind in the area.
Information from J-J Pittard's lake chronicles, Les Seiches: vagues d'oscillation fixe des lacs by F. A Forel and the Musée du Léman.
For further information: The birth and transformation of lakes
Photo credit : Lake Geneva Museum
