"Little by little, the lake's temperature is increasing," contributing to a lack of oxygen in its depths, said Nicole Gallina, corporate secretary of the International Commission for the Protection of the Waters of Lake Geneva (CIPEL).

CIPEL is a joint French-Swiss organisation that monitors the picturesque, crescent-shaped body of water shared between both countries.

France's National Research Institute for Agriculture, Food and the Environment analyses water samples collected by CIPEL.

"If there is less and less oxygen in the water, there is less and less viable space for living organisms," explained Viet Tran-Khac, laboratory manager at the research institute's facility in the Thonon-les-Bains, on the lake's southern shore.

Typically in winter, the surface temperatures cool to a density comparable with the lake's deeper layers, facilitating a mixing of waters between the levels.

This natural cycle is essential for maintaining aquatic ecosystems as it transfers oxygen to the lake's lower levels.

However, this full-scale winter mixing is becoming rare, as ever-milder winters, which scientists attribute to global warming, prevent the surface waters from cooling sufficiently.

"With climate change, we no longer have the extremely cold winters needed for this natural mixing to take place," Gallina told AFP.

In the current winter, the minimum average temperature measured in the top 10 metres (30 feet) of the lake was 7.8 degrees Celsius (46 degrees Fahreinheit): an increase of 1.5C compared to the 1991-2020 reference period.

- New record -

Data published Wednesday by CIPEL showed that, this year, the waters only mixed down to a depth of 110 metres -- but the lake's deepest point is 309 metres down.

Thirteen consecutive winters without complete mixing marks a "historic record", warned Gallina, surpassing the previous longest spell set between 1987 and 1999.

"During the last complete mixing in 2012, the oxygen level in the deep waters was seven milligrammes per litre," the biologist said.

It has now dropped to 2.4 mg per litre -- below the critical threshold of four mg necessary for living organisms.

However, Gallina stressed there remained "hope" for the re-oxygenation of deep waters.

Last year, a study by the Swiss Federal Institute of Technology university in Lausanne showed that the complete vertical mixing in 2012 also benefited from lateral mixing flows -- a phenomenon which was previously unknown.

However, an entire ecosystem is starting to change, warned CIPEL.

The lack of oxygen also impacts the growth of phytoplankton plants, which are eaten by zooplankton organisms -- which themselves serve as the food base for fish.

The future of Arctic char, fera and other emblematic Lake Geneva fish is under threat.

"Salmonids like fera need cold water to spawn. Before, it spawned at a depth of three to six metres; now it spawns at 20 to 25 metres," said Alexandre Fayet, president of the Swiss inter-cantonal association of professional Lake Geneva fishermen.

"For the moment, we are not too worried" but "we are trying to diversify and commercialise fish that like warmer and less oxygenated waters, such as carp, tench and bream," he told AFP.

- 'Transformation' phase -

LeXPLORE, a floating scientific platform, has been carrying out research on Lake Geneva since 2019, studying 44 different parameters down to a depth of 110 metres.

Natacha Tofield-Pasche, its project manager, said that besides the rising lake temperatures, global warming also leads to "extreme events" that wash a lot of polluting particles down into Lake Geneva, as witnessed during major floods last year in Switzerland's Wallis region.

Such events can also knock out wastewater treatment plants, while the lake provides drinking water to around a million people.

CIPEL is "very worried because it sees that Lake Geneva is going through a transformation phase," marked by long periods without complete mixing, said Gallina.

Added to this are other challenges, such as pollution invisible ot the naked eye, such as micropollutants and microplastics, or the invasion of quagga mussels.

In addition, high heat episodes increase the risk of proliferation of cyanobacteria, which can be toxic.

The lack of mixing also promotes the accumulation of nutrients such as phosphorus in deep waters.

In the event of increasingly exceptional complete mixing, the phosphorus could rise to the surface, causing algae blooms.