THONON-LES-BAINS, France, March 12, 2025 (BSS/AFP) - Fish in Lake Geneva --
Western Europe's largest lake -- are under threat as its waters warm and
become increasingly stagnant.

"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.