ScienceWhy do some high-altitude lakes turn green in the Pyrenees?

    Why do some high-altitude lakes turn green in the Pyrenees?

    The Areau pond, in the Hautes-Pyrenees, now has an unusual green color. The reason for this change in color is debated within the scientific community, but the proliferation of green algae is particularly pointed out.

    An unusual color. This summer, the Areau pond, in Haut-Salat, in Ariège, presented an unusual green color. An astonishing phenomenon, but which affects several high altitude lakes. It would be linked to several factors, including the introduction of a small fish by fishermen, according to some scientists.

    A campaign to sound the alarm

    Located at more than 1,800 meters above sea level, the lake of the Cirque d’Etom, near the town of Cauterets, has taken on a surprising color in recent months, going from crystalline to green. The phenomenon has been observed in many mountain lakes in the Pyrenees.

    The Pyrenees National Park and the Hautes-Pyrenees Departmental Fishing Federation have sounded the alarm via the advertising campaign Preserve mountain lakes and streams” published in July, in which they are concerned about this development.

    “The remarkable diversity of species in these ecosystems can be undermined by the deterioration of water quality and also by the introduction of fish, predators of insects and amphibians,” deplores the national park and the federation in a communicated.

    Lake Areau, in Seix, in the Hautes-Pyrenees, on August 29, 2023
    Lake Areau, in Seix, in the Hautes-Pyrenees, August 29, 2023 © Emmanuel CHARRET / AFP

    The introduction of fish

    In fact, scientists warn about the presence of marine species in high-altitude water bodies.

    “When we see fish in mountain lakes, we see an ecosystem that is disturbed,” said AFP Adeline Loyau, biologist and engineer at the National Polytechnic Institute of Toulouse.

    Fish were introduced into the mountains by humans several centuries ago, first as a source of protein for shepherds and then, on a more massive scale, to supply hotels and restaurants in spa towns.

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    proliferating algae

    Among these fish, the biologist, and her husband, a specialist in mountain ecology at the INP Dirk Schmeller, were particularly interested in the minnow, a species of less than ten centimeters which normally lives in cool rivers and which is used as live bait.

    When it manages to escape from the hook or when it is released by fishermen, it acclimatizes well, devouring amphibians and insects, as well as zooplankton, “small microscopic crustaceans whose role is to eat algae and to keep the water very clear, very pure”, explains Adeline Loyau.

    When a lake turns green, “it’s because the algae have won”, adds Dirk Schmeller.

    “A cocktail of factors”

    The proliferation of algae is however not only due to the minnow and the real impact of this small fish on the ecosystem is at the heart of lively debates between researchers.

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    For Didier Galop, research director at the National Center for Scientific Research (CNRS) specialist in the history and geography of the environment, “there is a cocktail of disturbance factors” that can explain the greening of lakes, such as the concentration of herds around these water points or the higher temperatures linked to global warming.

    In the eyes of the scientist, also a fisherman, greening is a phenomenon that remains quite marginal and is only one of the many symptoms of the deterioration of the quality of water in mountain lakes. “There are also lakes which are very blue, but which have zero biodiversity,” he emphasizes.

    Dirk Schmeller and Adeline Loyau believe that green lakes are more and more frequent, especially on small water surfaces. “We even have hikers who sometimes came thirty years ago and who point it out to us,” assures the researcher.

    The question of “wild” reintroductions

    On the other side of the Pyrenees, green lakes were observed as early as 2011 by Spanish researchers, who in 2014 began fish elimination programs using nets or electric fishing techniques.

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    In 2018, the Pyrenees National Park in France imitated them. But he found that fish had subsequently been reintroduced in a “wild” manner. He therefore counts on raising awareness among fishermen to find a balance between leisure and environmental preservation.

    Sebastien Delmas, president of an association bringing together the fishing federations of the Pyrenees, recognizes that the minnow poses a problem and wishes to “harmonize the regulations”, which differ from one department to another, to limit live fishing in the mountains. But he believes that other fish, such as trout, have a perfect place there.

    “Fish are also biodiversity: if they have been there for centuries, it is because they are there,” he maintains.

    Tourism also involved?

    According to him, we should also look at tourism to understand the poor health of the lakes, because swimming with sunscreen or mosquito repellent also has an effect on the ecosystem.

    “On a summer day, there may be three or four fishermen around a lake, but 300 bathers. But we always blame the fishermen”, he regrets.

    Dirk Schmeller, in favor of eliminating fish, also believes that the use of pollutants around lakes should be reduced. “Afterwards, we will just have to change global warming…”, he notes with irony.

    Juliette Desmonceaux with AFP

    Source: BFM TV

    This post is posted by Awutar staff members. Awutar is a global multimedia website. Our Email: [email protected]


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