From winter to spring, the French Alps are shrouded in stark white snow. But when spring turns into summer, the stoic slopes start to blush. Parts of the snow take on bright colors: deep red, rusty orange, lemonade pink. The locals call this “Sang de Glacier” or “Glacier Blood”. Visitors sometimes walk with “watermelon snow”.

In reality, this redness comes from an embarrassment of seaweed. In recent years, alpine habitats around the world have seen spikes in snow algae blooms – dramatic, oddly colored clusters of these normally invisible creatures.

While snow algae blooms are poorly understood, this is likely not a good sign. Researchers have started to measure the algae of the Alps to better understand which species live there, how they survive and what may be pushing them over the bleeding edge. Some of their first results were published in Frontiers in Plant Science this week.

The plant-like bacteria we call algae are tiny but powerful, and are “the foundation of all ecosystems,” says Adeline Stewart, a study author who worked on it as a PhD student at the University of Grenoble Alpes in France. Thanks to their photosynthesis abilities, algae produce a large part of the world’s oxygen and form the basis of most food webs.

But sometimes they overdo it and multiply until they throw things off balance. This can lead to toxic red tides, moldy freshwater blooms, or troubling glacier blood.

It is unclear what exactly drives the flowers, the color – often red, but sometimes also green, gray or yellow – comes from pigments and other molecules that the snow algae use to protect themselves from ultraviolet light. These hues absorb more sunlight, which causes the snow underneath to melt faster. This can change ecosystem dynamics and accelerate the shrinking of glaciers.

Inspired by increasing reports of the phenomenon, researchers from several alpine institutes decided to turn their attention from algae species in distant habitats to those “growing next door,” said Eric Maréchal, director of a plant physiology laboratory at Grenoble Alpes Alp University and a project leader.

Because so many different types of algae can live and bloom in the mountains, the researchers began counting parts of the French Alps to find out what is growing where. They took soil samples from five peaks, spread over different altitudes, and looked for algae DNA.

They found that many species tend to prefer certain high altitudes and most likely evolved to thrive in the conditions there. One key genus, aptly named Sanguina, only grows over 6,500 feet.

The researchers also brought some species back to the lab to study their potential flower triggers. Algal blooms occur naturally – the first written observation of glacier blood comes from Aristotle, who suggested that the snow had grown hairy red worms from lying around too long.

But man-made factors can make such outbreaks worse and more common. Extreme weather, unusually warm temperatures, and nutrient inputs from agricultural and sewage runoff all play a role in freshwater and marine algae bloom.

To see whether this also applies to glacier blood, the researchers exposed the algae to excess nutrients such as nitrogen and phosphorus. Although they haven’t found anything significant so far, they plan to continue this series of tests, said Dr. Stewart.

The limits of DNA sampling mean that even this study gives an incomplete picture of what lives in and under the snow, said Heather Maughan, a microbiologist and research scientist at the Ronin Institute in New Jersey who was not involved. Still, it showed the “incredible diversity” of alpine algae – underscoring how little we know about them and their potential “to serve as a beacon of ecosystem change,” she said.

In the coming years, researchers will track how species distribution changes over time, which could shed light on the overall health of the ecosystem, said Dr. Stewart. They will also try to find out whether temperature patterns correlate with flowers and begin to compare species compositions in white and colored snow. Eventually they hope to decipher the blood-red message.

“We know so little,” she said. “We have to dig deeper.”