Blood of the glaciers seen from space

In the Alps, the microalga Sanguina nivaloides proliferates in the spring snow, forming blooms. It accumulates astaxanthin, which colors the snow red (the blood of the glaciers) and accelerates its melting.

In a paper published in the journal PNAS, scientists used satellite images to map the blooms. They deduced the favorable conditions for their formation.


Snow cover projections based on IPCC scenarios suggest that these algae will not significantly exacerbate snowpack melting by 2100.

The conditions triggering snow algae blooms are poorly understood

At the end of spring in the alpine meadows, the surface of the snow sometimes turns color due to microscopic particles, which accelerate snowmelt. The hues taken on by the snow patches vary.


Ochre shades are often seen due to Saharan dust deposits from the previous winter. There are also more or less scattered red patches. These patches correspond to clusters of microalgae, made up of microscopic cells barely visible to the naked eye, which have proliferated in large numbers in the snow, forming what is called a "bloom" or "efflorescence."

In the Alps, the dominant species of snow algae is Sanguina nivaloides. It is commonly referred to as "blood of the glaciers," although this alga develops in seasonal snow, and it would be more accurate to call it "blood of the snow." The red color comes from a carotenoid molecule that Sanguina nivaloides accumulates, called astaxanthin.

Despite significant efforts made recently to understand its biology, Sanguina nivaloides remains mysterious and raises many questions. Where can blooms be found? Do they always occur in the same locations, linked to a persistent reservoir of algae from year to year, or are they found in different locations following a dispersal process, with microalgae possibly being transported by air? Why do the red blooms appear at this specific time? Are blooms more frequent due to climate change? What will be the impact of these algae on the snowpack in the coming decades?

Satellite monitoring to better understand the origin of blooms

Scientists from various disciplines within the AlpAlga consortium (https://alpalga.fr/) have coordinated their efforts and knowledge to try to answer these questions. Using satellite images captured by Sentinel-2, and applying classification methods to exclude the coloration caused by Saharan dust deposits, they were able to detect the carotenoid pigment astaxanthin, which colors the snow patches, and map the occurrences of intense blooms in the European Alps over five years.

Algal blooms appear between 6,500 and 9,800 feet (2000 and 3000 meters) in elevation, notably in Vanoise, the Swiss Valais, and the Italian Ruitor. Across the alpine massifs, they cover up to 1.3% of the surface above 5,900 feet (1800 meters).


This analysis resulted in the first "atlas" of red snow in the European Alps (https://umap.openstreetmap.fr/fr/map/red-algae-in-alpine-snow_936611#9/45.3454/7.2372).

This research also allowed, through the use of detailed snowpack simulations, the identification of previously unknown, very specific conditions that are most correlated with bloom appearance. Specifically, they appear after sufficiently long melting periods, around 50 days, which give the algae more time to develop in a snowpack saturated with liquid water.

Blooms seem to reoccur year after year in the same areas, reinforcing the hypothesis of persistent reservoirs of microalgae, particularly in the soil. Additionally, the study showed that blooms do not seem to develop on soils that remain frozen year-round or permafrost. This confirms the sensitivity of Sanguina nivaloides to freezing, observed in the laboratory, and supports the hypothesis that microalgae do not find snow to be an extreme environment but rather a thermally stable and protective one.

The impact of blooms on snowmelt

When blooms color the snow, the albedo decreases, leading to an increase in the amount of solar energy absorbed by the snowpack. As a result, the algae accelerate the melting of the snow on which they grow.

Can this phenomenon have a significant impact on the scale of a mountain range? It has been previously shown in the Arctic, particularly in Greenland, that the presence of microalgae significantly contributed to glacier melt. Given the changing climate, scientists have been wondering about the future of these blooms and their impacts, especially in the European Alps.

To explore this, they relied on climate evolution scenarios. Climate projections show that the amount of snow and the average melt durations will decrease at lower altitudes (around 4,900 feet or 1500 meters). However, the projections for mid-elevations (around 8,200 feet or 2500 meters), where the studied algae are found, are more moderate: the snowmelt durations and the amounts of snow will decrease less rapidly than at lower altitudes.

This study, therefore, predicts that the frequency of blooms and their impact on the snow should remain stable or slightly decrease by 2100.

References:
Snowmelt duration controls red algal blooms in the snow of the European Alps.
Proc. Natl. Acad. Sci. USA. 121 (41), L. Roussel, M. Dumont, S. Gascoin, D. Monteiro, M. Bavay, P. Nabat, J. Abdellatif Ezzedine, M. Fructus, M. Lafaysse, S. Morin, E. Maréchal (2024)
PNAS, September 23, 2024, DOI: https://www.pnas.org/doi/10.1073/pnas.2400362121