A living sunscreen
In the high-altitude snowfields of the Greater Chimgan mountains, patches of snow often take on a pink or reddish hue, a phenomenon sometimes called "watermelon snow". This coloration is not a mineral stain but a bloom of microscopic, cold-loving algae. The most common species responsible for this red snow is Chlamydomonas nivalis, a type of single-celled green alga that thrives in near-freezing water temperatures between 0°C and 10°C.
These organisms have a complex life cycle adapted to the harsh alpine environment. During the warmer, high-sunlight months of late spring and summer, the algae enter a dormant cyst stage. In this phase, they produce massive quantities of a secondary carotenoid pigment called astaxanthin to shield their chlorophyll and DNA from intense ultraviolet (UV) radiation. This red-orange pigment is so concentrated that it masks the green of the chlorophyll, giving the cells—and the snow they inhabit—their characteristic red color. Large blooms can reach cell densities of 100,000 to 1,000,000 cells per milliliter of melted snow.
Radar interference and melt acceleration
The astaxanthin pigment is an important protective function for the algae. It strongly absorbs light in the blue-green and ultraviolet parts of the spectrum, with peak absorbance generally between 475 and 495 nanometers. This absorption of solar energy, however, has a secondary consequence. By darkening the snow's surface, the algae reduce its ability to reflect sunlight, an effect known as lowering the albedo. This causes the snow to absorb more heat, accelerating melt rates. Some studies have linked algal blooms to as much as an 11 percent increase in the loss of glacier ice and snow.
This absorption of electromagnetic radiation also creates problems for scientists studying glaciers. Radar altimetry is an important technique used to measure the thickness of snow and ice sheets. Satellites and aircraft bounce radar waves off the surface and sub-surface layers to determine their depth. The presence of liquid water and biological impurities—like dense blooms of snow algae—can scatter and absorb the radar signal. The pigments and associated liquid water interfere with the radar's ability to penetrate the snowpack, leading to inaccurate measurements of glacier mass balance. This effect complicates efforts to monitor the health of these critical freshwater resources.