The Algae and Its Protective Pigment
The striking pink or red coloration of “watermelon snow” is caused by a species of cold-loving microalgae, most commonly identified as Chlamydomonas nivalis. This single-celled organism is a type of green alga that thrives in the harsh, sub-freezing environment of snow and ice. During the winter and early spring, the algae exist in a motile, green phase, using flagella to swim in the thin film of meltwater surrounding snow crystals. As summer approaches, challenging environmental conditions trigger the algae to transform into a dormant, resting stage known as a cyst.
This shift to the cyst stage produces the characteristic red color. The algae synthesize and accumulate a large quantity of a secondary carotenoid pigment called astaxanthin. Although the primary green pigment, chlorophyll, is still present, the intense red color of the astaxanthin completely masks it, turning the snow a deep pink, red, or dark orange hue. Astaxanthin serves a purpose analogous to sunscreen, absorbing the intense ultraviolet (UV) radiation that penetrates the snowpack at high altitudes and latitudes.
The pigment acts as a photoprotectant, shielding the cell’s internal machinery, particularly the chloroplasts, from damage caused by high light intensity. By accumulating this red pigment, the dormant cells can survive until the next season, even when exposed on the snow surface. The sheer density of these microscopic, red-pigmented cells, potentially reaching millions per milliliter of meltwater, is what makes the coloration visible across vast snowfields.
Geographic Distribution and Habitat
Watermelon snow is a cosmopolitan phenomenon, appearing across the globe in environments characterized by perennial snow cover. It is regularly sighted in both high-altitude alpine regions, such as the Rocky Mountains and the Himalayas, and in high-latitude polar environments, including the Arctic and the Antarctic Peninsula. The organism that causes the bloom, C. nivalis, is adapted to survive in extremely cold conditions.
The algae require a specific set of environmental parameters to initiate a significant bloom. Sufficient liquid water is a requirement, which is why the phenomenon is most commonly observed during the summer melt season when snow surfaces are wet. The presence of meltwater allows the motile cells to move, reproduce, and facilitates nutrient exchange.
The temperature must hover just around the freezing point, as the algae’s metabolic processes are optimized for these cold conditions. The combination of liquid water and high light intensity, typical of summer on a snowfield, are the two primary factors that trigger the mass transition into the protective red cyst stage. This allows the algae to establish colonies in snow depths ranging from the surface down to about 20 centimeters.
Impact on Snow Melt
The presence of red-pigmented snow algae has a measurable effect on the physical properties of the snowpack. Fresh, white snow has a very high albedo, which is the measure of how much solar radiation it reflects back into space. Pristine snow can reflect up to 90% of incoming sunlight, keeping the surface cool.
When the algae bloom and darken the snow surface, the albedo is substantially reduced. This darker, colored surface absorbs significantly more solar energy. The absorbed energy is converted into heat, causing the snow around the algal cells to warm and melt at an accelerated rate.
This process creates a positive feedback loop, often referred to as the “bio-albedo effect.” The melting snow provides more liquid water, which stimulates further growth and reproduction of the algae, leading to an even darker surface. Studies indicate that these pigmented cells can reduce the albedo of a snowfield by an average of 13% over a melt season, contributing to localized snow and glacier mass loss.
Safety and Taste
The name “watermelon snow” is derived from its color and the mild, fruity scent that emanates from a dense bloom. This faint aroma, sometimes described as sweet or like fresh squash, is caused by volatile organic compounds produced by the algae. This scent, however, does not indicate that the snow is safe for human consumption.
While the C. nivalis organism itself is not considered toxic, consuming the colored snow is discouraged. Ingesting large quantities can have a laxative effect, leading to digestive discomfort. Also, the algae often accumulate contaminants and pollutants from the environment, such as wind-blown dust, bacteria, and heavy metals, all of which are concentrated in the meltwater.