High in alpine and polar regions, snowfields can take on a reddish or pink hue, a phenomenon often called “watermelon snow.” This coloration isn’t a mineral deposit or a trick of the light, but a massive bloom of a microscopic, cold-loving freshwater alga. The most common species responsible for this display is Chlamydomonas nivalis, a type of green alga that thrives in freezing temperatures.
The Science of Survival in the Cold
Red snow algae have a two-stage life cycle allowing them to flourish in harsh conditions. In the spring and early summer, as meltwater becomes available, the algae exist as small, green, motile cells with flagella, allowing them to swim through the snowpack to find optimal light and temperature. This green stage is focused on growth and asexual reproduction, utilizing chlorophyll for photosynthesis much like other plants.
As conditions change, the algae transition into a dormant, non-motile cyst stage to survive the winter. During this phase, the cells produce a secondary pigment called astaxanthin, a type of carotenoid. This pigment masks the green chlorophyll and gives the cells, and thus the snow, their red color.
The astaxanthin pigment serves a dual purpose for survival. It acts as a protective shield, absorbing intense ultraviolet (UV) radiation that is prevalent at high altitudes. The dark red pigment also absorbs solar radiation and heat, which warms the snow immediately surrounding the cell, providing the alga with the liquid water it needs to remain viable.
Global Distribution and Appearance
Red snow algae are found on every continent, from the polar expanses of the Arctic and Antarctica to high-altitude mountain ranges. It is commonly sighted in the Sierra Nevada, the European Alps, the Himalayas, and the Rocky Mountains. These blooms appear in the late spring and summer months.
The emergence of watermelon snow depends on several environmental factors. The algae require temperatures warm enough to create some snowmelt, providing the liquid water for their life processes, but the snowpack must still be present to serve as their habitat. The blooms often appear as patches and streaks, concentrated in meltwater channels and sun cups, where both water and sunlight are readily available.
Ecological Impact
Within its frozen habitat, red snow algae form the base of a food web. They are a primary food source for other microscopic organisms, including ice worms, rotifers, and springtails. These creatures, in turn, can become food for larger organisms like spiders and birds.
The most significant ecological effect of these algal blooms is their influence on snowmelt. Clean, white snow has a high albedo, reflecting a large portion of incoming solar radiation. The red pigmentation from the algae darkens the snow surface, decreasing its albedo and causing it to absorb more solar energy, which accelerates the rate at which snow and glaciers melt.
This phenomenon can create a feedback loop where accelerated melting provides more liquid water, encouraging further algal growth and leading to even faster melting. This process has broader implications for glacial retreat and water resource management.
Safety and Human Interaction
For those who encounter these colorful snowfields, a common question is whether the snow is safe to consume. The algae itself, Chlamydomonas nivalis, is not considered toxic.
However, it is recommended to avoid eating “watermelon snow.” Ingesting the algae can act as a laxative and may cause digestive issues like diarrhea.
The meltwater where the algae thrive can also host other bacteria and microorganisms unsafe for consumption. While the snow may have a faintly sweet or fruity scent, which gives it its name, it is best appreciated visually. It is perfectly safe to walk through, though the pigments can stain clothing.