Watermelon Snow

Nature Notes by Dr. Frank Lang

Skiers have probably noticed lemon snow, but what about watermelon snow? In spring and summer, at higher elevations, receding snow banks often take on a pink or reddish tinge. It happens in the bowl on Mount Ashland, near the summit of Mount Eddy west of Mount Shasta, and on the slopes of Mount McLoughlin, to name a few places. You can find red snow in alpine and polar areas worldwide, even Australia, New Zealand and the glaciers of New Guinea. Once you look for watermelon snow you will discover that it is really quite common on high elevation snow banks where small particles of organic debris accumulate and snow begins to melt.

What causes snow to turn color? To find out, pack small vials of red snow in snow, to keep the samples cold, and bring the samples back to the laboratory for microscopic examination. Make a slide, put on a cover slip, and take a look. What you will see are tiny microscopic algal cells, most likely the resting cells of Chlamydomonas nivalis, the snow algae, although it could be one of several other related genera. The single, green, cup-shaped chloroplasts of individual cells are masked by the presence of red pigments. It is the collective presence of these tiny plants that give the snow its color.

These cryophilic, that is, cold-loving algae, carry out their lives and loves in the chilly water-filled spaces among ice crystals in melting snow. It is in these chilly waters that resting cells germinate, producing flagellated cells that swim about, reproducing asexually until motile gametes or asexual resting cells are formed. Gametes unite and zygotic resting cells develop. There is no heat generated by love or sex in these creatures.

Throughout the year, air currents blow the resting cells about, eventually to lodge in soil, trees, and snow. Only in the spring and summer, when melt water is present for at least 24 hours, does enough extensive vegetative growth of algal cells occur to color snow. Nutrients and minerals are leached from dust, conifer needles, and lichen scraps that litter the surface of the snow this time of year. Algae’s own photosynthesis provides for their energy needs and that of other organisms in a snowbound ecosystem. Grazers include snow worms, protozoans, spiders and insects that like it chilly. Nutrients tied up in the bodies of the algae and the grazers are recycled by the action of snow fungi and bacteria.

Next time you are hiking along snow fields in the spring and summer look for watermelon snow. The naturalist Daniel Mathews writes in Cascade Olympic Natural History that some say watermelon snow tastes like watermelon. I know from personal experience that it smells like watermelon. He writes that others warn of diarrhea. Except for pure and white, I avoid eating snow of any color.

— Dr. Frank Lang     

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