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Scientific Research, Sierra Nevada Red Snow


WILLIAM H. Thomas, Scripps Institution of Oceanography1

Author’s note: I am very grateful to the Gilkey Research Fund of the American Alpine Club, the Society of Sigma Xi, the University of California Water Resources Center and the Southern California Edison Company for supporting this research. I also thank many mountaineering friends for aiding in the field work and supplying distributional information via questionnaires. A detailed account of this work will appear in the Journal of Phycology.

Often a mountaineer reaches the point where he wishes he could spend all of his time in the hills. Out of such feelings expeditions, guide services, and mountaineering and ski schools are born. Sometimes the scientist-mountaineer reaches this point too, so he yearns to practice good science in the mountains. My professional experience and training are in biological oceanography with emphasis on the physiology and ecology of marine algae, and such science has led me away from the hills. Oceanographic cruises are similar to mountaineering expeditions in their logistical problems, but the environment is different.

In the Sierra Nevada I often noticed red snowfields, which are caused by the growth of algae in the snow, and I jokingly remarked to my climbing friends it would be interesting to apply oceanographic techniques to the study of these algae. Just as a climber looks up at a peak and formulates routes in his mind, so there were scientifically interesting questions about snow algae that stimulated my curiosity: What was their distribution; what factors affect their growth; how rapidly do they photosynthesize; how do they survive and live in such a harsh environment? It was as easy to formulate questions as it is to map out a climbing route from below; making a route go and obtaining answers to such questions are more difficult.

The study of snow algae is of practical interest. It was reported that the watermelon-scented snow containing algae had violent laxative properties. Increase in snowfield nutrients by air pollution might increase the abundance of algae and wash them into streams and lakes so water supplies would become toxic. Abundant snow algal blooms might change the melting characteristics of snow so that runoff and its prediction would be affected.

Much was known about the taxonomy and general distribution of snow algae, but answers to my ecological questions were few. Kol listed only eight occurrences of snow algae in California, and obviously we could learn more about their distribution. Only Fogg had measured photosynthesis in a snowfield.

First, I needed to know where best to find algae for intensive field work. Also the geographical distribution would be interesting in itself. To supplement my own travels, I enlisted the aid of mountaineers, forest rangers, and park rangers to report occurrences of colored snow via questionnaires.

Most reports of colored (nearly always red) snow were from the Sierra Nevada, but there were a few occurrences in Southern California and high desert ranges: Telescope Peak and the White Mountains. Colored snow was found most often from 10,000 to 12,000 feet although the range was from 4000 to 14,000 feet. The accompanying map shows the distribution in the High Sierra. There seemed to be no difference in distribution between 1969, a year of very heavy snowfall, and 1970, a year of less than normal snowfall, and algae were generally distributed along the high crests. Color was always found in old, wet snowfields that persisted into the summer, never in dryer winter snowfields. The color was generally faint and occurred in footsteps (where the algae are concentrated by crushing) or in suncups. Sometimes in the late season the color was continuous in given snowfields, but more often the color was found as patches in depressions.

One of my correspondents reported he had seen colored snow for the last 30 years near the Carnegie Experimental Station southeast of Mount Conness in the Tioga Pass area of the Sierra. A trip there in October, 1968, suggested this site as ideal for field work on the quantitative abundance and photosynthesis of snow algae. The site was readily accessible, and a cabin with electric power for microscope and instruments was located on the Tioga road. The Southern California Edison Company made the cabin available for field work in 1969 and 1970. Several mountaineering friends assisted me in the work: my wife Sara Thomas, Jerry and Ann Hooper, Peter Young, Phil and Gretchen Bettler, Walter Moffitt and Paul Seramur.

Our first trips were in July and August 1969. We backpacked laboratory equipment — flasks and bottles, a hand vacuum pump, light meter, pH meter, snow corers, etc. — into the site and set up a field laboratory on bare ground near north-facing snowfields containing reddish patches of algae. Quantitative measurements of chlorophyll and numbers of algal cells demonstrated this patchiness very well. Questionnaire observations suggested that algae were most abundant in the upper few centimeters of snow. Our quantitative measurements substantiated this impression, but algae were also found at depths ranging down to 10 cm. Algae may be concentrated near the snow surface by ablation, but light intensities optimum for photosynthesis may only occur near the surface. Light measurements showed great attenuation of intensity by the snow cover, but suggested that photosynthesis could occur at depths as great as 50 cm. Later measurements verified this suggestion.

During the July trip we attempted to stimulate algal growth by spraying a snowfield with nutrient solutions. However, when we returned in August, our marking wands had been disrupted by downslope snow creep. All we were able to show was that the snowfield, which was not obviously red, contained a considerable amount of chlorophyll. This suggested that any persistent Sierra snowfield, like any lake or stream, would contain algae even though it was not colored.

We sampled greenish-brown material on the frozen snow-covered surface of Green Treble Lake. On the ice I was well belayed in case of a cold bath. The samples from the lake consisted of pine pollen, not algae.

Red snow in the Tioga Pass area contained entirely red, spherical cells of the alga Chlamydomonas nivalis. No other algae were observed, and this lack of diversity contrasts strongly with studies by Garric in the Cascades and in Colorado by Stein and Amundsen where many more species were found.

Summer minimum air temperatures at Tioga Pass were generally above freezing, and the algae develop when there is a high water content in the snow and no alternate freezing and thawing.

In May and June, 1970, we sampled the early season algae. Patches were few and far between; on a ski tour to our 1969 site, we saw red patches only about every kilometer, but at the exact site of our 1969 work, algae were again developing.

A nutrient enrichment experiment in 1970 was successful, yielding a tenfold increase of chlorophyll in a few days. In August, 1970, the snowfield we had sprayed in 1969 was continuously red and stained our hands and feet. I do not think this was due to nutrient addition the year before; rather, algae develop in the same places from year to year.

In both years, we measured photosynthesis by exposing snow samples to radioactive carbon dioxide, filtering off the cells after incubation in snowfields, and then measuring the amount of cellular radioactivity in the laboratory. From this, I calculated the increase in cell carbon due to photosynthesis. Algae kept in the dark took up a smaller fraction of the radioactivity than that taken up by illuminated algae, so photosynthesis occurred. Photosynthesis was inhibited when snow was melted only slightly. In unmelted snow, the process was particularly intense, suggesting that the growth of the algae, like that of land plants, was carbon dioxide-limited.

Near melting snowbanks, reddish crusts of algae may appear on rocks or soil once covered by snow, and we found traces in soil. The algae must overwinter in soil and be available to grow up again the next season when conditions are proper. Since algae preceding the red cell stage are motile, cells can swim up through waterlogged snow to where light intensities are favorable.

Pollack has reported the presence of various bacteria and protozoa in snow. The protozoa may feed on algae, and algae may supply food for mosquito and other insect larvae in snow. In the Sierra, the apex predator of this simple food chain is probably the rosy finch, which feeds upon insects in snow and is commonly observed around high altitude snowfields.

Further research could include studies of laboratory physiological requirements, and we could follow algal development in the field continuously over a whole season. We need to test algal toxicity with laboratory animals. The effects of algae on runoff also need to be further investigated.


Fogg, G. E. 1967. “Observations on snow algae of the South Orkney Islands.” Phil. Trans. Roy. Soc. London B252: 279-287.

Garric, R. K. 1965. “The cryoflora of the Pacific Northwest.” Amer. J. Botany 52: 1-8.

Kol, E. 1964. “Cryobiological research in the Rocky Mountains.” Arch. Hydrobiol. 60: 278-285.

Pollack, R. 1970. “What colors the mountain snow?” Sierra Club Bull. 55, No. 4: 18-20.

Stein, J. R. and C. C. Amundsen. 1967. “Studies on snow algae and fungi from the Front Range of Colorado.” Canadian J. Botany 45: 2033-2045.

1Institute of Marine Resources, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California.