Hot Springs: An Introduction


Cold. Spring freshwater environments hold a place in literature, song, and art, serving as a metaphor for a source, a point of origin. Springs have been studied by naturalists, geologists, ecologists, microbiologists, and taxonomists. Springs are defined as water emerging from the ground. The source of that ground water, the depth of the source, the chemistry of the rocks through which the water flows all create unique spring environments. Nearly all springs emerge at temperatures more constant than found in surface water. Cold springs are not cooled by any particular process, rather they simply retain the relatively cold temperatures of the earth during warmer seasons. Deep cool springs are inviting on the hottest of summer days, promising relief from the heat. However, during winter, the water emerges at temperatures warmer than surrounding environment. Springs and spring fed streams rarely freeze during winter. They let off steam, often being mistaken for warm or hot springs.

Hot spring, Yellowstone National Park, observe the microbial mats along the edge of the spring

Hot.  Some springs are heated by subterranean magma or other heat sources and emerge at temperatures much greater than the surrounding ambient water and atmospheric temperatures. These hot or thermal springs are unique to the point of rarity, but are found throughout the world, mostly near fault and tectonic zones and near volcanoes. Thermal spring water may emerge from the ground at temperatures approaching boiling or may emerge at warm temperatures ranging from 35 to 40°C, cool enough for safe contact with the skin.

Fountain Geyser 2
Hot springs surround Fountain Geyser, Yellowstone National Park

Variety. Hot springs are as varied as the ground chemistry and heat sources that define them. Some are acidic, characterized by sulfur fumes and low productivity. Others are basic, characterized by high concentrations of salts and precipitates. Many thermal springs support large colonies of algae and bacteria that mesh together to form extensive mats (See the review by Castenholz 1969). A small set of these microbes survive, even thrive in the highest temperatures.

Yeroogiin Khaluun RashaanA
Yeroogiin Khaluun Rashaan, Mongolia (Photo: John Morse)

Extremophiles. Brock (1985) and others studied the microbes, those thermophilic algae and bacteria, in Yellowstone National Park for at least ten years. They identified several species, extremophiles, living at temperatures in excess of 100°C. One species, Thermus aquaticus, has important uses in biotechnology. The enzyme that strings together nucleotides, the building blocks of DNA, can break apart or denature at high temperatures for most species. Yet, since Thermus aquaticus is adapted to the hottest temperatures on Earth, its DNA polymerase enzyme is not denatured by high temperatures. Researchers use this feature to create copies of DNA from small original concentrations of sample DNA in the process called the Polymerase Chain Reaction or PCR.

Ponca Hot Spring, Colorado, USA

Diversity in the heat. The temperature is greatest at the source of hot springs, but it cools as a function of distance from that source, along the spring runoff. A succession of life is found along this thermal gradient. Species of algae and bacteria occupy niche spaces defined by temperature, chemistry, and competitive exclusion, sometimes in a predictable pattern. Invertebrates also may vary along the thermal gradient, sometimes moving up or down the runoff as the temperatures change to exploit the best temperature ranges for the species, their thermal optimum.

Firehole River
Firehole River, Yellowstone National Park. This thermally influenced river exhibits a thermal gradient.

Of pools and parks. When I first started studying hot springs, I envisioned the clear, colorful springs of Yellowstone National Park. In fact, what I found when I went searching for hot springs were pools crowded with happy people soaking in the heat and watching the golden sun set over mountain peaks. Most hot springs are not protected in North America, rather they have been modified as pools or spas. Some thermal water is harnessed and sent by pipes to hot springs swimming pools. The majority of research has focused on those few springs on conservation lands. Microbes have been studied extensively in Yellowstone National Park while thermophilic invertebrates have been studied in hot springs in Iceland, Japan, and New Zealand.  However, few studies of thermal springs have consisted of surveys of large geographic areas. Such studies could answer whether thermophilic species of blue green algae or cyanobacteria are truly cosmopolitan with only a few species found at the extreme limits of life worldwide.  Or the research could find new species of invertebrates.

Future topics. The world of thermal springs is fascinating and beautiful. Future topics will include microbial mats, thermal Diptera, and the geophysical environment.

Spring source, Ponca Hot Springs

Further Readings

Brock, T.D., 1985. Life at high temperatures. Science230(4722), pp.132-138.

Castenholz, R.W., 1969. Thermophilic blue-green algae and the thermal environment. Bacteriological Reviews33(4), p.476.

Lamberti, G.A. and Resh, V.H., 1985. Distribution of benthic algae and macroinvertebrates along a thermal stream gradient. Hydrobiologia128(1), pp.13-21.

Hayford, H. and Herrmann, S.J., 1998. Migration patterns of four macroinvertebrates along a rheocrene thermal spring. Studies in crenobiology: the biology of springs and springbrooks. Backhuys Publishers, Leiden7584.

Polymerase Chain Reaction.

Pielou, E.C. 2000. Freshwater. University of Chicago Press. 256 pp.

Tüxen, S.L., 1944. The hot springs of Iceland. Zoology of Iceland.

Pritchard, G., 1991. Insects in thermal springs. The Memoirs of the Entomological Society of Canada123(S155), pp.89-106.

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