Geothermal springs

the unusual ecology of a unique habitat

This essay takes me rather far afield from my normal trails, but there are so many fascinating aspects to these springs, and so many things still to be discovered, that I could not resist trying to write about them.

Geothermal sites are scattered over North America, with concentrations in the mountainous west. Water from rain or snowmelt sinks far underground, where the very high temperatures near the center of the earth heat the water. When that water rises to the surface through weak spots in the rocks, its temperature ranges from tepid to very hot. Depending on the amount of water emerging, the spring may be a geyser (if water is under strong pressure), or a warm pool, a seep or mud puddle, or just some steam. Some springs have notable concentrations of dissolved minerals, such as sulfur and salts.

Alaska has its share of geothermal sites—at least 79 of them. Most lie on a line from the Seward Peninsula eastward or on an arc from the Aleutians over to Southeast. Southeastern Alaska is well-endowed with hot springs, whose temperatures range up to about 180 degrees.

Who lives in hot springs? Of course that depends on the temperature (as well as water chemistry and the availability of oxygen); in general, the higher the temperature, the fewer the kinds of things that can live there. If the water is merely warm, there might be algae, some small crustaceans or molluscs, along with some beetles, true bugs, or flies—even a dragonfly! A certain kind of fly (in the genus Ephydra) likes it pretty hot: up to about a hundred and thirteen degrees F. Some hot-springs critters are such specialized ‘thermophiles” (meaning heat-loving) that they live only at higher temperatures. Unfortunately, there appear to be no studies of hot-spring faunas in Alaska, although Ephydra flies have been found in one of them. An obligately thermophilic water-mite is recorded from northern B.C., and I wonder if it might occur in Alaska too.

Few vertebrates can tolerate the conditions in thermal springs, but in North America, some little fish in desert pools have become quite famous—and perilously endangered. These are two species of pupfish (of the widespread genus Cyprinodon). The desert pupfish is less than three inches long, living in springs that may get as hot as a hundred and ten degrees F., reportedly feeding on snails that also must tolerate such temperatures. The Devil’s Hole pupfish is even smaller, about one and a half inches long; it’s found in only one pool in a Nevada cavern at about ninety three degrees F., and there are very few of them left now.

The only organisms known to thrive in extremely hot springs, with temperatures over a hundred and eighty degrees F, are certain bacteria and some similar-looking micro-organisms known as Archaea. These all have metabolisms that are very different from other organisms—their enzymes obviously have to be very heat-tolerant.

Thermal springs influence the vegetation that grows around them, in part because the annual frost-free period is longer. Species such as cow parsnip can grow taller than usual. A study in central Alaska near a hot spring (temperature about a hundred forty-one degrees F) discovered three species (a grass, a fern, and a violet) living around the spring hundreds of kilometers north of their usual geographic range. (That, of course, leaves open the question of how they got there…). Soil temperatures near a spring are often warmer, maybe allowing better growth of species (such as white spruce) whose roots don’t do well in cold soil.

Geothermal springs across the continents seem to attract more public attention as places for people to bathe than as subjects for scientific study. In contrast, the geothermal vents in the deep ocean have made headlines for their rich diversity in exceptionally extreme conditions.

Deep-sea hydrothermal vents are found principally in places where tectonic plates are separating and the sea floor is spreading as new material arises from deep in the earth. They are sometimes also found where two tectonic plates collide and one plate is pushed beneath the other, creating a deep trench. Underwater volcanos create some hot vents too. Some (not all) of these hydrothermal vents spew forth water at phenomenally high temperatures—up to more than seven hundred degrees F; at the great pressures of the ocean deeps, that water doesn’t boil. As the emerging hot water meets the extremely cold water of the deep sea, minerals are precipitated out, sometimes creating tall chimneys around the vent itself.

Nothing lives at those extremely hot temperatures inside the vents, but around the vents and chimneys there is often a very rich community of organisms. The base of the food chain there is made up of bacteria and archaea that metabolize sulfur instead of oxygen (which is scarce down there). An assortment of small crustaceans, shrimp, snails, mussels, limpets, tube worms, clams, and no doubt other things can be found on the warm sides of the chimneys. Farther up the food chain, crabs of several types are predators on the others. Certain species of fish called eelpouts are known from some vent systems, where they feed on tubeworms, crabs and other crustaceans. A tiny octopus (Vulcanoctopus) in the Pacific feeds from the sides of vent chimneys on swarms of amphipods.

In 2015, a deep-sea exploration near the Galapagos discovered a cluster of egg cases of the Pacific white skate near a geothermal vent in an area where the temperature was distinctly warmer than the ambient thirty-seven degrees F. Scientists thought that perhaps the warmer temperatures would speed up the normally very slow (over four years!) embryonic development time (although faster development might require more oxygen, which is limited in supply there). We don’t, apparently, know the fates of those eggs and embryos. A cautionary note is provided, however, by observations of a certain octopus on a deep-sea lava outcrop off Costa Rica. This octopus likes to lay its eggs in the lava crevices, where the mothers tend the eggs until they hatch. But on the geothermally warm parts of the outcrop, the embryos did not develop and the mothers looked stressed, leading the scientists to wonder if all the good, cool incubation sites on the lava were already occupied, leaving no room for the stressed, failed mothers, who had to use what space was left.

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