Splendid Isolation in Thingvallavatn
Campbell, David G., Natural History
Our planet has been graced by life for at least three and a half billion years. Today, most places have a long and convoluted history of interactions with living things. Even the frozen interior of Antarctica hides a medley of fossils. But the earth is not entirely without newborn places. One is in Iceland, where the Mid-Atlantic Ridge rises above the surface of the sea and traverses the country from south to north, splitting the land asunder. Here, where two tectonic plates pull away from each another, magma is extruded through the earth's crust, pushed up through an uneasy suture between the plates. Iceland's rifting activity produce some of the youngest rocks on earth, a few millennia--in some places, just a few years--old. (Millions of years from now, these flows will be swallowed by subsidence, as one tectonic plate slides beneath another, in a world that will have oceans and continents diferent in shape from those of today and that, undoubtedly, will have long forgotten humans.)
The centerpiece of this youthful land is Thingvallavatn in southwestern Iceland. This clear and deep lake was born ten thousand years ago in a valley gouged from volcanic rock and ash by Langjoekull Glacier, which today has retreated twenty-four miles to the north. Thingvallavatn--Iceland's largest lake--is one of the most thoroughly studied bodies of water on earth, thanks to a monumental effort by a team of more than a dozen scientists of various disciplines, led by Petur Jonasson, of the Freshwater Biology Laboratory, University of Copenhagen. These researchers have found that as a result of its youth, its isolation on an island away from the continents, and its high latitude, Thingvallavatn is impoverished in animal species. It supports only three species of fish: three-spined stickleback, brown trout, and arctic char. Cut off from the ocean, it was never colonized by Atlantic salmon or by European and American eels (whose ranges overlap elsewhere in Iceland). Other vertebrates that in varying degrees depend on the lake include the common loon, the red-breasted merganser, the arctic tern, and an introduced species, the mink.
Young ecosystems like Thingvallavatn are the delight of ecologists because their simplicity helps elucidate fundamental principles of ecology and evolution. The chars of Thingvallavatn have been of special interest to evolutionary biologists because in only a few millennia, they have diversified from a common ancestor into a splendid array of four varieties, or morphs; all, however, are still members of the same species. (Icelanders have long recognized at least two morphs. Fish-eating chars were sought by the Vikings, who used baited hooks on hand lines dropped through holes cut in the ice. By the eighteenth century, Icelanders were using woolen nets to capture the murta, a plankton-eating variety of char whose pink, firm flesh tastes to me of fresh-ground walnuts.)
Arctic char (Salvelinus alpinus) is a notoriously plastic species. Northern lakes in Canada and Europe commonly have two or three varieties of this relative of trout and salmon, but Thingvallavatn is the only lake to support four. To understand why the chars of Thingvallavatn have diversified to such a degree, we must first understand this rift lake's short, but dynamic, history, which distinguishes it from most ofits counterparts in the Arctic.
Thingvallavatn is a flooded graben, a piece of the earth's crust stuck in the middle of a fault zone. As the rift widens, the graben sinks. Today the bottom of the lake subsides an average of four or five millimeters a year. Since its formation, four separate volcanic eruptions have flowed into its basin, resulting in a variety of terrains: a shallow, tapering shoreline, tiered shelves of lava on its flanks, and a relatively flat bottom strewn with boulders. Much of the lake's shoreline consists of lava, with stones and boulders strewn on top, although there are a few sandy beaches and pockets of wetlands. …