Many tundra and taiga aquatic habitats freeze solid during the long winter. Surface ice may form temporarily at any time during the warm season, but this event occurs more commonly at higher latitudes and higher altitudes. Obviously, there is a gradient from north to south with regard to temperature and light exposure. Winters are longest, coldest, and darkest, and the growing season is the shortest in the High Arctic. Continuous (or nearly so) summer sunshine partially compensates for the short summer everywhere in both biomes. While these climatic characteristics are generally applicable across the tundra and taiga, they are not uniformly applicable. This is because local variations in topography, hydrology, soil type, snow accumulations, protection from winds, and favorable exposure to sunshine all work together to produce microclimates and microhabitats where plants and animals can flourish to a far greater degree than would be possible in nearby but less favorable situations. As plants and animals flourish in these microhabitats, they tend to further alter the local conditions, usually in the direction of making them even more favorable for living things.
Were it not for permafrost in both tundra and taiga, which largely prevents downward movement of water, the tundra would be a desert (since it gets only around 250 mm total precipitation annually) and the taiga, with only about 250-500 mm total precipitation annually, would be clothed in xerophytes (actually, to a large extent, the taiga is clothed in xerophytes: members of the pine family are generally more resistant to drought—and to cold—than are many species of deciduous trees).
In the early days of mineral and petroleum exploration in the North, it was common practice during the warm season to deploy survey crews in heavy, usually tracked, vehicles. Evidence of those journeys may often be seen even now in the form of two parallel lines of puddles stretched out across the land. When the weight of these vehicles compressed the surface layer of peat moss, the insulation efficiency of the moss diminished, resulting in shallow melting of the underlying permafrost. Scars formed in this manner usually take decades to heal. Eventually, however, as is the case with most tundra and taiga ponds, these thermokarst depressions begin to be filled in with growing plants, dead plants and animals, and wind-blown mineral and plant debris. Such materials gradually displace the water and very slowly build up to the point where they begin to have some insulating properties. With the overlying insulating materials restored to some extent, the permafrost, which always reformed beneath the scar in winter, during summer now begins to persist longer and longer with each successive season until the underlying ground remains permanently frozen, a condition roughly comparable to its original state.
Chemical and botanical studies of a series of High Arctic ponds (81.49° N 70.18° W) on Ellesmere Island, Canada, revealed that the water is hard, rich in dissolved solids, and alkaline (pH 6.8-8.9), and that the ponds are populated by several kinds of vascular and primitive plants (e.g., moss, Drepanocladus brev-ifolius; cottongrass, Eriophorum spp.; and water sedge, Carex aquatilis) that are either aquatic or tolerant of water-saturated substrates.
The vast majority of the millions of tundra and taiga ponds are rather small in size, many being no more than puddles, often in thermokarst depressions or patterned ground such as polygons. There are, however, many thousands of large and deep lakes. Lake Baikal in southeastern Siberia, the most famous of the taiga lakes (for its great depth, biological diversity, and numerous endemic species), covers 31,500 km2 and is 1741 m deep. The seventh largest lake in the United States is Alaska's Lake Iliamna—2590 km2. Many other very large lakes are spread across the circumpolar North.
Many lakes are indispensable as spawning and nursery areas for salmon (Oncorhynchus spp.) and serve as nesting areas for many kinds of migratory waterfowl. Some of the more accessible lakes have been damaged by industrial pollution and to a lesser extent by recreational housing, boating, and fishing. Within their range, moose (Alces alces) utilize the aquatic plants associated with ponds and lakes of all sizes.
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