Europe's most famous alpine lakes are located in the Alps. Lakes in the Alps are often associated with alpine lakes but when the term alpine is based on a biogeographical region division, true alpine lakes are here restricted to mountainous regions above the tree-line. Such alpine lakes are located at various altitudes in Europe, varying from the coastline in northern Norway to above 2000 m in the Alps. Lakes in the alpine region are usually headwater lakes (Figure 7), many of them without human settlements in the catchment and thus normally ultraoligotrophic and pristine in the sense that there is little local anthropogenic disturbance, with two exceptions especially in the Alps: one is the presence of sheep, a tradition that goes back 7000-9000 years, and the other is the atmospheric deposition. Atmospheric deposition includes not only natural dust, for instance of Saharan origin, but also acid rain and snow, and
organic pollutants, such as polychlorinated biphenyls (PCBs), which undergo global distillation and cold trapping, i.e., they accumulate in cold areas such as the alpine regions and the poles. Saharan dust has been identified as an important source of limiting nutrients (phosphorus) in lakes of the southernmost alpine lakes of Europe.
The numerous lakes in the Scandinavian alpine region are less known than the alpine lakes in the Alps, probably because they are very remote from larger human settlements. The lakes in the Scandinavian alpine region have much in common with the alpine lakes in the Alps as they are also mainly ultraoligotrohic and less influenced by local anthropogenic disturbance. However, there are also considerable differences in the biogeochemical recycling, resulting from a colder climate and another geology in the Scandinavian alpine region. In addition, lakes in the Scandinavian alpine region are not affected by sheep but by reindeer herding that has a great influence on the vegetation of many watersheds, with unclear effects on lakes.
Most of the alpine lakes are ice covered during winter. The lakes usually show a well-defined thermal winter stratification and clear thermal summer stratification, making them dimictic. Additionally, the lakes are often deep and nutrient-poor, giving them a romantic crystal clear appearance. Since the lakes in the alpine region are located above the tree-line, where organic soils are thin and export only small amounts of dissolved organic substances, they mostly show low concentrations of humic substances. This special feature makes those lakes very transparent to UV radiation that increases by 10-15% with each kilometer altitude. Major threats to lakes in the alpine region are changes in the climate and in atmospheric deposition. Especially, lakes in crystalline rock basins are often very sensitive to acidification, with natural (preindustrial) alkalinities of much less than 100 mequiv T1.
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