Lakes are most abundant in high northern latitudes and in mountain regions, particularly those that were covered by glaciers in recent geologic times. The primary sources of lake water are melting ice and snow, springs, rivers, runoff from the land surface, and direct precipitation. In the upper part of lakes there is a good supply of light, heat, oxygen, and nutrients, well distributed by currents and turbulence. As a result, a large number of diverse aquatic organisms can be found there. The most abundant forms are plankton (chiefly diatoms), algae, and flagellates. In the lower levels and in the sediments, the main forms of life are bacteria. See also limnology. Heavy Water
Water from most natural sources contains about 0.015% deuterium oxide; this can be enriched or purified by distillation, electrolysis, or chemical processing. Heavy water is used as a moderator in nuclear power plants, slowing down the fast neutrons so that they can react with the fuel in the reactor. Heavy water is also used in research as an isotopic tracer for chemical reactions and biochemical pathways. Water with tritium (T2O) rather than deuterium may also be called heavy water.
Coastal lagoons have low to moderate tides and constitute about 13% of the world's coastline. Their water is colder than the sea in winter and warmer in summer. In warm regions, evaporation may more than balance any freshwater input and may result in hypersaline water and even the buildup of thick salt deposits. Coral-reef lagoons occur on marginal reefs such as the Great Barrier Reef, but the most spectacular examples, some more than 30 mi (50 km) across, are associated with Pacific atolls.
U.S. senator (2005– ).
Obama graduated from Columbia University (1983) and Harvard Law School (1991), where he was the first African American to serve as president of the Harvard Law Review. He moved to Chicago, where he served as a community...
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