One of the many remarkable properties of water is the unwillingness of bodies of water of substantially different temperature to mix together. This property is responsible for the formation in fresh-water lakes of a phenomenon known as the thermocline, a phenomenon that can play an important role in a lake's summertime ecology. Consider the annual temperature fluctuations in a typical deep-water impoundment in the southern United States. In late winter, usually sometime in February or March, whatever ice may have previously formed at the surface of the lake melts, and the water temperature measures a uniform 38-42 degrees. Wave action stirs oxygen into the water at the lake's surface, and the temperature uniformity allows distribution of this dissolved oxygen to all depths. With oxygen plentiful, many of the reservoir's fish species, both predator and forage, are found throughout the water column. The windy, sunny days of early spring quickly warm the lake's surface. As the surface temperature increases, that water expands. Because 50-degree water is lighter than 40-degree water, a layer of warmer water builds at the surface of the lake, resting like a pillow on the mass of colder water below. The pillow of warm surface water slowly increases in thickness, as heat is transferred into the depths by the limited stirring of wave action. By early summer, a remarkable stratification has occurred. A sharp boundary separates two independent bodies of water within the lake. The boundary is a temperature gradient called the thermocline, and it acts as a barricade to prevent any further mixing of oxygen into the chilly depths. The depth of the thermocline fluctuates with air temperatures and the prevailing winds. On July 1, the water temperature might be 86 degrees at the surface, 84 degrees at 10 feet, and 82 degrees at 20 feet―but at 24 feet, the temperature has plunged to 65 degrees.   When winter ended, the depths were well-oxygenated and populated with fish. But as the thermocline set up in late spring, the supply of oxygen to the depths was abruptly shut off. In summer, the temperature barrier prevents oxygen from circulating down-ward from the surface to replace the oxygen consumed by fish and dying zooplankton. In order to survive, fish are forced upward into the relatively narrow zone between the thermocline and the surface. The cold nights of autumn reverse the trend, as the surface cools to the point that it is heavier than the water below the thermocline. This initiates a process known as the turnover: A current of richly oxygenated water plunges to the bottom of the reservoir, forcing stagnant water back to the surface. The lake reaches equilibrium by early winter and remains there until the process repeats itself the following spring. there　= equilibrium(平衡・均衡状態) itself　=　 the process（冬の終わりに氷が解けて、波が酸素を湖全体に行き渡らせ、春には温かい水と冷たい水で上下に分かれる層を形成して、秋には表面の冷えた水が下に下がって下の暖かい水は上に上がって、冬の始まり頃には氷が張って平衡状態になる。で、春にはまた氷が解けて…同じプロセスが繰り返される。 )