2011م - 1444هـ
نبذه عن الكتاب:



the concentration of ions—charged atoms or

molecules—present in water. Certain lakes, such as

Great Salt Lake in Utah (U.S.) and the Dead Sea in the

Middle East, have salinity levels higher than that of the

world’s oceans. The majority of lakes, however, have

ion concentrations so low that salinity level becomes

negligible. Soluble salts in the surrounding rock, which

gradually erodes and flows into lakes via streams and

rivers, determine salinity. Many other inorganic compounds such as various plant nutrients (e.g., phosphates

and nitrates), heavy metals (e.g., mercury), and polychlorinated hydrocarbons (e.g., DDT) may also find

their way into a lake by these means, although rainfall

and wind-borne air particulates are culprit methods of

addition as well.

Numerous interacting variables influence the life

cycle of a lake, among them temperature variants within

lake strata and the extent to which light penetrates into

the deeper waters. Solar radiation, inflows and outflows

of water, and the effects of wind cause temperature

change; currents and wave action each directly affects

the temperature map of a lake. These temperature zones

within lakes are further affected by clastic sediments

(clay, silt, sand), organic debris, and chemical precipitates. In general, lakes undergo heat transfers vertically,

which causes mixing of the waters to the extent that

thermocline, or heat-gradient, regions will “flip.” For

example, the uppermost lake waters, characterized by

temperatures near freezing (0 °C [32 °F]) in the early

spring, absorb heat as the weather warms. The density

of the water in these upper layers will increase until the

water temperature exceeds (4 °C [39.2 °F]). If the density

of the upper layers of the lake is greater than that of the

lower layers, the water in the upper layers sinks, and the

lake water overturns.