- Origins of lakes
- Aging of lakes
- Salinity, wind, temperature, and light
- Water circulation
- Lake threats
Lakes are inland bodies of freshwater. They range in size from small bodies of water that can be entirely navigated using a rowboat to the Great Lakes—the connected lakes that form the largest collection of freshwater on Earth.
Lakes are plentiful —millions of them are found over Earth’s surface. Lakes are classified on the basis of origin, age, salinity, fertility, and water circulation. They can be formed by glaciers, tectonic plate movements, river and wind currents, and even by volcanic activity or in the aftermath of a meteorite impact.
Lakes can also be a phase of evolution in the aging process of a bay or estuary. Some lakes are only seasonal, drying up during parts of the year. As a lake changes over time, it can become a marsh, bog, or swamp. Part of the temporal change has to do with the composition of the lake water. Young lakes have clear water with less organic matter, while older lakes have murkier water and higher levels of organic matter as well as nitrogen, phosphorous, and detritus (decaying material).
Another aspect of a lake’s composition is salinity. Salinity is a measure of the dissolved ionic components in lake water. High salinity lakes, that is, lakes whose salt content is elevated (although no to the level of saltwater), have high levels of precipitates and less organic matter, whereas lower salinity lakes have fewer precipitates and more organic matter.
Lake shape, climate, and salinity each effect water movement within a lake, contributing to an individual lake’s annual circulation patterns. Most lakes exchange surface water with bottom water at least once during the year, but multiple factors influence
this complex process. Life within a lake is also determined by multiple factors. The study of fresh water, including lakes and ponds, is called limnology.
Origins of lakes
In northern latitudes, most lakes were formed as a result of glacier activity. Earth’s glacial ice formed and extended into what is now Canada, the northernmost United States, and northern Europe. As the heavy, thick ice pushed along, it scoured out topsoil, creating crevices in the former landscape. Glacial growth peaked about 20,000 years ago, after which time the ice slowly began to melt. As the ice melted and the glaciers retreated, basins formed by glaciers remained. These filled with water from the melting glaciers. Lake basins formed at the edge of glaciers were generally not as deep as basins underneath glaciers. The shallower lakes are called ice-block or depression lakes; the lakes formed under glaciers (some more than 1 mile or 1.6 kilometers high) are called ice-scour lakes.
Movements of earth, water, and wind can also form lakes. Rock deformations of Earth’s crust occur as folds, tilts, or sinking, usually along fault lines. Depressions created can fill with water, forming lakes such as Lake Baikal in Siberia. It may seem peculiar to state that water forms lakes also, but water currents and land erosion by water form specific types of lakes: oxbow and solution lakes. Oxbow lakes are created as old and winding rivers change course and establish new channels that isolate the bending portions. An example is the Mississippi River, which contains an oxbow lake called Lake Whittington. Solution lakes result from ground water eroding the bedrock above it, creating a sinkhole. Sinkholes are the predominant type in Florida and on the Yucatan Peninsula. Wind can also create lake basins called blowouts; such lakes usually occur in coastal or arid areas. Blowouts created by sand shifted in arid regions are typical of lakes in northern Texas, New Mexico, southern Africa, and parts of Australia.
Less commonly, lakes can form due to the impact of a meteorite or because of volcanic activity. Gases at high pressure under crests of volcanic lava can explode, forming basins that collect water. Volcanic basins up to 1 mile (1.6 kilometer) in diameter are called craters, and those with greater diameters are called calderas. Crater Lake in Oregon is a caldera of Page 2436 | Top of Articlegreat depth; indeed, it the seventh deepest lake in the world. The largest well-documented meteorite-formed lake in the world is Chubb Lake in Quebec. Lake Chubb is 823 feet (250 meters) deep inside a crater 10,990 feet (3,350 meters) wide.
Lakes can be created when a watercourse is blocked. Natural dams can be made by streams or beavers. Humans also deliberately dam a watercourse; the controlled flow of the lake water through turbines positioned in the dam can be used to generate electricity.
Aging of lakes
Lake formation and aging are natural periods in the lifespan of a lake. Some lakes have a short lifespan of 100-1,000 years, although many lakes will exist for 10,000 years or longer, but there are lakes that only exist in damper seasons of the year. As water tends to support life, lakes are often assessed based on their fertility—the life they can and do support. The deposits in lake basins have layers (strata) that reveal details about a lake’s history. At any point in time, a lake’s fertility is related to water stratification by regions of similar temperature and light penetration.
Fertility is governed by a number of biological and chemical factors. The photosynthetic plankton that grow on a lake’s surface are eaten by zooplankton; these plankton make up the primary link in the lake’s food chain. Photoplankton contribute to a lake’s fertility as a food source and as an oxygen source through photosynthesis. Plankton are consumed by aquatic invertebrates which are, in turn, eaten by small and larger fish.
Minerals such as phosphorous and oxygen are also required for life to flourish. Oxygen concentration is primarily due to photosynthesis in lake plants and surface wind agitation. Some oxygen can also come from streams that empty into the lake.
Lakes are classified as oligotrophic, mesotrophic, or eutrophic depending on age and whether they have little, some, or a lot of life, respectively. Oligotrophic lakes are the youngest and are typically the least fertile lakes; they tend to be deep with sparse aquatic vegetation and few fish. Mesotrophic lakes are middle-aged lakes that are less deep and more fertile than oligotrophic lakes. Eutrophic lakes (the oldest lakes) are most fertile and even more shallow than mesotrophic lakes. Eutrophic lakes eventually reach the point where demand for oxygen exceeds the oxygen supply. Eutrophic lakes have many aquatic life forms that eventually die and decompose; decomposition uses up oxygen that could have supported additional life. Decomposing material collects on the lake bottom, which decreases the depth of the water body. As oxygen becomes scarcer, less life is able to exist.
Salinity, wind, temperature, and light
One can focus on almost any characteristic of water in a lake and see that the particular factor influences and is influenced by other characteristics of the same water. A profile of any given lake must take several of these factors into account. For example, salinity and temperature are two factors that affect life in opposing ways. High salinity does not favor most life other than some algal and shrimp growth. An example is the Dead Sea; this lake has a salinity seven times that of seawater, which makes it very inhospitable to most life (although salt-loving bacteria thrive). Temperature effects fertility both directly and indirectly. Most fish species prefer certain temperatures. While largemouth bass flourish at 75°F (24°C), trout prefer 50°F (10°C), for example. Indirectly, temperature affects fertility by playing a large part in determining oxygen capacity of water. Warmer water holds less oxygen than colder water.
Salinity remains relatively constant in some lakes, while it tends to increase significantly in others. A lake that has outflow, such as a runoff stream, keeps within a normal salinity range for that lake. But lakes that have no runoff lose water over time to evaporation, and a higher salinity results.
Sunlight can only penetrate water to a limited depth. Both murky and choppy water decrease light penetrance. Submerged regions receiving light throughout are called euphotic. Since light is required by plants for photosynthesis, which produces oxygen, cloudy water generally has less oxygen. However, plants vary in how much light they require for growth. Some aquatic plants, such as hydrilla, can grow on the lake’s littoral zone (part of the lake that slopes from the shore toward the benthos) 50 ft (15 m) under clear water. Other plants, like cattails, maiden cane, wild rice, and lily pads grow in 3 ft (1 m) or less of water closer to shore.
Water circulation is the mixing of water in a lake. Water mixes at the surface, within the top layer (epilimnion) and among layers. The bottom layer of water is called the hypolimnion, and the water between the hypolimnion and epilimnion makes up the metalimnion. The metalimnion is also called the thermocline, because a drastic temperature change occurs the lower one goes in it. Mixing is facilitated by wind at the epilimnion and is possible due to water density Page 2437 | Top of Articlevariation between layers. When layers mix and change places, a lake is said to turn over. Turnover occurs when water in an upper layer is heavier, or denser than the layer of water underneath it. Lakes that turn over once a year are described as being monomictic. Lakes that turn over twice a year, once in spring and once in fall, are dimictic. Lakes that turn over at least once a year are holomictic. Some lakes do not fully turn over at all due to high salt content; the high salt lower layer prevents hypolimnic turnover in these meromictic lakes.
The most controlling factors in lake circulation are changes and differences in water temperature; however, salinity, wind, and lake shape each have a role in circulation as well. Bowl-shaped lakes tend to turn over more easily than oxbow lakes. Water temperature determines water density, which, in turn, accounts for turnover. Water is at its minimum density in the form of ice. Warmer water is less dense than cooler water until cold water reaches 39.2°F (4°C), when it gets lighter. Deeper water is generally both denser and colder than shallower water—other than ice.
Tremendous variability exists in turnover patterns and date of onset. Polar lakes warm later in spring and cool sooner in fall than similar lakes in tropical regions. Ice may only melt away from some lakes for two months a year, resulting in slow fish growth compared to warmer climates. High altitude lakes also warm later and cool sooner than equivalent low altitude lakes. Tropical, high altitude lakes lose heat continuously, do not develop layers, and overturn continually, whereas sub-tropical, low altitude lakes that never freeze only layer in summer and turn over in winter.
Aside from the natural aging process, major threats to the longevity of lake fertility include pollution (including acid rain), eutrophication, and shoreline overdevelopment. Acid rain is formed by sulfates and nitrates emitted from coal-burning industries and automobile exhaust pipes. These chemicals combine with moisture and sunlight and are converted into sulfuric and nitric acids that enter lakes via precipitation. Acid rain has a pH of 4.5, contrasting with the normal rain pH value of 5.6. Since a single digit pH difference represents a 10-fold change in acidity (the pH scale is logarithmic), acid rain is more than 10 times more acidic than normal rain. Freshwater life generally prefers alkaline (above pH 7) conditions, but lake fertility is usually fairly functional down to a pH
Eutrophic—Older, fertile lakes with thick basin deposits and several life forms.
Mesotrophic—Middle-aged lakes with intermediate levels of basin deposits and numbers of life forms.
Oligotrophic—Young lakes with the least amount of basin detritus and numbers of life forms.
Turnover—The mixing and flip-flopping of thermal layers within a lake that results in nutrient mixing within the lake.
of 6.0. However, when pH drops to 5.0 and below, as the effects of acid rain accumulate, life forms are severely effected. Plants, plankton, insects, and fish all gradually disappear. Young and old organisms die first, followed by the young and middle-aged adults. Many bacteria even die. Other chemical pollutants include fertilizers and pesticides that drain into lakes through soil and enter through streams. Pesticides are toxic to fish, while fertilizers can cause eutrophication.
Eutrophication is the abundance of nutrients for fertile growth. It is a natural phenomenon in mature lakes. However, chemical pollutants, including phosphorous and nitrogen compounds, can artificially propel lakes to this state where the demand by aquatic animals on lake oxygen is great. Human-made eutrophication threatens to deplete lake oxygen which can kill most of a lake’s fish. Some eutrophigenic lakes are now aerated by man to increase available oxygen.
Shore overdevelopment disrupts natural habitats and increases pollution. Shorelines that are built up with dirt to support construction of buildings can crush wet, rocky areas that some lake species use for spawning. In addition, shoreline plant life is sometimes removed to create sandy, recreational areas, and the influx of people usually increases pollution.
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