Lake

Lakes are inland bodies of water—millions of which are scattered over the earth's surface. Lakes are classified on the basis of origin, age, salinity, fertility, and water circulation. Lakes can be formed by glaciers , tectonic plate movements, river and wind currents, and volcanic or meteorite activity. 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 reaches old age, it can become a marsh, bog, or swamp. 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 or decaying matter. Salinity is a measure of the dissolved ionic components in lake water. High salinity lakes, salt lakes, have high levels of precipitates and less organic matter, whereas freshwater or low 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 any given lake is determined by multiple factors as well and is of considerable interest to fishermen and marine biologists. Lakes are used for several purposes other than for the food they contribute to the food chain: they are used for recreation and enhance scenic beauty. The study of fresh water, including lakes and ponds, is called limnology.

Origins

Although several geological phenomenon account for the formation of numerous lakes on Earth , 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 top soil , 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, the glaciers retreated, but the basins formed by glaciers remained and 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 mi [1.6 km] high) are called ice-scour lakes.

Movements of earth, water, and wind can also form lakes. Rock deformations of the 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 windy rivers change course. Windy rivers such as the Mississippi meander, carrying water through twists and turns; when they change direction at a particular twist or turn in the river, a loop can become separate from the main water flow. As deposits build up and separate the loop from the river, an oxbow lake such as Lake Whittington in Mississippi forms. 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 .

A few lakes result from meteors or volcanic activity. Gases at high pressure under crests of volcanic lava can explode, forming basins that collect water. Volcanic basins up to 1 mi (1.6 km) in diameter are called craters, and those with diameters greater than 1 mi (1.6 km) are called calderas. Crater Lake in Oregon is a caldera 1,932 ft (590 m) deep, 20 ft (6 m) in length, and 16 ft (5 m) in width. This makes 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 ft (250 m) deep inside a crater 10,990 ft (3,350 m) wide.

Dams made by streams, beavers , and humans have also created lakes. Natural dams can be formed as a stream deposits debris at the point that it enters a river; the accumulated material can close off the stream, creating a lake. Man-made lakes have many characteristics in common with natural lakes, although water level can be less consistent in man-made lakes.

Age

Lake formation (or birth) and evolution (or aging) are natural periods of lake existence as they are for all living things. 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. Because people who study lakes have considerably shorter life spans, the chemical, physical, and life-supporting properties of water are used to classify lake age. As water tends to support life, lakes are often assessed based on what life they can and do support: their fertility. The deposits in lake basins have strata , or layers, that reveal details about a lake's history. And a lake's present 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. Photo plankton 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. Phosphorous levels can vary over a range of parts per billion (ppb). Most fish require an oxygen concentration of at least 5 parts per million (ppm). Oxygen concentration is primarily due to photosynthesis in lake plants and surface wind agitation. Some oxygen can also come from tributary streams.

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, usually, 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. And 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, detritus, collects on the lake's benthos (basin bottom), making the lake shallower. As oxygen becomes sparse, lakes approach senescence, full maturity to death.

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 seriously inhibit or promote life. High salinity does not favor most life other than some algal and shrimp growth. The Dead Sea, which has a salinity seven times that of seawater, has very low fertility. 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). Indirectly, temperature affects fertility by playing a large part in determining oxygen capacity of water. Warmer water holds less oxygen than colder water. Water at 45°F (7°C) can hold up to 12 ppm of oxygen; while 75°F (24°C) water can only hold a maximum of 8.5 ppm of oxygen.

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.

Wind plays an important role in water circulation, wave action, and surface temperature. On a warm, windy day, the surface water may be considerably warmer than the water beneath it. Since wind is directional and effects surface water more readily than lower water, it pushes warm water in the direction of the wind. The result can be a dramatic temperature difference between opposing shores of the same lake. The downwind shore may be as much as 30°F (16.7°C) warmer than the upwind shore. Strong winds can also create choppy waves that effectively decrease light penetrance.

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 , maidencane, wild rice , and lily pads grow in 3 ft (1 m) or less of water closer to shore.

Water circulation

Water circulation is the mixing of water in a lake. Water mixes at the surface, within the top layer, the 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 variation 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 said to be monomictic. Lakes that turn over twice a year, once in spring and once in fall, are called dimictic. Lakes that turn over at least once a year are called 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.

In fall, the surface is cooled in proximity to the surrounding air. As this surface water cools, it sinks, mixing throughout the epilimnion. The epilimnion continues to cool and eventually matches the metalimnion in temperature. Wind mixes these two water layers, which then cool to temperatures lower than the hypolimnion temperature. Then the hypolimnion water mixes in with the rest of the water and rises to surface. If hypolimnion water was oxygen-depleted, then it will obtain more oxygen at the surface during the winter. During winter, the hypolimnion is warmer than the epilimnion-unless the entire lake freezes. This process is called fall turnover.

In spring, the ice warms, melts, and mixes within the epilimnion. As the entire epilimnion warms, it becomes denser than the hypolimnion, the whole lake turns over, and mixing takes place. This is spring turnover. As summer progresses, the metalimnion warms and the three temperature layers are apparent until fall. If snow has piled high onto the surface over the winter and blocked photosynthesis, then much lake life may die, resulting in a phenomenon called winterkill.

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.

Lake threats

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 acid that enter lakes via precipitation . Acid rain has a pH of four and half, contrasting with the normal rain pH value of 5.6. Since a single digit pH difference (say, from eight to nine) represents a 10-fold change in acidity, acid rain is more than 10 times more acidic than normal rain. Freshwater life generally prefers alkaline (basic, non-acidic) conditions, but lake fertility is usually fairly functional down to a pH of six. However, when pH drops to five and below, as the effects of acidrain 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. Man-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.

See also Ecosystem; Ice ages.

Resources

books

Cvancara, A. At the Water's Edge. New York: John Wiley & Sons, 1989.

Sternberg, D. Fishing Natural. Lakes. R. R. Donnelley & Sons Company, 1991.

Louise Dickerson

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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.

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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.