Pond atCornjum,NetherlandsA man made pond at sunset in Montgomery County, Ohio.Stereoscopic image of a pond in Central City Park, Macon, GA,c. 1877.
Apond is a small, still, land-basedbody of water formed by pooling inside adepression, either naturally orartificially. A pond is smaller than alake[1] and there are no official criteria distinguishing the two, although defining a pond to be less than 5 hectares (12 acres) in area, less than 5 metres (16 ft) in depth and with less than 30% of its area covered byemergent vegetation helps in distinguishing theecology of ponds from those of lakes andwetlands.[2][3]: 460 Ponds can be created by a wide variety of natural processes (e.g. onfloodplains as cutoffriver channels, by glacial processes, by peatland formation, in coastal dune systems, bybeavers), or they can simply be isolated depressions (such as akettle hole,vernal pool,prairie pothole, or simply natural undulations in undrained land) filled by runoff, groundwater, or precipitation, or all three of these.[4] They can be further divided into four zones: vegetation zone, open water, bottom mud and surface film.[3]: 160–163 The size and depth of ponds often varies greatly with the time of year; many ponds are produced by spring flooding from rivers. Ponds are usuallyfreshwater but may bebrackish in nature.Saltwater pools, with a direct connection to the sea to maintain full salinity, may sometimes be called 'ponds' but these are normally regarded as part of the marine environment. They do not support fresh or brackish water-based organisms, and are rathertidal pools orlagoons.
Ponds are typically shallow water bodies with varying abundances ofaquatic plants and animals. Depth, seasonal water level variations, nutrient fluxes, amount of light reaching the ponds, the shape, the presence of visiting large mammals, the composition of anyfish communities and salinity can all affect the types of plant and animal communities present.[5]Food webs are based both on free-floatingalgae and upon aquatic plants. There is usually a diverse array of aquatic life, with a few examples including algae,snails, fish,beetles,water bugs,frogs,turtles,otters, andmuskrats. Top predators may include large fish,herons, oralligators. Since fish are a major predator upon amphibian larvae, ponds that dry up each year, thereby killing resident fish, provide important refugia for amphibian breeding.[5] Ponds that dry up completely each year are often known asvernal pools. Some ponds are produced by animal activity, including alligator holes andbeaver ponds, and these add important diversity to landscapes.[5]
Ponds are frequently man made or expanded beyond their original depths and bounds byanthropogenic causes. Apart from their role as highly biodiverse, fundamentally natural, freshwater ecosystems ponds have had, and still have, many uses, including providingwater for agriculture, livestock and communities, aiding in habitat restoration, serving as breeding grounds for local and migrating species, decorative components oflandscape architecture,flood control basins, general urbanization, interception basins for pollutants and sources and sinks ofgreenhouse gases.
The technical distinction between a pond and a lake has not been universally standardized.Limnologists andfreshwater biologists have proposed formal definitions forpond, in part to include 'bodies of water where light penetrates to the bottom of the waterbody', 'bodies of water shallow enough for rooted water plants to grow throughout', and 'bodies of water which lack wave action on the shoreline'. Each of these definitions are difficult to measure or verify in practice and are of limited practical use, and are mostly not now used. Accordingly, some organizations and researchers have settled on technical definitions ofpond andlake that rely on size alone.[6]
Some regions of the United States define a pond as a body of water with a surface area of less than 10 acres (4.0 ha).Minnesota, known as the "land of 10,000 lakes", is commonly said to distinguish lakes from ponds, bogs and other water features by this definition,[7] but also says that a lake is distinguished primarily by wave action reaching the shore.[8] Even among organizations and researchers who distinguish lakes from ponds by size alone, there is no universally recognized standard for the maximum size of a pond. The internationalRamsar wetland convention sets the upper limit for pond size as 8 hectares (80,000 m2; 20 acres).[9] Researchers for the British charity Pond Conservation (now called Freshwater Habitats Trust) have defined apond to be 'a man-made or natural waterbody that is between 1 m2 (0.00010 hectares; 0.00025 acres) and 20,000 m2 (2.0 hectares; 4.9 acres) in area, which holds water for four months of the year or more.' Other European biologists have set the upper size limit at 5 hectares (50,000 m2; 12 acres).[10]
In North America, even larger bodies of water have been called ponds; for example,Crystal Lake at 33 acres (130,000 m2; 13 ha),Walden Pond inConcord, Massachusetts at 61 acres (250,000 m2; 25 ha), and nearbySpot Pond at 340 acres (140 ha). There are numerous examples in other states, where bodies of water less than 10 acres (40,000 m2; 4.0 ha) are being called lakes. As the case of Crystal Lake shows,marketing purposes can sometimes be the driving factor behind the categorization.[11]
In practice, a body of water is called a pond or a lake on an individual basis, as conventions change from place to place and over time. In origin, a pond is a variant form of the word pound, meaning a confining enclosure.[12] In earlier times, ponds were artificial and utilitarian, asstew ponds,mill ponds and so on. The significance of this feature seems, in some cases, to have been lost when the word was carried abroad with emigrants. However, some parts of New England contain "ponds" that are actually the size of a small lake when compared to other countries. In the United States, natural pools are often called ponds. Ponds for a specific purpose keep the adjective, such as "stock pond", used for watering livestock. The term is also used for temporary accumulation of water fromsurface runoff (ponded water).
There are various regional names for naturally occurring ponds. In Scotland, one of the terms islochan, which may also apply to a large body of water such as a lake. In the South Western parts of North American, lakes or ponds that are temporary and often dried up for most parts of the year are calledplayas.[13] These playas are simply shallow depressions in dry areas that may only fill with water on certain occasion like excess local drainage, groundwater seeping, or rain.
Pond formation through seeping groundwater in South Tufa, California
Anydepression in the ground which collects and retains a sufficient amount of water can be considered a pond, and such, can be formed by a variety ofgeological,ecological, and humanterraforming events.
Ornamental pond withwaterfall in Niagara Falls Rock Garden
Natural ponds are those caused by environmental occurrences. These can vary from glacial, volcanic, fluvial, or even tectonic events. Since the Pleistocene epoch, glacial processes have created most of the Northern hemispheric ponds; an example is thePrairie Pothole Region of North America.[14][15] When glaciers retreat, they may leave behind uneven ground due to bedrockelastic rebound and sediment outwash plains.[16] These areas may develop depressions that can fill up with excess precipitation or seeping ground water, forming a small pond. Kettle lakes and ponds are formed when ice breaks off from a larger glacier, is eventually buried by the surrounding glacial till, and over time melts.[17]Orogenies and other tectonic uplifting events have created some of the oldest lakes and ponds on the globe. These indentions have the tendency to quickly fill with groundwater if they occur below the local water table. Other tectonic rifts or depressions can fill with precipitation, local mountain runoff, or be fed by mountain streams.[18] Volcanic activity can also lead to lake and pond formation through collapsed lava tubes or volcanic cones. Naturalfloodplains along rivers, as well as landscapes that contain many depressions, may experience spring/rainy seasonflooding and snow melt. Temporary or vernal ponds are created this way and are important for breeding fish, insects, and amphibians, particularly in large river systems like theAmazon.[19] Some ponds are solely created by animals species such asbeavers,bison,alligators and othercrocodilians through damning and nest excavation respectively.[20][21] In landscapes withorganic soils, local fires can create depressions during periods of drought. These have the tendency to fill up with small amounts of precipitation until normal water levels return, turning these isolated ponds into open water.[22]
Manmade ponds are those created by human intervention for the sake of the local environment, industrial settings, or for recreational/ornamental use.
Many ecosystems are linked by water and ponds have been found to hold a greater biodiversity of species than larger freshwater lakes or river systems.[23] As such, ponds are habitats for many varieties of organisms including plants,amphibians, fish,reptiles,waterfowl,insects, and even somemammals. Ponds are used for breeding grounds for these species but also as shelter and even drinking/feeding locations for other wildlife.[24][25] Aquaculture practices lean heavily on artificial ponds in order to grow and care for many different type of fish either for human consumption, research, species conservation or recreational sport.
In agriculture practices,treatment ponds can be created to reduce nutrient runoff from reaching local streams or groundwater storages. Pollutants that enter ponds can often be mitigated by natural sedimentation and other biological and chemical activities within the water. As such, waste stabilization ponds are becoming popular low-cost methods for general wastewater treatment. They may also provideirrigationreservoirs for struggling farms during times of drought.
A Retention pond guarded by concrete wall and surrounded by Taro plants in an urban area
As urbanization continues to spread,retention ponds are becoming more common in new housing developments. These ponds reduce the risk of flooding and erosion damage from excess storm water runoff in local communities.[26]
Experimental ponds are used to test hypotheses in the fields of environmental science, chemistry, aquatic biology, and limnology.[27]
Some ponds are the life blood of many small villages in arid countries such as those in sub-Saharan Africa where bathing, sanitation, fishing, socialization, and rituals are held.[28] In theIndian subcontinent,Hindu temple monks care for sacred ponds used for religious practices and bathing pilgrims alike.[29] In Europe duringmedieval times, it was typical for manymonastery andcastles (small, partly self-sufficient communities) to havefish ponds. These are still common in Europe and inEast Asia (notably Japan), wherekoi may be kept or raised.
In Nepal artificial ponds were essential elements of theancient drinking water supply system. These ponds were fed with rainwater, water coming in throughcanals, their own springs, or a combination of these sources. They were designed to retain the water, while at the same time letting some water seep away to feed the localaquifers.[30]
Azalea flowers around a still pond in London's Richmond Park
A defining feature of a pond is the presence ofstanding water which provideshabitat for a biological community commonly referred to aspond life. Because of this, many ponds and lakes contain large numbers of endemic species that have gone through adaptive radiation to become specialized to their preferred habitat.[18] Familiar examples might includewater lilies and other aquatic plants,frogs,turtles, and fish.
Apumpkinseed, a common species ofsunfish found in ponds in North America
Often, the entire margin of the pond is fringed bywetland, and these wetlands support the aquaticfood web, provide shelter for wildlife, and stabilize the shore of the pond. This margin is also known as the littoral zone and contains much of the photosynthetic algae and plants of this ecosystem calledmacrophytes. Other photosynthetic organisms such asphytoplankton (suspended algae) andperiphytons (organisms includingcyanobacteria,detritus, and othermicrobes) thrive here and stand as the primary producers of pond food webs.[18] Some grazing animals likegeese andmuskrats consume the wetland plants directly as a source of food. In many other cases, pond plants will decay in the water. Many invertebrates and herbivorous zooplankton then feed on the decaying plants, and these lower trophic level organisms provide food for wetland species including fish,dragonflies, and herons both in the littoral zone and the limnetic zone.[18] The open water limnetic zone may allowalgae to grow as sunlight still penetrates here. These algae may support yet another food web that includes aquatic insects and other small fish species. A pond, therefore, may have combinations of three different food webs, one based on larger plants, one based upon decayed plants, and one based upon algae and their specific upper trophic level consumers and predators.[18] Hence, ponds often have many different animal species using the wide array of food sources though biotic interaction. They, therefore, provide an important source ofbiological diversity in landscapes.
Opposite to long standing ponds arevernal ponds. These ponds dry up for part of the year and are so called because they are typically at their peak depth in the spring (the meaning of "vernal" comes form the Latin word forspring). Naturally occurring vernal ponds do not usually have fish, a major higher tropic level consumer, as these ponds frequently dry up. The absence of fish is a very important characteristic of these ponds since it prevents long chained biotic interactions from establishing. Ponds without these competitive predation pressures provides breeding locations and safe havens for endangered or migrating species. Hence, introducing fish to a pond can have seriously detrimental consequences. In some parts of the world, such as California, the vernal ponds have rare and endangered plant species. On the coastal plain, they provide habitat for endangered frogs such as theMississippi Gopher Frog.[20]
Often groups of ponds in a given landscape - so called 'pondscapes' - offer especially high biodiversity benefits compared to single ponds. A group of ponds provides a higher degree of habitat complexity and habitat connectivity.[31][32]
Lakes are stratified into three separate sections: I. TheEpilimnion II. TheMetalimnion III. TheHypolimnion. The scales are used to associate each section of the stratification to their corresponding depths and temperatures. The arrow is used to show the movement of wind over the surface of the water which initiates the turnover in the epilimnion and the hypolimnion.
Many ponds undergo a regular yearly process in the same matter as larger lakes if they are deep enough and/or protected from the wind. Abiotic factors such as UV radiation, general temperature, wind speed, water density, and even size, all have important roles to play when it comes to the seasonal effects on lakes and ponds.[33] Spring overturn, summer stratification, autumn turnover, and an inverse winter stratification, ponds adjust their stratification or their vertical zonation of temperature due to these influences. These environmental factors affect pond circulation and temperature gradients within the water itself producing distant layers; theepilimnion,metalimnion, andhypolimnion.[18]
A pond in winter experiencing inverse stratification
Each zone has varied traits that sustain or harm specific organisms and biotic interactions below the surface depending on the season. Winter surface ice begins to melt in the Spring. This allows the water column to begin mixing thanks to solar convection and wind velocity. As the pond mixes, an overall constant temperature is reached. As temperatures increase through the summer, thermal stratification takes place. Summer stratification allows for the epilimnion to be mixed by winds, keeping a consistent warm temperature throughout this zone. Here,photosynthesis andprimary production flourishes. However, those species that need cooler water with higher dissolved oxygen concentrations will favor the lower metalimnion or hypolimnion. Air temperature drops as fall approaches and a deep mixing layer occurs. Autumn turnover results inisothermal lakes with high levels of dissolved oxygen as the water reaches an average colder temperature. Finally, winter stratification occurs inversely to summer stratification as surface ice begins to form yet again. This ice cover remains until solar radiation and convection return in the spring.
Due to this constant change in vertical zonation, seasonal stratification causes habitats to grow and shrink accordingly. Certain species are bound to these distinct layers of the water column where they can thrive and survive with the best efficiency possible.
For more information regarding seasonal thermal stratification of ponds and lakes, please look at "Lake Stratification".
Ponds provide not only environmental values, but practical benefits to society. One increasingly crucial benefit that ponds provide is their ability to act as greenhouse gas sinks. Most natural lakes and ponds are greenhouse gas sources and aid in theflux of these dissolved compounds. However, manmade farm ponds are becoming significant sinks for gasmitigation and the fight againstclimate change.[34] These agriculture runoff ponds receive highpH level water from surrounding soils. Highly acidic drainage ponds act as catalysis for excessCO2 (carbon dioxide) to be converted into forms of carbon that can easily be stored in sediments.[35] When these new drainage ponds are constructed, concentrations of bacteria that normally break down dead organic matter, such as algae, are low. As a result, breakdown and release of nitrogen gases from these organic materials such asN2O does not occur and thus, not added to our atmosphere.[36] This process is also used with regulardenitrification inanoxic layer of ponds. However, not all ponds have the ability to becomesinks forgreenhouse gasses. Most ponds experienceeutrophication where faced with excessive nutrient input from fertilizers and runoff. This over-nitrifies the pond water and results in massalgae blooms and localfish kills.
Some farm ponds are not used for runoff control but rather for livestock likecattle or buffalo as watering and bathing holes. As mentioned in the use section, ponds are important hotspots for biodiversity. Sometimes this becomes an issue with invasive or introduced species that disrupt pond ecosystem dynamics such asfood-web structure,niche partitioning, and guild assignments.[37] This varies from introduced fish species such as theCommon Carp that eat native water plants or NorthernSnakeheads that attack breeding amphibians, aquatic snails that carry infectious parasites that kill other species, and even rapid spreading aquatic plants likeHydrilla andDuckweed that can restrict water flow and cause overbank flooding.[37]
Ponds, depending on their orientation and size, can spread their wetland habitats into the local riparian zones or watershed boundaries. Gentle slopes of land into ponds provides an expanse of habitat for wetland plants andwet meadows to expand beyond the limitation of the pond.[38] However, the construction of retaining walls, lawns, and other urbanized developments can severely degrade the range of pond habitats and the longevity of the pond itself. Roads and highways act in the same manor, but they also interfere with amphibians and turtles that migrate to and from ponds as part of their annual breeding cycle and should be kept as far away from established ponds as possible.[39] Because of these factors, gently sloping shorelines with broad expanses of wetland plants not only provide the best conditions for wildlife, but they help protect water quality from sources in the surrounding landscapes. It is also beneficial to allow water levels to fall each year during drier periods in order to re-establish these gentile shorelines.[39]
In landscapes where ponds are artificially constructed, they are done so to provide wildlife viewing and conservation opportunities, to treat wastewater, for sequestration and pollution containment, or for simply aesthetic purposes. For natural pond conservation and development, one way to stimulate this is with general stream and river restoration. Many small rivers and streams feed into or from local ponds within the same watershed. When these rivers and streams flood and begin to meander, large numbers of natural ponds, includingvernal pools andwetlands, develop.[40]
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