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A pond is a body of standing water, either natural or artificial, that is usually smaller than a lake.〔http://www.merriam-webster.com/dictionary/pond〕 They may arise naturally in floodplains as part of a river system, or they may be somewhat isolated depressions (examples include vernal pools and prairie potholes). Usually they contain shallow water with marsh and aquatic plants and animals. A few animals also make their home in ponds, including both alligators and beavers. The type of life in a pond is generally determined by a combination of factors including water level regime (particularly depth and duration of flooding) and nutrient levels, but other factors may also be important, including presence or absence of shading by trees, presence or absence of streams, effects of grazing animals, and salinity. Ponds are frequently human-constructed. In country side farmers and villagers dig a pond in their backyard or increase the depth of an existing pond by removing layers of mud during summer season. A wide variety of artificial bodies of water are classified as ponds. Some ponds are created specifically for habitat restoration, including water treatment. Others, like water gardens, water features and koi ponds are designed for aesthetic ornamentation as landscape or architectural features. Fish ponds are designed for commercial fish breeding, and solar ponds designed to store thermal energy. Standing bodies of water such as puddles, ponds, and lakes are often categorized separately from flowing water courses, such as a brook, creek, stream or river. Nutrient levels and water quality in ponds can be controlled through natural process such as algal growth, or through artificial filtration, such as an algae scrubber. ==Technical definitions== The technical distinction between a pond and a lake has not been universally standardized. Limnologists and freshwater biologists have proposed formal definitions for ''pond'', 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 has met with resistance or disapproval, as the defining characteristics are each difficult to measure or verify. Accordingly, some organizations and researchers have settled on technical definitions of ''pond'' and ''lake'' which rely on size alone. Even among organizations and researchers who distinguish lakes from ponds by size alone, there is no universally recognised standard for the maximum size of a pond. The international Ramsar wetland convention sets the upper limit for pond size as ,〔(【引用サイトリンク】title=Information Sheet on Ramsar Wetlands (RIS) )〕 but biologists have not universally adopted this convention. Researchers for the British charity Pond Conservation have defined a ''pond'' to be 'a man-made or natural waterbody which is between 1 m2 and 20,000 m2 in area (2 ha or ~5 acres), which holds water for four months of the year or more.'〔 Other European biologists have set the upper size limit at . 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 North America, even larger bodies of water have been called ponds; for example, Walden Pond in Concord, Massachusetts measures , nearby Spot Pond is , while in between is Crystal Lake at . There are numerous examples in other states of bodies of water less than being called lakes. As the case with Crystal Lake shows, marketing purposes may be the driving factor behind some names.〔(【引用サイトリンク】title=Newton Park and Conservation Lands: Crystal Lake )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「pond」の詳細全文を読む スポンサード リンク
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