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is a 
karst
Karst () is a topography formed from the dissolution of soluble carbonate rocks such as limestone and Dolomite (rock), dolomite. It is characterized by features like poljes above and drainage systems with sinkholes and caves underground. Ther ...
pond on Minamidaitōjima
, also spelt as Minami Daitō or Minami-Daitō, is the largest island in the Daitō Islands group southeast of Okinawa, Japan. It is administered as part of the village of Minamidaitō, Okinawa. Shimajiri District, Okinawa and has a populat ...
, Okinawa Prefecture
is the southernmost and westernmost prefecture of Japan. It consists of three main island groups—the Okinawa Islands, the Sakishima Islands, and the Daitō Islands—spread across a maritime zone approximately 1,000 kilometers east to west an ...
, Japan. Ōike is the largest pond containing both freshwater and seawater. It covers , the circumference is , and it is at its deepest point. Fresh water is mixed with seawater beneath.
Geology
The island is composed of elevatedlimestone
Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
from its atoll
An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical parts of the oceans and seas where corals can develop. Most ...
. Rainwater has dissolved the limestone leading to a depression in the ground (sinkhole
A sinkhole is a depression or hole in the ground caused by some form of collapse of the surface layer. The term is sometimes used to refer to doline, enclosed depressions that are also known as shakeholes, and to openings where surface water ...
). This has filled with water to become a pond.
Natural history
On the north of the pond extends an area of wetland with large communities of Bruguera gymnorrhiza, amangrove
A mangrove is a shrub or tree that grows mainly in coastal saline water, saline or brackish water. Mangroves grow in an equatorial climate, typically along coastlines and tidal rivers. They have particular adaptations to take in extra oxygen a ...
plant, which was designated as a Natural monument
A natural monument is a natural or cultural feature of outstanding or unique value because of its inherent rarity, representative of aesthetic qualities, or cultural significance. They can be natural geological and geographical features such as w ...
since it is unique in freshwater on March 18, 1975. Other plants include ''Scirpus tabemaemontani C. C. Gemlin, Panicum repens L. Najas marina'' and ''Vallisneria natans''.
Common carp
The common carp (''Cyprinus carpio''), also known as European carp, Eurasian carp, or simply carp, is a widespread freshwater fish of eutrophic waters in lakes and large rivers in Europe and Asia.Fishbase''Cyprinus carpio'' Linnaeus, 1758/ref>Ark ...
and '' Carassius auratus langsdorfii'' were introduced in 1910 and tilapia
Tilapia ( ) is the common name for nearly a hundred species of cichlid fish from the coelotilapine, coptodonine, heterotilapine, oreochromine, pelmatolapiine, and tilapiine tribes (formerly all were "Tilapiini"), with the economically mos ...
in 1955. Birds include mallard
The mallard () or wild duck (''Anas platyrhynchos'') is a dabbling duck that breeds throughout the temperate and subtropical Americas, Eurasia, and North Africa. It has been introduced to New Zealand, Australia, Peru, Brazil, Uruguay, Arge ...
, Pacific reef heron, common moorhen
The common moorhen (''Gallinula chloropus''), also known as the waterhen, is a bird species in the Rail (bird), rail family (Rallidae). It is distributed across many parts of the Old World, across Africa, Europe, and Asia. It lives around well-ve ...
and cattle egret
The cattle egret (formerly genus ''Bubulcus'') is a cosmopolitan distribution, cosmopolitan clade of heron (family (biology), family Ardeidae) in the genus ''Ardea (genus), Ardea'' found in the tropics, subtropics, and warm-temperate zones. Ac ...
.Environmental Agency''The 4th Reports on Japanese Lakes and Ponds- Chugoku, Shikoku, Kyushu and Okinawa'' 1993
Footnotes
References
*Ministry of the Environment, ''Environment of lakes and ponds in Japan part 2'' 1995, Nature and Environment Research Center, *Masaaki Tanaka, ''Lakes and ponds in Japan 2'' 2004, University of Nagoya Press, {{DEFAULTSORT:Oike Landforms of Okinawa Prefecture Karst Ponds of Japan Wetlands of Japan