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Jellyfish Lake (, "Fifth Lake") is a marine lake located on Eil Malk island in Palau. Eil Malk is part of the Rock Islands, a group of small, rocky, mostly uninhabited islands in Palau's Southern Lagoon, between Koror and Peleliu. There are about 70 other marine lakes located throughout the Rock Islands. Millions of golden jellyfish migrate horizontally across the lake daily. Jellyfish Lake is connected to the ocean through fissures and tunnels in the limestone of an ancient Miocene reef. However the lake is sufficiently isolated and the conditions are different enough that the diversity of species in the lake is greatly reduced from the nearby lagoon. The golden jellyfish, ''Mastigias'' cf. ''papua etpisoni'', and possibly other species in the lake have evolved to be substantially different from their close relatives living in the nearby lagoons. == Lake stratification == Jellyfish Lake is stratified into two layers, an oxygenated upper layer (mixolimnion) and a lower anoxic layer (monimolimnion). The oxygen concentration in the lake declines from about 5 ppm at the surface to zero at 15 meters (at the chemocline). Stratification is persistent and seasonal mixing does not occur. The lake is one of about 200 saline meromictic lakes that have been identified in the world. However most of these lakes are of freshwater origin. Permanently stratified marine lakes are unusual, but on Eil Malk and on other nearby islands there are eleven other apparently permanent stratified marine lakes. The stratification of the lake is caused by conditions which prevent or restrict the mixing of water vertically. These conditions include:〔 1. The lake is surrounded by rock walls and trees which substantially block the wind flow across the lake that would cause mixing. 2. The primary water sources for the lake (rain, runoff and tidal flows through tunnels) are all close to the surface. 3. The lake is in the tropics where seasonal temperature variation is small, and so the temperature inversion that can cause vertical mixing of lakes in temperate zones does not occur. The oxygenated layer extends from the surface to about . All organisms that require oxygen live in this layer including the jellyfish, a few species of fish and copepods. This layer is somewhat turbid. Visibility is limited to about . The salinity of this layer down to about is affected by rain and runoff, and below this, salinity levels are unaffected by freshwater inputs. The lake is connected to the sea via three tunnels that lie near the surface. The tunnels channel tidal water in and out of the lake. Tide levels in the lake are damped to about a third of the lagoon tidal levels. The tidal peaks are delayed from the lagoon tidal peaks by about 1 hour and forty minutes. Biologist William Hamner estimated that about 2.5% of the lake's volume is exchanged during a tidal flow. However, because the tidal water enters at the surface, the lower anoxic layer is largely unaffected by tidal flows. The anoxic layer extends from about below the surface to the bottom of the lake. The oxygen concentration in this layer is zero. The hydrogen sulfide concentration rises from about zero at the top of this layer to over 80 mg/liter at the bottom of the lake. The top three meters of this layer contains a dense population of bacteria, at least one species of which is a purple photosynthetic sulfur bacterium. This bacterial layer absorbs all sunlight so that the anoxic layer below the bacterial plate is dark, but transparent. Hamner estimated the visibility to be about 30 meters. The anoxic layer also contains high concentrations of ammonia and phosphate which are almost completely absent from the upper layer. The anoxic layer is potentially dangerous for divers, who can be poisoned through their skin. This risk is mitigated as scuba diving equipment is not allowed in the lake, thus limiting the depths to which individuals may dive. 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「Jellyfish Lake」の詳細全文を読む スポンサード リンク
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