Jellyfish Lake
The Ongeim'l Tketau, among diving enthusiasts and in the popular press and Jellyfish Lake or Jellyfish Lake called [Note 1], is a marine lake which lies on the Koror island offshore island Eil Malk in the South Pacific island nation of Palau. He has become known by the mass occurrence of a sub-species of Scyphozoa jellyfish Mastigias papua. It has been calculated that at peak times inhabit the lake to about 30 million jellyfish.
Formation
The lake was formed about 12,000 years ago, when after the end of the last ice age rise in sea level. Due to the superficial strongly karstified, Miocene limestones of the island sea water penetrated into an isolated valley from the sea. The lake level rose continuously, coupled with sea level rise up to the present level of.
Physical characteristics of the lake
The Ongeim'l Tketau is about 420 m long and max. about 200 m wide. He is about 30 m deep at its lowest point in the western half of the lake. An isolated by a slight threshold, rotund basin in the eastern half of the lake is about 20 m deep. The lake level is at sea level. Extrapolating the sedimentary cover from the pelvis may have been at its lowest point of only 60 m deep. The margins fall to the surface of the flat-lying sediments steeply.
The lake is stratified. He has a top layer of oxygen-rich water and a lower anoxic layer. The oxygen content decreases from about 5 to 6 ppm to the surface to 0 ppm in the depth of approximately 15 meters. The stratification is permanent, that is, a seasonal mixing or intermixing during and shortly after storms do not happen .. therefore are organisms that need oxygen to live on this 15 m limited. A stable, about 3 m thick floating mat of purple bacterium Chromatium has formed at the chemocline. It also absorbs nearly all the sunlight. Underneath the water is relatively clear and the visibility is again about 30 m.
The salinity varies in the upper three meters by precipitation and inflowing fresh water from the immediate area from 19.6 to 26 ‰. After a severe typhoon on 16 April 1979 over 51 cm of rain per square meter, the salinity decreased to 19.6 ‰. But again already after three days had set "normal" salinity of 26 ‰. Below the upper three meters of the salt content is no longer affected by flowing fresh water and rain. The salinity remains constant at 30 ‰ here. The upper three meters and are almost always somewhat agitated and the visibility is usually limited to about 5 m.
Above the chemocline, the levels of nutrients are very low. The levels of nitrates and nitrites are hardly measurable. The content of ammonium (NH4 ) is above the chemocline less than 0.5 micrograms per liter. He descends below the chemocline rapidly and reaches a depth of 28 m is about 600 g / l The phosphate content is above the chemocline with <0.4 g / l also vanishingly low and decreases below the chemocline at 25 ug / l. However, the content of the silicates from the surface of <1 ug and decreases continuously from the chemo Kline little affected at approximately 400 microns. An entirely different curve point to the present in the aquatic particles of organic carbon. On the surface, the water contains about 800 g / l at depth to about 8 m to 350 microns decrease. In the chemocline to about 15/16 m depth, a sharp increase is then observed up to 7,000 to 8,000 ug / l. Among them, the content of suspended organic carbon particles drops sharply and reaches a depth of 28 m, a minimum of 1.5 ug / l The content of hydrogen sulphide can not be measured above the chemotherapeutic Kline, it rises to the bottom of the lake rapidly to 80 mg / l ( ).
The temperature at the surface of about 31 ° C to 32 ° C. Takes you to about 27 ° C. in the deepest areas of the lake. Here, a noticeable kink is observed in the field to the chemocline. While the temperature of 32 ° C decreases in the top 15 m only about 31 ° C above the chemocline, the temperature drops from just above the chemocline to just below ( about 17 m) rapidly to 28 ° C. to less than 17 m slow decrease at 27 ° C to the lowest point.
The lake is connected to the sea through three tunnels near the surface, that is, the tides are felt in the lake. But are delayed by approximately one hour and forty minutes, and only reach about one-third of the tide height as the adjacent lagoon (rarely greater than 50 cm). Approximately 2.5 % of the lake water is exchanged daily. Because the tunnels are near the surface, the lower anoxic layer is not affected by the tides.
Organisms
The lake is inhabited by only a few organisms. A summary does not yet exist. At larger organisms so far only one species of the genus Pranesus, two types of copepods [Note 2] and on the banks of a sea anemone were medusenfressende ( Oithona oculata and Acrocalanus inermis ), two scyphozoe jellyfish ( Mastigias cf papua ssp. Etpisoni and Aurelia sp. ( Entacmaea medusivora ). Besides, there are also specifically not certain snails, also on the banks of the lake. at microorganisms chain-forming diatoms ( Chaetoceras sp.) called, large dinoflagellates ( Diceratium ) and not to specific Mikroflagellaten. at the chemocline has a stable, floating mat of the bacterium Chromatium formed. absorbs nearly the full sunlight. the many visitors to the lake are problematic for the ecology of the lake. in 2006, at least three invasive species have been found in the lake. a sea anemone ( Aiptasia sp. ), a sponge ( Haliclona sp. ) and a Zooxanthelle ( as a symbiont of the sea anemone Aiptasia sp.). The previous effects on the ecosystem are not yet investigated.
" Mastigias cf papua ssp etpisoni -. The Golden Jellyfish "
In the popular press and in circles of diving enthusiasts, the lake is famous as " Jellyfish Lake ". Most throughout the year, the lake is populated by about 1.5 million jellyfish. It was first defined by William and Peggy Hamner as Mastigias papua ( Lesson, 1830). In addition to the Ongeim'l Tketau come jellyfish of the same kind even in four other marine lakes in Palau before and in the lagoons to express Malk. Recent molecular biological studies have shown a much more complex picture. Each lake has its own subspecies and distinguished not only genetically but also morphologically distinct from the original species by their behavior. The molecular genetics, intraspecific variability is expected in the Lagunenart, the putative ancestral species at the highest in the sub-species in the five lakes very low. However, among the populations of different sites from the distribution area of Mastigias papua such a large genetic variability were observed as they come only species. Mastigias Papua is therefore likely to be a Artkomplex of at least three, but probably more of five or more cryptic species. The occurring in Palau nature is therefore not to be determined at the moment. Michael Dawson, therefore, certain as Mastigias cf papua and retired five subspecies, each of which is limited to a lake. The occurring in Ongeim'l Tketau subspecies is as Mastigias cf papua ssp. etpisoni Dawson, 2005 respectively. The medusae are golden brown, without blue pigmentation, fine white spots rarely occur. But otherwise no stains are present. The thickened end portions of the eight arms oral relatively short and wide. The largest jellyfish grow up to 23 cm. The medusae are planktonic and usually the whole year round in the water column from 0 to 13 m depth available. They live on symbiotic zooxanthellae, which they owe the color, but also on zooplankton. Laboratory experiments have shown that they do not grow with zooxanthellae alone. Perform daily walks. At night they accumulate in the central part of the western lake basin and lead several up-and - walks between the chemocline and the surface by. From early morning to about 9 clock 30 medusae migrate from the central western part of the lake basin to the eastern basin. From early afternoon to about 15 clock 30 You then migrate to the western end of the lake. At sunset, they migrate back to the pelvic region in the western part of the lake. During their travels the jellyfish rotate counterclockwise, probably to send to the zooxanthellae light evenly. The medusae avoid shadows and the shady shore areas, because sitting there quite numerous sea anemones of the endemic only in this lake type Entacmaea medusivora, which mainly feeds on jellyfish.
The endemic in Ongeim'l Tketau subspecies is most closely related to Mastigias cf papua, who lives in the open marine lagoons Eil Malk. This shape is characterized by white spots on the screen. Moreover, it has a blue pigment, overlying the golden brown color of zooxanthelles in many places. In contrast Mastigias missing cf papua ssp. etpisoni the club-shaped end members of the eight oral arms almost completely.
In the fall of 1998, a significant decrease in the number of medusae was observed in the lake. By December 1998, all medusae were gone. Michael Dawson and his colleagues suspect that probably is responsible by the caused by an El Niño - configuration of the ocean currents in the Pacific very hot weather for the disappearance of medusae. The zooxanthellae die at these temperatures. In January 2000, medusae appeared again in greater numbers in Ongeim'l Tketau. In January 2005, was found to be approximately 31 million, the highest ever number established.
Snorkeling / Diving
Snorkeling in Ongeim'l Tketau is now a very popular activity of tourists to Palau. From Koror tourist tours are offered to the lake. The boat ride to the island of Koror Eil Malk takes about 45 minutes. Tourists need a permit from Koror State Department of Conservation and Law Enforcement. The permit costs $ 35 and is valid for 10 days (2007). Scuba diving is not permitted in the lake. It is the only one around. 70 marine lakes of Palau, which is publicly available at present.