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Jellyfish Lake

Jellyfish Lake

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Jellyfish Lake is a marine lake located on Eil Malk
Eil Malk
Eil Malk or Mecherchar is the main island of the Mecherchar Islands, an island group of Palau in the Pacific Ocean. In a more narrow sense, just the southeastern peninsula of Mecherchar is called Eil Malk...

 island in Palau
Palau
Palau , officially the Republic of Palau , is an island nation in the Pacific Ocean, east of the Philippines and south of Tokyo. In 1978, after three decades as being part of the United Nations trusteeship, Palau chose independence instead of becoming part of the Federated States of Micronesia, a...

. 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. Jellyfish Lake is one of Palau's most famous dive (snorkeling only) sites. It is notable for the millions of golden jellyfish which migrate horizontally across the lake daily.

Jellyfish Lake is connected to the ocean through fissures and tunnels in the limestone of ancient Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

 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
Stratification (water)
Water stratification occurs when water masses with different properties - salinity , oxygenation , density , temperature - form layers that act as barriers to water mixing...

 into two layers, an oxygenated
Oxygenation (environmental)
Environmental oxygenation can be important to the sustainability of a particular ecosystem. Insufficient oxygen may occur in bodies of water such as ponds and rivers, tending to suppress the presence of aerobic organisms such as fish...

 upper layer (mixolimnion) and a lower anoxic
Hypoxia (environmental)
Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...

 layer (monimolimnion). The oxygen concentration in the lake declines from about 5 ppm at the surface to zero at 15 meters (at the chemocline
Chemocline
A chemocline is a cline caused by a strong, vertical chemistry gradient within a body of water. A chemocline is analogous to a thermocline, the border at which warmer and cooler waters meet in an ocean, sea, lake, or other body of water...

). Stratification is persistent and seasonal mixing does not occur. The lake is one of about 200 saline meromictic
Meromictic
A meromictic lake has layers of water that do not intermix. In ordinary, "holomictic" lakes, at least once each year there is a physical mixing of the surface and the deep waters...

 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 so that 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 15 metres (49.2 ft). 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 5 metres (16.4 ft). The salinity of this layer down to about 3 metres (9.8 ft) 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
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

 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 15 metres (49.2 ft) below the surface to the bottom of the lake. The oxygen concentration in this layer is zero. The hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

 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
Purple bacteria
Purple bacteria or purple photosynthetic bacteria are proteobacteria that are phototrophic, that is capable of producing energy through photosynthesis...

. 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
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 and phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...

 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.

Age of the lake


Jellyfish lake is around 12,000 years old. This age estimate is based on the depth of the lake (about 30 meters), an estimate of the thickness of the sediment (at least 20 meters) and the rising sea level since the end of the last ice age. About 12,000 years ago, the sea level had risen to the point that sea water began to fill the Jellyfish Lake basin.

Jellyfish species



Two species of scyphozoan jellyfish live in Jellyfish Lake, moon jellyfish (Aurelia
Aurelia (genus)
Aurelia is a genus of scyphozoan jellyfish . There are at least 13 species in the genus Aurelia including many that are still not formally described...

sp.) and the golden jellyfish (Mastigias
Mastigias
Mastigias is a genus of true jellyfish in the family Mastigiidae. It contains eight described species.-Species:*Mastigias albipunctatus*Mastigias andersoni*Mastigias gracilis*Mastigias ocellatus*Mastigias pantherinus...

sp.).

Golden jellyfish


The golden jellyfish are most closely related to the spotted jellyfish (Mastigias papua) that inhabit the nearby lagoons. They are similar to the spotted jellyfish in that they derive part of their nutrition from symbiotic algae (Zooxanthella
Zooxanthella
Zooxanthellae are flagellate protozoa that are golden-brown intracellular endosymbionts of various marine animals and protozoa, especially anthozoans such as the scleractinian corals and the tropical sea anemone, Aiptasia....

) that live in their tissues and part of their nutrition from captured zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...

. However, the golden jellyfish are morphologically, physiologically, and behaviorally distinct from the spotted jellyfish. They are easily distinguished from the spotted jellyfish by the almost complete loss of spots on the exumbrella and the almost complete loss of their clubs, an appendage attached to the oral arms.

Marine biologist, Michael Dawson proposed that the golden jellyfish that inhabit Jellyfish Lake be classified as a subspecies (Mastigias cf. papua etpisoni) of the spotted jellyfish living in the nearby lagoons. The species identification is uncertain (denoted by cf.
Cf.
cf., an abbreviation for the Latin word confer , literally meaning "bring together", is used to refer to other material or ideas which may provide similar or different information or arguments. It is mainly used in scholarly contexts, such as in academic or legal texts...

 in the name) because the Mastigias papua local to Palauan lagoons may be only one of several cryptic species that make up the M. papua group, and in the future, the M. papua local to Palau may be identified as a separate species from other M. papua. He also proposed that the jellyfish living in four other Palauan marine lakes were distinctive enough to deserve recognition as unique subspecies.

Moon jellyfish


The moon jellyfish were identified as Aurelia aurita
Aurelia aurita
Aurelia aurita is one of a group of more than ten morphologically nearly identical jellyfish species in the genus Aurelia...

by Hamner. However, since the release of that report in 1981, genetic testing has been done on specimens of Aurelia collected from locations throughout the world. The results of that testing indicate in addition to the three named species of Aurelia there are at least six other cryptic species in the Aurelia genus. Three of the cryptic species identified were from Palau. One of these cryptic species is common to four of Palau's marine lakes with jellyfish populations including Jellyfish Lake. Hence, the most accurate designation for the moon jellyfish in Jellyfish Lake (as of February 2001) is Aurelia sp. Despite the close proximity of Palau's moon jellyfish cryptic species, Dawson and Jacobs stated that the molecular data suggested that they had not interbred for millions of years.

Golden jellyfish



The golden jellyfish migration pattern is as follows:
  • Night - For about 14 hours a day the jellyfish make repeated vertical excursions between the surface and the chemocline
    Chemocline
    A chemocline is a cline caused by a strong, vertical chemistry gradient within a body of water. A chemocline is analogous to a thermocline, the border at which warmer and cooler waters meet in an ocean, sea, lake, or other body of water...

     in the western basin possibly to acquire nutrients from near the chemocline for their symbiotic algae.
  • From early morning to about 0930 - The jellyfish move from center of western basin to the eastern basin
  • From early afternoon to about 1530 - The jellyfish move from eastern basin to near western end of lake
  • As the sun sets - The jellyfish move briefly eastward from western end to western basin where they remain through the night


The golden jellyfish rotate counterclockwise as they swim at the surface, presumably to provide even exposure to the sun for the symbiotic algae in their bodies.

The Jellyfish Lake golden jellyfish migration pattern is similar to Mastigias sp. in other Palauan marine lakes and coves which all migrate west to east in the morning. However the migration patterns in other Palauan coves and marine lakes are less well defined than that of the Jellyfish Lake golden jellyfish and the east to west migration in all these other lakes except for Clear Lake
Clear Lake (Palau)
Clear 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...

 on Eil Malk does not begin until the late afternoon.

Hamner and Dawson proposed that the difference is caused by evolutionary change driven by the jellyfish-eating anemones (Entacmaea medusivora) that inhabit the eastern regions of Jellyfish Lake and Clear Lake. The jellyfish instinctively avoid shadows and in the morning with the shadows on the eastern end the jellyfish also avoid the anemones. By moving east to west in the early afternoon the jellyfish avoid the time of day when the setting sun would eliminate shadows on the lake in the eastern end and thereby avoid the anemones in the afternoon.

Moon jellyfish


The moon jellyfish do not have an organized horizontal migration pattern. At night they migrate to the surface presumably to feed. The copepods that make up a significant portion of the moon jellyfish diet in Jellyfish Lake also migrate to the surface at night.

Golden jellyfish


Beginning in about the fall of 1998, a precipitous decline in the golden jellyfish medusa
Medusa (biology)
In biology, a medusa is a form of cnidarian in which the body is shaped like an umbrella, in contrast with polyps. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below...

 population was detected in Jellyfish Lake. By December 1998, the medusa population had declined to zero.

Based on their extensive investigation of the disappearance of the golden jellyfish medusae, Dawson et al. determined that the most likely cause was an El Niño weather event that raised the water temperature so that the symbiotic algae (zooanthellae) that live within the golden jellyfish medusae and the syphistomae (scyphozoan polyps) could not survive.

In January 2000, the golden jellyfish medusae were observed in Jellyfish Lake for the first time since April 1999. Today the medusae population has returned to pre-decline levels.

Dawson et al. also surveyed the golden jellyfish populations in three other Palauan marine lakes. They found significant changes in the medusa population in two of these lakes (Clear Lake on Eil Malk and Goby Lake
Goby Lake
Goby Lake is a marine lake located on Koror island in Palau. There are about 70 other marine lakes located throughout the Rock Islands and Koror...

 on Koror). The golden jellyfish population in Big Jellyfish Lake
Uet era Ngermeuangel
Uet era Ngermeuangel is a marine lake located on Koror island in Palau. There are about 70 other marine lakes located throughout the Rock Islands and Koror...

, Koror did not seem to be affected. The reason for this was not clear but Big Jellyfish Lake experienced lower temperature increases than the other lakes and there was experimental evidence that the golden jellyfish medusa from Big Jellyfish Lake were more tolerant of higher temperatures.

Although Clear Lake did not seem to have experienced the complete medusa population die-off that Jellyfish Lake did in 1998, the golden jellyfish medusae in Clear Lake are not always present. When conditions are not favorable for the short lived medusae stage or for syphistomae strobilation the medusae disappear in Clear Lake. The medusa population is reestablished by syphistomae strobilation when conditions are favorable for strobilation and medusae again.

A significant reduction in the Jellyfish Lake medusa population had also been noted in 1987. This was previously attributed to turbulence generated by scuba diving
Scuba diving
Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....

 that caused disturbance of the toxic layer. However, given that it occurred within the time frame when an unusually high sea surface temperature had been detected, it might reasonably be surmised to have been caused by a rise in water temperature that was the most likely cause of the 1998 die-off.

Moon jellyfish


The moon jellyfish exhibited unusual damage in the 1998 time frame; however, the population seemed no smaller than usual.

Snorkeling in Jellyfish Lake



Snorkeling
Snorkeling
Snorkeling is the practice of swimming on or through a body of water while equipped with a diving mask, a shaped tube called a snorkel, and usually swimfins. In cooler waters, a wetsuit may also be worn...

 in Jellyfish Lake is a popular activity for tourists to Palau. Several tour operators in Koror offer trips to the lake. Eil Malk island is approximately a 45 minute boat ride from Koror. The lake is accessed by a short trail from the beach on Eil Malk to the lake.

Tourists require a pass to access to Jellyfish Lake. The Rock Islands/Jellyfish Lake pass is $35 and is good for 10 days.

Scuba diving by tourists in the lake is not allowed. Two reasons are put forward for this:
  • The bubbles from scuba tanks can harm the jellyfish if they collect beneath their bell.
  • The anoxic layer that begins at about 15 meters contains high concentrations of hydrogen sulfide which can be absorbed through the skin of a diver which can lead to death.


Jellyfish Lake is currently the only one of Palau's marine lakes open to tourists.

Safety considerations


Although both species of jellyfish living in the lake have stinging cells (nematocytes), they are not in general powerful enough to cause harm to humans. It has been reported that it is possible to notice the stings on sensitive areas like the area around the mouth. The Fish n' Fins tour guide recommended that people with allergies to jellyfish consider wearing protective clothing.

Saltwater crocodiles
Saltwater Crocodile
The saltwater crocodile, also known as estuarine or Indo-Pacific crocodile, is the largest of all living reptiles...

 are native to Palau but there has only been one death attributed to them in recent times and they are generally not considered a threat to divers.

The hydrogen sulfide in the anoxic layer is a serious risk to scuba divers entering this layer. The gas can be absorbed through the skin. In 1977, the maximum safe threshold level for hydrogen sulfide was set at 10 ppm. The concentrations exceed that by eightfold at the bottom of the anoxic layer. However, the hydrogen sulfide concentration down to the chemocline at about 15 meters is reported to be zero, and if the anoxic layer is avoided, the hydrogen sulfide in the lake does not pose a risk for snorkelers.

Jellyfish Lake in popular culture

  • In two episodes of Survivor
    Survivor (TV series)
    Survivor is a reality television game show format produced in many countries throughout the world. In the show, contestants are isolated in the wilderness and compete for cash and other prizes. The show uses a system of progressive elimination, allowing the contestants to vote off other tribe...

    (Survivor: Palau
    Survivor: Palau
    Survivor: Palau is the tenth season of the United States reality show Survivor. Its preview appeared during the final episode of Survivor: Vanuatu. Survivor: Palau premiered on February 17, 2005. The complete season, including the Live Reunion Show, was released on DVD by CBS Home Video on August...

    and Survivor: Micronesia
    Survivor: Micronesia
    Survivor: Micronesia — Fans vs. Favorites is the sixteenth season of the American CBS competitive reality television series Survivor, being the show's third to include contestants from past seasons, after the all-returning contestant pool from Survivor: All-Stars, and Survivor: Guatemala in which...

    ), as a reward for winning a challenge, winners got to swim in Jellyfish Lake.
  • Jellyfish Lake was featured in the documentaries The Living Sea
    The Living Sea
    The Living Sea is a 70mm American documentary film exploring marine locales intended to show the importance of protecting the ocean, released to IMAX theaters in 1995...

    , The Private Life of Plants
    The Private Life of Plants
    The Private Life of Plants is a BBC nature documentary series written and presented by David Attenborough, first shown in the UK from 11 January 1995....

    , and Great Migrations
    Great Migrations
    Great Migrations is a seven-episode nature documentary television miniseries that airs on the National Geographic Channel, featuring the great migrations of animals across the globe. This is truly one of the greatest pieces of nature documentary there is. The producer, David Hamlin, has won 3...

    .
  • Matt Harding
    Matt Harding
    Matthew "Matt" Harding , is an American traveler, video game designer, and Internet celebrity known as Dancing Matt for his viral videos that show him dancing in front of landmarks and street scenes in various international locations...

    , known for his videos where he dances in locations worldwide, "danced" in Jellyfish Lake.

External links