Jellyfish Lake

Jellyfish Lake
Ongeim'l Tketau

Aerial view, looking west
Location Eil Malk, Rock Islands, Palau
Coordinates 7°09′40″N 134°22′34″E / 7.16111°N 134.37611°E / 7.16111; 134.37611Coordinates: 7°09′40″N 134°22′34″E / 7.16111°N 134.37611°E / 7.16111; 134.37611
Type Meromictic
Basin countries Palau
Max. length 460 m (1,510 ft)
Max. width 160 m (520 ft)
Surface area 5.7 ha (14 acres)
Average depth 30 m (100 ft)
Water volume 1.71×10^6 m3 (60×10^6 cu ft)
Frozen Never
Islands None

Jellyfish Lake (Palauan: Ongeim'l Tketau, "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

Stratification diagram

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.[1]

The stratification of the lake is caused by conditions which prevent or restrict the mixing of water vertically. These conditions include:[1]
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 15 metres (49 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 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 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.[2]

The anoxic layer extends from about 15 metres (49 ft) 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.[3]

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[4]) 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.[5]

Jellyfish species

Golden and spotted jellyfish comparison. Note the loss of spots, color and greatly reduced clubs in the golden jellyfish from Jellyfish Lake

Two species of scyphozoan jellyfish live in Jellyfish Lake, moon jellyfish (Aurelia sp.) and the golden jellyfish (Mastigias sp.).

Golden jellyfish

The golden jellyfish are most closely related to the spotted jellyfish (Mastigias papua) that inhabit the nearby lagoons.[6] They are similar to the spotted jellyfish in that they derive part of their nutrition from symbiotic algae (Zooxanthellae) that live in their tissues and part of their nutrition from captured zooplankton.[7]

However, the golden jellyfish are morphologically, physiologically, and behaviourally 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.[8]

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. 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.[8]

Moon jellyfish

The moon jellyfish were identified as Aurelia aurita by Hamner.[9] 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.[10] 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.[10]

Quotidian migration

Golden jellyfish

Map of Eil Malk with Jellyfish Lake in the eastern part of the main island

The golden jellyfish migration pattern is as follows:[9]

The golden jellyfish rotate counter-clockwise as they swim at the surface, presumably to provide even exposure to the sun for the symbiotic algae in their bodies.[2][11]

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. The east to west migration in all of these other lakes (except for Clear Lake 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.[12]

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.[2]

Jellyfish die-off

Golden jellyfish

Beginning in about the fall of 1998, a precipitous decline in the golden jellyfish medusa population was detected in Jellyfish Lake. By December 1998, the medusa population had declined to zero.[6]

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, with the result that the symbiotic algae (Zooxanthellae) that live within the golden jellyfish medusae and the syphistomae (scyphozoan polyps) could not survive.[6]

In January 2000, the golden jellyfish medusae were observed in Jellyfish Lake for the first time since April 1999.[6] By May, 2012, the medusae population had returned to pre-decline levels.[13]

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 on Koror. The golden jellyfish population in Big Jellyfish Lake, 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.[6]

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 medusa 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.[6]

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 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,[14] 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.[6]

Moon jellyfish

The moon jellyfish exhibited unusual damage in the 1998 time frame; however, the population seemed no smaller than usual.[6]

Tourism

A swimmer snorkeling in Jellyfish Lake.

Tourists are required to obtain a pass to access Jellyfish Lake. The Rock Islands/Jellyfish Lake pass is $100 and is good for 10 days.[15]

Snorkeling 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.[16] The lake is accessed by a short trail from the beach on Eil Malk to the lake.[17]

Scuba diving by tourists in the lake is not allowed. Two reasons are put forward for this:[17]

Jellyfish Lake is currently the only one of Palau's marine lakes open to tourists.[18]

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.[17][18] The Fish 'n Fins tour guide recommended that people with allergies to jellyfish consider wearing protective clothing.[17]

Saltwater crocodiles 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.[19]

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.[20] 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 metres (49 ft) 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.

References

  1. 1 2 Hamner, William M; Hamner, Peggy P (1998). "Stratified marine lakes of Palau (Western Caroline Islands)". Physical Geography. 19 (3): 175–220. doi:10.1080/02723646.1998.10642647.
  2. 1 2 3 4 Hamner, William M; Gilmer, R W; Hamner, Peggy P (September 1982). "The Physical, Chemical, and Biological Characteristics of a Stratified, Saline, Sulfide Lake in Palau". Limnology and Oceanography. 27 (5): 896–909. doi:10.2307/2835973.
  3. ↑ Davis, Patricia Z R; Kearns, Colby M (2007) [2005]. Koror State Government Marine Tour Guide Certification Manual (PDF) (2nd ed.). Koror State Department of Conservation and Law Enforcement.
  4. ↑ Orem, W H; Burnett, W C; Landing, W M; Lyons, W B; Showers, W (May 1991). "Jellyfish Lake, Palau: Early Diagenesis of Organic Matter in Sediments of an Anoxic Marine Lake". Limnology and Oceanography. 36 (3): 526–543. doi:10.2307/2837516.
  5. ↑ Gornitz, Vivien (January 2007). "Sea Level Rise, After the Ice Melted and Today". NASA.gov. Goddard Institute for Space Studies. Retrieved 14 September 2009.
  6. 1 2 3 4 5 6 7 8 Dawson, Mike N; Martin, Laura E; Lolita K, Lolita K.; Penland (May 2001). "Jellyfish swarms, tourists, and the Christ-child". Hydrobiologia. Springer Netherlands. 451: 131–144. doi:10.1023/A:1011868925383. ISBN 978-0-7923-6964-6.
  7. ↑ Dawson, Michael N (2000). "Variegated mesocosms as alternatives to shore-based planktonkreisels: notes on the husbandry of jellyfish from marine lakes". Journal of Plankton Research. 22 (9): 1673–1682. doi:10.1093/plankt/22.9.1673.
  8. 1 2 Dawson, Michael N (2005). "Five new subspecies of Mastigias (Scyphozoa: Rhizostomeae: Mastigiidae) from marine lakes, Palau, Micronesia" (PDF). Journal of the Marine Biological Association of the United Kingdom. 85 (3): 679–694. doi:10.1017/S0025315405011604.
  9. 1 2 Hamner, William M; Hauri, Ivan R (1981). "Long-distance horizontal migrations of zooplankton (Scyphomedusae: Mastigias)" (PDF). Limnology and Oceanography. 26 (3): 414–423. doi:10.4319/lo.1981.26.3.0414.
  10. 1 2 Dawson, Michael N; Jacobs, David K (February 2001). "Molecular Evidence for Cryptic Species of Aurelia aurita (Cnidaria, Scyphozoa)". Biological Bulletin. 200 (1): 92–96. doi:10.2307/1543089. PMID 11249217.
  11. 1 2 Murphy, Richard C (2002). Coral Reefs: Cities Under the Sea. Princeton, NJ: Darwin Press. ISBN 087850138X.
  12. ↑ Dawson, Michael N; Hamner, William M (December 2003). "Geographic variation and behavioral evolution in marine plankton: the case of Mastigias (Scyphozoa, Rhizostomeae)" (PDF). Marine Biology. 143 (6): 1161–1174. doi:10.1007/s00227-003-1155-z.
  13. ↑ Dawson, Michael N. "Marine Lakes: Experiments in Ecology and Evolution". University of California, Merced. Retrieved 20 May 2012.
  14. ↑ Brown, B E; Dunne, R P; Chansang, H (September 1996). "Coral bleaching relative to elevated seawater temperature in the Andaman Sea (Indian Ocean) over the last 50 years". Coral Reefs. Springer Berlin. 15 (3): 151–152. doi:10.1007/BF01145885.
  15. ↑ "KSPL K9-248-2011: Rock Island Permit Fee Increase" (PDF). Koror State Government. December 12, 2011.
  16. ↑ Merrill, Jessica (October 2008). "Make your dream trip a reality". Shape.
  17. 1 2 3 4 Martin, Laura; Dawson, Michael (2009). "Jellyfish Lake". Fish 'n Fins. Archived from the original on 20 May 2012.
  18. 1 2 "Jellyfish Lake Information Sheet" (PDF). Coral Reef Research Foundation. Retrieved 5 September 2009.
  19. ↑ Brazaitis, Peter; Eberdong, Joshua; Brazaitis, Peter John; Watkins-Colwell, Gregory J (2009). "Notes on the Saltwater Crocodile, Crocodylus porosus, in the Republic of Palau". Bulletin of the Peabody Museum of Natural History. 50 (1): 27–48. doi:10.3374/014.050.0103.
  20. ↑ Buzzi, Ruth Ann (1992). Chemical Hazards at Water and Wastewater Treatment Plants. Boca Raton, FL: Lewis Publishers. p. 10. ISBN 978-0-87371-491-4.
  • Turner, Pamela S (August 2006). "Darwin's Jellyfishes". National Wildlife. 44 (5). Accession number 21887173. 
  • Fautin, D G; Fitt, W K (June 1991). "A jellyfish-eating sea anemone (Cnidaria, Actiniaria) from Palau: Entacmaea medusivora sp. nov.". Hydrobiologia. 216/217 (1): 453–461. doi:10.1007/BF00026499. 
  • Dawson, Michael N; Martin, Laura E (2007). "Migrations by Mastigias". The Scyphozoan. University of California, Merced. 
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