Often when people speak of Palau, they can't help but mention their delightful visit to Jellyfish Lake. It is like no other experience - swimming among the beautiful golden jellies. Snorkeling this lake is a popular tourist activity and an unusual phenomenon not to be missed.
Several tour operators in Koror will take groups to Eil Malk island, just a 45 minute boat ride from Koror, to this land-locked marine lake. Eil Malk is part of the Rock Islands, a group of uninhabited islands in Palau's southern lagoon located between Koror and Peleliu. The lake is accessed by a short, somewhat steep trail from the beach over a rocky limestone hill. Bring good footwear that you don't mind getting wet, such as hard-soled dive booties or Teva sandals for the hike.
Jellyfish lake is connected to the ocean through fissures and tunnels in this island's ancient limestone reef. The isolation of this lake has led to an unusual transformation of its resident jellies. Over the course of a millennia, with no predators to fend off, the golden jellyfish have lost their sting. They spend their days leisurely pulsating across the lake, following the sun and feeding on algae.
The age of the lake is estimated to be around 12,000 years old. Only snorkeling is allowed in the lake for two reasons: 1) because bubbles from scuba are believed to get trapped under the jellies' bell and damage them, and 2) because of an anoxic (non-oxygenated) layer that at exists at an approximate depth of 50 feet.
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 being 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, and 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.
All organisms that reside in Jellyfish Lake live in the upper oxygenated, somewhat turbid layer, including the jellyfish, a few species of fish and copepods. The salinity of this layer is affected by rain and runoff. The lake is connected to the sea via three tunnels that lie near the surface. which channel tidal water in and out of the lake. Tidal peaks are delayed from the outer lagoon tidal changes by about 1 hour and forty minutes. 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 50 feet below the surface to the bottom of the lake and 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 ten feet of this layer contains a dense population of bacteria that absorbs all sunlight so that the anoxic layer below the bacterial plate is dark, but transparent. The anoxic layer also contains high concentrations of ammonia and phosphate which 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]
Both golden jellies and moon jellies exist in the lake, but a sighting of a moon jelly is rare during the daylight hours because they generally only migrate to the surface in the evening to feed. Although these species living in the lake have nematocysts (stinging cells), they are not powerful enough to harm humans. However, if you are known to have an allergic reaction to jellyfish stings, it is suggested you might consider wearing a skin for protection.
[1] Hamner, William M; Hamner, Peggy P (1998). "Stratified marine lakes of Palau (Western Caroline Islands)". Physical Geography 19 (3): pp. 175-220.
[2] 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): pp. 896-909. doi:10.2307/2835973.
[3] Davis, Patricia Z R; Kearns, Colby M (2007) [2005]. Koror State Government Marine Tour Guide Certification Manual (2nd ed.). Koror State Department of Conservation and Law Enforcement.
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