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 million m3 (60 million cu ft)
Surface elevation	Sea level
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.

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.