Jellyfish Lake
Koror
Jellyfish Lake, locally known as Ongeim'l Tketau, is a remarkable marine lake located on Eil Malk Island within Palau's Rock Islands. This isolated lake is renowned for its vast population of golden jellyfish (Mastigias papua etpisoni) that migrate horizontally across the water daily. The lake is stratified into two distinct layers: an oxygen-rich upper layer supporting diverse life including jellyfish and fish, and a lower anoxic layer devoid of oxygen but rich in bacteria and hydrogen sulfide. Connected to the ocean through underwater limestone tunnels, the lake maintains unique ecological conditions that have driven the evolution of its jellyfish species, differentiating them from their lagoon relatives. The lake's calm, wind-shielded environment, tropical climate, and limited tidal exchange contribute to its permanent stratification. The golden jellyfish play a vital role in mixing the lake's waters, sustaining its delicate ecosystem. Due to the anoxic lower layer's toxicity, scuba diving is prohibited, ensuring visitor safety while allowing snorkeling in the upper waters. This marine lake is one of about 70 in the Rock Islands, but its combination of biological uniqueness, geological formation, and cultural significance makes it an extraordinary destination for nature lovers and scientists alike.
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Tip: The best time to visit Jellyfish Lake is during daylight hours when the golden jellyfish are most active in their daily migration. Visitors should purchase tickets in advance as access is limited to protect the environment. Snorkeling is allowed in the upper oxygenated layer, but scuba diving is prohibited due to the toxic lower layer. Wearing reef-safe sunscreen is recommended to preserve the lake's delicate ecosystem. Guided tours can enhance the experience by providing ecological and cultural insights. Early morning visits may offer calmer waters and better visibility.
Interesting facts
- •Jellyfish Lake is one of about 200 known saline meromictic lakes worldwide, but one of the few permanently stratified marine lakes in the tropics.
- •The golden jellyfish in the lake have evolved differently from their close relatives in nearby lagoons due to the lake's isolation.
- •The lake's anoxic lower layer contains high concentrations of hydrogen sulfide and bacteria that absorb all sunlight, creating a dark but transparent zone.
- •Approximately 2.5% of the lake's volume is exchanged during each tidal cycle through three underwater tunnels connecting it to the ocean.
- •The jellyfish population actively mixes the lake's waters, influencing oxygen and nutrient distribution.
History
Jellyfish Lake formed approximately 12,000 years ago when rising sea levels after the last ice age flooded a limestone basin on Eil Malk Island.
Over millennia, the lake became isolated from the ocean except for narrow underwater tunnels, leading to unique ecological conditions.
The golden jellyfish species evolved distinctly within this environment, adapting to the lake's stratified layers.
The lake has long held cultural significance for the Palauan people, who know it as Ongeim'l Tketau, meaning 'Fifth Lake'.
Its scientific importance was recognized in recent decades, attracting researchers studying marine biology and limnology.
Place Guide
Upper Oxygenated Layer
This layer extends from the surface to about 15 meters and supports all oxygen-dependent life including the golden jellyfish, fish, and copepods. Visibility is limited to about 5 meters due to turbidity.
Lower Anoxic Layer
Extending from about 15 meters to the lake bottom, this layer contains no oxygen but is rich in hydrogen sulfide and sulfur bacteria. It is dark yet transparent and potentially toxic to divers.
Underwater Limestone Tunnels
Three tunnels near the surface connect the lake to the ocean, allowing tidal water exchange that affects about 2.5% of the lake's volume per cycle and helps regulate salinity and water levels.
