A Lake in Layers
Jellyfish Lake, known locally as Ongeim'l Tketau, is a marine lake that formed approximately 12,000 years ago as rising sea levels flooded a depression in the island's limestone rock. Though connected to the ocean via small fissures and tunnels, the lake is isolated enough to have developed a unique and highly stratified ecosystem. The lake is meromictic, meaning its water is permanently separated into two distinct layers that do not mix.
The upper layer, extending from the surface down to about 15 meters (49 feet), is oxygenated and supports the lake's visible animal life. Below this is a sharp transition zone, or chemocline, which contains a dense, two-meter-thick pink layer of photosynthetic purple bacteria, primarily of the genus Chromatium. This bacterial plate absorbs all sunlight, leaving the bottom layer completely dark. The lower layer, from 15 meters to the lakebed at 30 meters, is anoxic—it contains no oxygen. Instead, it has high levels of dissolved sulfide, a gas produced by bacterial decomposition that is toxic to most forms of life, including humans. Only certain microbes can survive in this lower, inhospitable zone.
Golden Jellyfish and Their Daily Migration
The lake's most famous inhabitants are millions of golden jellyfish, a subspecies named Mastigias papua etpisoni that is endemic to this specific lake. These jellyfish evolved from an ancestral population of spotted jellyfish trapped in the lake thousands of years ago. They have a symbiotic relationship with single-celled algae called zooxanthellae (Symbiodinium spp.) that live within their tissues. The algae photosynthesize, providing the jellyfish with a major source of energy.
To ensure their symbiotic algae receive enough sunlight, the entire population of jellyfish performs a synchronized horizontal migration across the 400-meter-long lake every day. As the sun rises, they swim from the western basin towards the east. Around noon, they turn and follow the sun back to the western side of the lake, where they remain overnight. This daily migration also keeps them away from the shadowy lake edges where their primary predator, the sea anemone Entacmaea medusivora, lives. While often called "stingless," the jellyfish do possess stinging cells (nematocysts), but their sting is too mild for humans to feel.
The jellyfish population fluctuates dramatically. At its peak in 2005, the lake contained over 30 million individuals, while typical numbers are around 5 million. The population has experienced severe crashes, notably in 1998-99 and 2016, linked to El Niño events that raise water temperatures. The population has shown resilience, rebounding from just a few thousand individuals as the tiny polyps—the jellyfish's hardy larval stage—survive the warmer conditions to repopulate the lake.