Jellyfish Lake, Eil Malk Island, Palau, Pacific Ocean

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 so that 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.