Types Of Marine Lakes

There are ~57 marine lakes in Palau and each is unique, with the majority concentrated in the Rock Islands Southern Lagoon of Koror State, a UNESCO World Heritage Site. Jellyfish Lake is the only lake open to visitors.  Marine lakes can be grouped into three broad categories –  mixed ”holomictic,” transitional, and stratified “meromictic.”  Holomictic lakes, directly connected to the ocean, have a uniform and well-oxygenated water column, and share similarities to that of the reefs found in the surrounding lagoon, often harboring corals and colorful fishes.  Meromictic lakes, often found further inland with indirect connections to the ocean have damped and delayed tides, are stratified from top to bottom, and anoxic at the bottom with poisonous hydrogen sulfide. The stratification is maintained by the reduced salinity on the surface due to high annual rainfall (~4 m per year), coupled with relatively low water exchange with the lagoon. Transitional lakes can change between being mixed or stratified, depending on weather conditions. A detailed map of Palau’s marine lakes can be found in Chapt. 10, Marine Environments of Palau.

Holomictic (mixed) Lakes

Meromictic (stratified) Lakes

Meromictic, or stratified, lakes have little mixing between layers in the water column. While many of the lakes are located farther inland in the rock islands, with steep and long hiking trails, a few are found close to the ocean, such as Jellyfish Lake. However, all of these lakes retain indirect connections to the ocean, with damped and delayed tides, through small tunnels, cracks and fissures. Stratified lakes are typically characterized by fine muddy slopes and bottoms, do not have corals and usually have mangroves along their perimeter. Instead they have colorful sponges and sometimes jellyfish that look very different from those found in the ocean. Stratification of the water column means that temperature, salinity, oxygen and density are different between surface and deeper waters, and the stagnant bottom layer is usually anoxic with poisonous hydrogen sulfide.  A 1 m thick pink bacterial layer can be found at the transition zone between anoxic and oxygenated waters.

Transitional Lakes

Lakes With Jellyfish

Many marine lakes in Palau have jellyfish of various species, but only a few have persistent populations of them. Five of Palau’s marine lakes have distinct subspecies of the golden jelly, Mastigias papua, the ancestral population found in Palau’s lagoon. These five endemic subspecies were named after the first five elected Palauan presidents, in succession with the age of the lake where they occur (determined by maximum depth of the lake). Ngermeuangel Lake is the oldest (and deepest) with Mastigias papua remeliiki, followed by M. papua saliii in Clear Lake, M. papua etpisoni in Jellyfish Lake, M. papua nakamurai in Goby Lake, and M. papua remengesaui in Ongael Lake.

This function has been disabled for Coral Reef Research Foundation.