Stratified Marine Lakes of Palau (Western Caroline Islands)

Abstract

The physical geography of the limestone islands of Palau permits permanent water-column stratification in 13 tropical sea-level marine lakes, each with unique watercolumn physics, chemistry, and biology. Embayments and lagoons amid the coral became isolated as marine lakes after Miocene uplifting. Surface mixing of lake water by wind is reduced by jungle-covered karst ridges. Surface tidal exchange through fissures in fenes trated karst is slow while midwater exchange through submarine tunnels is fast, but both produce damped, delayed tides with modest seawater exchange from the barrier-reef lagoon. Topographic protection from wind, heavy regular rain throughout the year, with precipitation exceeding evaporation, and modest tidal exchange produce stratified water columns with brackish waters above permanently anoxic saline hypolimnia. Permanent lake stratification is documented for 18 years; sediment cores (by others) show stratifica tion for >100 years, and recent constant sea level implies ecosystem stability for thousands of years. Therefore, the marine lakes in Palau are small, closed, simple ecosystems that do not change over time-steady-state chemostats permitting replicate field measurements of biological and physical attributes from day to day, month to month, or decade to decade.[

Genetic Diversity and Evolution of Marine Animals Isolated in Marine Lakes

“…We hypothesized that the different pattern between the NLK and the other marine lake populations may be related with areas and depth to anoxic layer of each marine lake because the population size and its stability are dependent on such spatial factors. However, the areas and depth of the marine lakes we studied are very similar (area: JFM = 50,000 m 2 ; NLK = 43,000 m 2 ; CLM = 39,000 m 2 , depth to anoxic layer: JFM = 15 m; NLK = 10 m; CLM = 15 m; Dawson & Hamner, 2005;Hamner & Hamner, 1998). In this study, we cannot conclude why only the NLK population demonstrates the different pattern and whether the pattern was caused by chance or not.…”

“…Secondly, the islands were eroded by rain and wind, and numerous depressions were formed. Finally, with the rising sea level of the end of the Last Glacial Maximum, seawater flooded the depressions through fissures or tunnels (Hamner and Hauri, 1981;Hamner and Hamner, 1998). Geologically, the marine lakes were estimated to have formed in chronological series, namely, the deeper depressions flooded first (~12,000 years ago) and the shallower depressions flooded later (~5,000 years ago) (Dawson and Hamner, 2005).…”

“…In the meromictic lakes, thorough water-circulation does not occur because mixing by wind is reduced by the jungle-covered karst ridges. Topographic protection from wind, heavy regular rain throughout the year with precipitation exceeding evaporation, and modest tidal exchange produce stratified brackish waters above permanently anoxic saline hypolimnia (Hamner and Hamner, 1998). Mangrove forests develop on their coasts, and some fish species (e.g., Apogonidae spp., Atherinidae spp., and Gobiidae spp.…”

“…18,000 years ago); these waters were isolated from the surrounding barrier-reef lagoons and became sealevel marine lakes (Dawson and Hamner, 2005). Most of these lakes have been avoided by the local population due to the treacherous surface of the surrounding karst, lack of fresh water on the islands, and persistent myths that the marine lakes are haunted (Hamner and Hamner, 1998). Therefore, the ecosystems of most of the marine lakes have not yet been www.intechopen.com…”

“…The Palau archipelago, which consists of volcanic islands to the north and a large number of elevated limestone islands to the south (U.S. Geological Survey, 1956;Hamner and Hamner, 1998), is surrounded by multiple hard-coral atolls. The adjacent waters of the Palau Islands are known as a hotspot with the highest level of marine species diversity (Allen, 2008).…”