Stratified marine lakes of Palau (Western Caroline Islands)

The physical geography of the limestone islands of Palau permits permanent water-column stratification in 13 tropical sea-level marine lakes, each wit h unique water-column physics, chemistry, and biology. Embayments and lagoo ns amid the coral became isolated as marine lakes after Miocene uplifting. Surface mixing of lake water by wind is reduced by jungle-covered karst rid ges. Surface tidal exchange through fissures in fenestrated karst is slow w hile midwater exchange through submarine tunnels is fast, but both produce damped, delayed tides with modest seawater exchange from the barrier-reef l agoon. Topographic protection from wind, heavy regular rain throughout the year, with precipitation exceeding evaporation, and modest tidal exchange p roduce stratified water columns with brackish waters above permanently anox ic saline hypolimnia. Permanent lake stratification is documented for 18 ye ars; sediment cores (by others) show stratification for >100 years, and rec ent constant sea level implies ecosystem stability for thousands of years. Therefore, the marine lakes in Palau are small, closed, simple ecosystems t hat do not change over time-steady-state chemostats permitting replicate fi eld measurements of biological and physical attributes from day to day, mon th to month, or decade to decade.