Foraminifera occur in the peculiar, meromictic Jellyfish Lake on Mecherchar Island in the Palau Archipelago (Western Caroline Islands). The lake is marine, 30 m deep, and characterized by both oxic and anoxic water layers separated by a distinct chemo-and thermocline. Above 15 m, the lake is well lighted, oxic, and with normal marine salinity and temperatures. Below that, the lake is dark and anoxic with high sulfide content in the water and sediment. Sediment surface samples, collected by SCUBA from sites within both layers, contain few foraminifera. The foraminiferal fauna is characterized by a particularly low-diversity and thin-shelled assemblage and is devoid of typical diverse reef and lagoonal species commonly present in South Pacific reef complexes, including those surrounding Mecherchar Island just 100 m or so from Jellyfish Lake. A total of 15 species was recovered from surface sediments in the upper oxic zone of the lake, but no foraminifera were found in those samples collected in the deeper, anoxic water of the lake. Two species, Helenina sp. and Ammonia sp., dominate in the shallow-water lake habitats (0-10 m), while the agglutinated species Glomospira fijiensis and Rheophax scorpiurus appear to be most tolerant to decreasing oxygen conditions at the anoxic/oxic interface. The foraminiferal faunal association most closely resembles assemblages that have been described from mangrove environments of the South Pacific. The specific distribution pattern and the absence of foraminifera within the anoxic zone is consistent with findings from volcanic sulfide-rich environments. The physico-chemical properties of this stratified lake, in particular the lack of dissolved oxygen and the high concentration of hydrogen sulfide within the anoxic water layer, appear to control this particular presence/absence pattern of benthic foraminifera.

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