Introduction: Marine Lakes Research

 

Marine lakes are exceptional ecosystems because they contain atypically small and isolated populations of divergent marine organisms that have inhabited peripheral, unusual, environments for at least many thousands of years. The marine lakes in Palau are uniquely valuable because nowhere else in the world is there known to be such an high concentration or variety of these novel ecosystems. These lakes are the marine analogs of the Galapagos Islands, the origins of modern evolutionary thought in terrestrial systems.

Marine lakes form when depressions in porous, fissured, karst (limestone) landscapes are flooded by rising sea-level. Damped tidal flux, shelter from wind, high rainfall, and the lack of distinct seasons in the tropics can lead to vertical stratification by salinity and temperature and anoxic, foul, bottom waters. However, not all lakes have such extreme physical structure. Marine lakes are variously connected to the lagoon, forming a continuum from completely isolated to almost lagoon-like, and constitute a wide variety of habitats and assemblages. Such environmental heterogeneity yields a range of novel selective regimes with the potential to foster rapid evolution and adaptive radiations of marine lake organisms.

Hamner et al. (1982) described the unique water column physics, chemistry, and biology of "Jellyfish Lake" [Ongeim'l Tketau], emphasizing its unique ecological structure. These features were compared with "strange" aspects of other marine lakes in Palau (Hamner, 1982; Hamner & Hamner, 1998) which may have contributed to the unprecedented, lake-specific, behaviors, physiologies, and morphologies of golden jellyfish, Mastigias sp. indicative of adaptive evolution and speciation in these novel environments (Hamner & Hauri, 1981; Muscatine & Mariam, 1982; McCloskey et al., 1994). Biodiversity in the marine lakes also has been discovered in the forms of a new actinarian species, Entacmaea medusivora (Fautin & Fitt, 1991), and novel compounds from sponges (Reddy et al., 1997, 1998). However, the importance of marine lakes is not limited to biological studies. Jellyfish Lake also has been the setting for studies of novel diagenetic processes and sedimentary records of short-term climatic variations in the tropics (Burnett et al., 1989; Landing et al., 1991; Orem et al., 1991; Bates et al., 1993; Lyons et al., 1996). In summary, Hamner & Hamner (1998) wrote that the marine lakes through "a comparative acquaintance with the bizarre ... clarify the familiar" (p.217).

With the bizarre and familiar in mind, we have been investigating the ecology and evolution of the marine lakes and other coastal ecosystems in Palau since 1994. In the following pages we present a short review of our work.