Microbes versus Metazoans as Dominant Reef Builders: Insights from Modern Marine Environments in the Exuma Cays, Bahamas

Abstract

Mixed microbial–metazoan reefs, with distinct lateral zonation of microbialites, coralline algae and coral, fringe the windward-facing margin of two islands in the Exuma Cays, Bahamas: Stocking Island and Highborne Cay. These reefs offer a unique opportunity to identify and characterize controls governing the spatial distribution of microbial versus metazoan reef builders in a modern setting. Distribution patterns characterized by prolifi c stromatolite development in back-reef lagoons, prominent coralline algal growth along reef crests, and rare coral development on seaward reef edges are a direct response to intensity and frequency of sediment stress, sand abrasion and burial of reef biota. The recent discovery and dating of an outcropping coral reef underlying stromatolites in the southern Highborne Cay reef serves as a basis for interpreting the late Holocene evolution of this reef and addressing the question: what led to the demise of a metazoan and rise of a microbial reef? Comparison of growth histories of Highborne and Stocking Island reefs allows us to explore processes that have governed microbial versus metazoan reef building from mid-Holocene to present, and to speculate on future reef development at these sites. Antecedent topography and late Holocene sea-level transgression determined the timing of reef nucleation at both sites. Initial reef development was relatively unaffected by sedimentation due to a lag in sediment production relative to rising sea level. Once produced, sediment was deposited on the Pleistocene terrace, which due to rapid sea-level rise was below average wave base and sediment suspension. Slowing of sea-level rise combined with increasing sediment production decreased accommodation space. Mobile sediment progressively impinged upon the reef environment and, following the emergence of an algal ridge, sediment was trapped in the back-reef area. Although corals can tolerate some sediment stress, they cannot cope with extended periods of burial. Increasing sediment stress and more importantly increasing amplitude and frequency of sediment burial are interpreted as dominant factors leading to the demise of metazoan reef builders and the prolifi c growth of microbialites in the back-reef lagoon. Microbial buildups are surprisingly young (<1000 years) and a recent addition to the fringing reef systems along the Exuma margin. The study of modern analogues is important for our processorientated understanding, which in turn provides concepts and hypothesis to apply and validate in the rock record.