A unique algal ridge system in the Exuma Cays, Bahamas

Abstract.

 An algal ridge system discovered along the Exuma Cays, Bahamas constructs bioherms to a thickness of at least 1.5 m and is associated with modern intertidal stromatolites. These algal ridges are unique because they grow in atypical environments characterized by relatively low wave energy, high rates of sedimentation and low rates of herbivory. They also are composed primarily of the branching crustose coralline alga, Neogoniolithon strictum, which heretofore was not known to form algal ridges. Lateral growth rates of crusts, vertical growth rates of branches and survivorship of transplanted N. strictum were greatest in the shallow fore reef zone of the algal ridge. The alga is also capable of surviving and growing when covered with sediment for at least 100 days. Under such conditions it transforms from a branched to an unbranched morphology. Parrotfish grazing, which is said to limit the abundance of branched corallines and algal ridges, was two orders of magnitude lower than in published accounts from other reef systems of the Caribbean and one order of magnitude less than that found on nearby coral reefs of the west Exuma Sound. Neogoniolithon strictum, a delicate and open-branched coralline, persisted for over a year without grazing damage when transplanted to a depth of 2.3 m. This algal ridge-building coralline becomes a well-indurated limestone following submarine lithification of sediment that infills the open branch framework. As a result, N. strictum ridges are comparable to the dense frameworks associated with most algal ridges. Observations of N. strictum -associated bioherms along Central America suggest this ridge system may exist elsewhere under conditions similar to those described for the Bahamas.