A UNIQUE ALGAL RIDGE SYSTEM IN THE EXUMA CAYS, BAHAMAS

An algal ridge system discovered along the Exuma Gays, Bahamas constru cts bioherms to a thickness of at least 1.5 m and is associated with m odern intertidal stromatolites. These algal ridges are unique because they grow in atypical environments characterized by relatively low wav e 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 bran ches and survivorship of transplanted N. strictum were greatest in the shallow fore reef zone of the algal ridge. The alga is also capable o f surviving and growing when covered with sediment for at least 100 da ys. Under such conditions it transforms from a branched to an unbranch ed morphology. Parrotfish grazing, which is said to limit the abundanc e of branched corallines and algal ridges, was two orders of magnitude lower than in published accounts from other reef systems of the Carib bean and one order of magnitude less than that found on nearby coral r eefs of the west Exuma Sound. Neogoniolithon strictum, a delicate and open-branched coralline, persisted for over a year without grazing dam age when transplanted to a depth of 2.3 m. This algal ridge-building c oralline becomes a well-indurated limestone following submarine lithif ication of sediment that infills the open branch framework. As a resul t, N. strictum ridges are comparable to the dense frameworks associate d with most algal ridges. Observations of N. strictum -associated bioh erms along Central America suggest this ridge system may exist elsewhe re under conditions similar to those described for the Bahamas.