Article: Construction and preservation of two modern coralline algal reefs, St. Croix, Caribbean
The internal structures of two coralline algal reefs from St. Croix are described. The primary framebuilders are Lithophyllum congestion, which dominates in exposed mid-intertidal situations, and Porolithon pachydermum from the high intertidal. Secondary frameworks are constructed by one of the following corallines: Tenarea, Lithothamnium ruptile, Mesophyllum syntrophicum, Lithophyllum congestion, and Neogoniolithon sp., together with Homotrema and vermetid gastropods. The environmental preferences of these corallines and their recognition in slabbed reef sections permits a reconstruction of past reef morphologies and environments. Predictable ecological successions are found within preserved coralline sequences which correspond with previous settlement plate experiments. The main agents of reef destruction are sponge and echinoid bioerosion. Inter-reef sediments are winnowed by wave currents and reflect the composition of surrounding coral reefs in addition to debris from the coralline algal reefs. A relatively low proportion of coralline algal debris in sediments around the reefs is thought to result from deposition of silt-sized sponge chips of corallines in quieter water elsewhere. Internal reef sediments reflect the composition of the reef constructors.