Direct underwater methods were used to carry out an ecological and oceanographic study of a coral-reef tract. The reef tract, called the Hopetown reef, is located off Elbow Cay, one of a chain of small islands that parallels the coasts of Little and Great Abaco islands. The geological evidence gathered suggests that the chain of small islands is part of a ridge formed from a lithified wind-laid sand dune over a line of coral remains. The reef studied is established at the edge of a wave-cut bench.
Analyses of the reef sediments disclose a gradient of sediment size from the reef barrier to the shore, with the greatest concentration of benthic animals in the finer, more even-sized sediments in the shoreward half of the reef area. Temperature and salinity gradients did not exceed the limits of tolerance for the coral animals. Light, through its influence on the symbiotic zooxanthellae, limits the development of the coral-reef growth to less than 50 feet. Light also influences the depth and position of many of the corals. The data gathered indicate that wave activity plays a significant role in coral growth and hence in horizontal zonation. Tidal currents flush the reef area completely twice daily and are important in controlling the amount of sediment deposited throughout the reef.
The zones of the reef were established by sampling and measuring along a transect across the reef area. The significant organisms and the ecological factors most important to each zone were established. The zones identified were the Sea-Fan–Surf Zone, Montastrea–Deep-Water Zone, Acropora palmate–Active-Wave Zone, Porites–Moderate-Wave Zone, and the Echinoid–Weak-Wave-Action Zone. Several distinctive phases may be found in the various zones of which the cavern phase is most distinctive.
Within the lagoon area between the line of small islands and the main island shore, six major types of ecological association were identified. Tidal flow and bottom structure are the most important physical factors in these relationships.
In summary, the study indicates the importance of the slope of the bottom in the reef area to the over-all structure of the developing reef. Of major importance to the development of Bahamian coral reefs is the presence of a ridge seaward of the main island masses and whether or not the ridge is submergent or emergent. This factor will also regulate the position and development of the coral reef. The conclusion is reached that seaward of the main reef barrier the reef masses may be entirely self-built. In the remainder of the reef, the reef structures may have had a nucleus of either a truncated section of the island mass or a collapsed portion of a cliff that had fallen into the sea. Finally, the rapid erosion of Elbow Cay indicates that all the land masses may eventually be truncated.