Physiography, Ecology, and Sediments of Two Bermuda Patch Reefs

In order better to characterize the features of patch reefs, we studied two reefs located in Bermuda's north lagoon. Both are less than 150 m wide and rise steeply from the relatively featureless lagoon floor at 18 m depth, to within 1 or 2 m of sea level. On the rough mesa-like reef tops, coral- and algae-covered knobs are interspersed with sediment-filled valleys and hollows. There is no pronounced zonation of the organisms or reef structures across the reefs, although one reef is atoll-like, having a coral-algal margin surrounding a sandy plain. Massive corals, principally Montastrea annularis and Diploria strigosa, are the most important reef-framework builders, as revealed in sections made by blasting. Uneven coral growth is thought to be the major factor influencing the shape of reef cavities which are common throughout the reef mass. The walls of these cavities are veneered with distinctive wall growths built by the skeletons of shade-loving organisms, principally encrusting ectoprocts, coralline algae, bivalves, and foraminifera. Sediments of the reef top are generally coarse-to-very-coarse sands with good-to-moderate sorting. All the particles are derived from the skeletons of organisms dwelling on the reefs, although the proportion of Halimeda fragments increases with an increase in sandy substrate on the reef. Cavity sediments are generally finer grained; their contribution to the total mass of reef sediment is difficult to estimate, although it is thought to be large. Some reef sands are washed down the reef face to build a steep reef sediment slope, but at distances greater than 10 m from the reef, the reef-derived constituents become quantitatively unimportant. On the near-reef lagoon floor, fine sediments winnowed from the reef settle from suspension to form a "halo of fines" around the reef.