Benthic Foraminifera at Platform-Margin Reefs, San Salvador Island, Bahamas

Although foraminifera in the Florida-Bahamas region have been the subject of numerous actualistic studies, those found at the shelf edge have received relatively little attention, presumably because of water depth and distance from shore. However, the small, isolated carbonate platform at San Salvador Island provides excellent opportunities for such research. Platform-margin sites currently under study begin at 12-18m deep and are located as close to shore as 300m. A variety of profiles, ranging from sandy slopes to shear “walls” with or without ledges, crevasses, and caves, are found in depths available to SCUBA divers before the seafloor gives way to bathyal depths.

Wave and tidal action focused at the shelf edge preclude the establishment of seagrass-macroalgal communities in the unstable sandy seafloor. Halimeda and non-calcareous algae typically grow only on hard substrates, such as coral heads and rubble. Epiphytal foraminifera found on Halimeda are rare and are dominated by Cornuspiramia, Acervulina, and Planorbulina. Those found on soft algae resemble platform-top assemblages except for high proportions of the rotaliids Amphistegina gibbosa and Asterigerina carinata compared to the miliolids Archaias angulatus and Cyclorbiculina compressa.

Large tests found in the sediment on slopes at the platform margin are dominated by platform-top taxa (chiefly Archaias and Cyclorbiculina), which are commonly highly altered taphonomically. Ledge assemblages, in contrast, contain a suite of free-living agglutinates that are not found in shallow-water habitats. Also present are a significant number of attached species, some cemented on loose grains and others that have been broken free of their attachment sites on the wall above. Indigenous assemblages (found live) can be recovered from protected areas such as reentrants in the wall, where introduction of transported tests is minimized. These include Nubecularia(?), Haddonia(?), and Sporadotrema. Thus, platform-margin foraminiferal faunas are distinctive and may reveal the nature of the platform-edge bathymetric profile.