COMPARISON OF ENCRUSTING FORAMINIFERA AT BANK BARRIER REEFS AND LAGOONAL PATCH REEFS FOUND AT SAN SALVADOR AND CAT ISLAND, BAHAMAS

Benthic foraminifera that are cemented by calcium carbonate or are otherwise firmly fixed to hard surfaces are known as encrusting or attached foraminifera. Relatively few actualistic studies focus on the use of encrusting foraminifera as paleoenvironmental indicators compared to the vast literature on free foraminifera. However, their sensitivity to environmental factors such as light and water energy, and the fact that they are fixed in place and therefore are likely to remain in the original habitat after death makes encrusting foraminifera valuable in paleoecology.

The present study draws from over a decade of work at San Salvador island, supplemented by research at Cat Island, Bahamas, in March 2014 and June 2015. The primary data set comes from reef-rubble cobbles, with additional data from deployed travertine tiles and sea-floor sediment samples. The objective of this research is to investigate the value of assemblages of encrusting foraminifera by contrasting two distinct environments: (1) bank barrier reefs (BBR), characterized by breaking waves and dense stands of Acropora palmata and Millepora, and (2) lagoonal patch reefs (PR), which are found in quieter waters behind the barrier reefs, where the corals Montastrea annularis and Diploriaare common.

BBR assemblages at both islands are dominated by the well-known red foraminifer Homotrema rubrum, which typically occurs in dense growths on the undersides of cobbles. On cobbles and on deployed tiles, the high-profile, inflated (globular) morphotype of the species is more common here than it is at the patch-reef sites. The large, crustose Gypsina plana is also characteristic of BBR assemblages. Sediment samples reflect the high densities of Homotrema by their large percentages in grain counts. Patch-reef assemblages are more varied than are BBR assemblages, but typically are very diverse with Planorbulina acervalis as the most abundant species followed by Homotrema rubrum (morphotype: “multiple”). Density (foraminifera per area on surfaces) is lower at PR than at BBR sites, and this is mirrored in sediment-sample grain counts. The consistent differences between BBR and PR assemblages at the two islands seem to indicate a pattern that can be used in paleoenvironmental analysis and warrants continued research.