ZONATION, GROWTH RATES, AND CALCIUM-CARBONATE PRODUCTION OF ATTACHED (ENCRUSTING) FORAMINIFERA AT SAN SALVADOR, BAHAMAS

Despite the long history of research on benthic foraminifera in the Florida-Bahamas region, not much attention has been given to species that are firmly attached to a hard substrate as opposed to those that are free (mobile). However, attached (encrusting) foraminifera are good paleoenvironmental indicators and play a significant role in calcium-carbonate production.

In the first phase of our research, cobbles were collected along shore-to-shelf-edge transects, and foraminifera attached to the undersides were examined in 10-cm² quadrats: individuals were counted, their taphonomic conditions were recorded, and their areas were assessed using ImageJ. Our findings demonstrated a marked zonation: (1) Homotrema was very abundant and well preserved in near-shore sites, (2) Planorbulina dominated a diverse community of attached species at mid-shelf, and (3) large Gypsina plana was extensive at platform-margin (wall) locations. To test the hypothesis that environmental differences led to differences in growth rates (fastest near-shore, slowest at the wall), concrete blocks with travertine tiles attached were deployed in water depths ranging from 1.5 to 31.4 meters in March 2011 and recovered in June and September. The results largely confirm the zonation seen on cobbles and underscore the slow rate of growth at the wall, where very few attached organisms of any kind were seen even after one year; among these were small (4.3mm) G. plana. Shaded environments in platform-top reefs have an abundance of attached foraminifera after 1-2 years.

In terms of calcium-carbonate production, the current phase of our work, attached foraminifera play a substantial role when compared to other encrusters. This is demonstrated by relative surface area and by their ranking with regard to mass-per-area values: low-profile taxa with thin walls (e.g., crustose coraline algae and bryozoa) are the lightest; corals and tubular serpulid polychaetes are intermediate; while certain serpulids and foraminifera, especially those with high profiles (e.g., the globular morphotype of Homotrema rubrum) and/or thick walls (e.g., Nubecularia sp.) are the heavyweights. Further assessment of the role of attached foraminifera will enhance our understanding of shallow-water carbonate systems, providing a baseline for anticipated global changes.