DISTRIBUTION OF ENCRUSTING FORAMINIFERA AT MAYAGUANA, BAHAMAS: DETERMINING ASSEMBLAGE COMPOSITION AND RELATIONSHIP TO FOOD AVAILABILITY

Benthic foraminifera that are cemented to hard substrates are known as encrusting or attached foraminifera. Their fixed nature makes them more likely than free foraminifera to remain in their original habitat after death; this, combined with their sensitivity to environmental variables such as light and water energy, makes encrusting foraminifera potentially valuable for paleoecology. A pattern of onshore-to-offshore distribution with Homotrema rubrum dominating assemblages nearshore, Planorbulina common in diverse mid-shelf assemblages, and large Gypsina plana found at relatively deep sites at platform margins has been recorded at San Salvador and at Cat Island, Bahamas. In addition, density and size of individuals was shown to decrease from onshore to offshore at Cat Island, suggesting that conditions for growth and reproduction diminish.

The objectives of this actualistic study was (1) to see if the same pattern of distribution occurs at Mayaguana, located some 215 km southeast of San Salvador, and (2) to take water and sediment samples to see if food availability could be involved in the distribution noted. We sampled 7 sites representing a range of environments and collected 6 cobbles per site. We used syringes to take 20-ml water samples under each cobble and collected 100-ml samples of sediment under the cobbles where possible.

As predicted based on past research, (1) cobbles at the mid-shelf patch reef have a diverse assemblage dominated by Planorbulina; (2) the bank barrier reef site is Homotrema-rich, including a notable amount of the globular morphotype along with erect Carpenteria and sparse, relatively small Gypsina plana; and (3) cobbles at the wall site at the platform margin (water depth: 22m) are dominated by large Gypsina plana. Homotrema are few and are very small. Assemblages at one of the near-shore sites are dominated by Homotrema and Nubecularia as we have seen previously; another site has a higher than expected percentage of Planorbulina and relatively few Homotrema. Two additional sites can be described as mid-shelf shoals, an environment not previously encountered by our research team. In these offshore, shallow-water (1-2m) sites, foraminifera can be very dense and are dominated by Homotrema. Meiofauna and microbial remains in the water and sediment samples are currently being analyzed.