Distribution of encrusting foraminifera at Mayaguana, Bahamas: Determining assemblage composition and relationship of test size and density to food availability
Type of DegreeMaster's Thesis
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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. Previous research on San Salvador and Cat Island, Bahamas, has created models for the density and occurrence of these encrusting foraminifera on carbonate platforms. The data in this study, which focuses on nearby Mayaguana, are compared to those of the previous experiments done on San Salvador and Cat Island. Cobbles were collected by both SCUBA and snorkeling at seven different sites that ranged from nearshore reefs to shoals to mid-shelf patch reefs and the platform margin. The sites vary considerably across localities regarding the size, density, and relative proportion of encrusting foraminifera. Nearshore localities contained considerable amounts of Homotrema rubrum and Planorbulina, with considerable site-to-site variability. Shoal environments not examined in previous Bahamian studies, were dominated by Homotrema rubrum in count and area, as was a bank barrier reef. Mid-shelf patch reefs were dominated by Planorbulina spp., whereas the platform margin was characterized by numerous large Gypsina plana. The size and density of foraminifera decreased from onshore to offshore, which compares well with prior results from Cat Island. All five morphotypes of Homotrema rubrum were encountered at the shoals and bank barrier reef, although morphotype data gathered on Planorbulina spp. was inconclusive. Almost all data are consistent with previous research performed on San Salvador and Cat Island, which supports the model of encrusting foraminifera distribution developed by Tichenor and Lewis (2009). Water samples were collected from beneath cobbles using a syringe and were buffered with a solution of 5% formalin. Three ml of each sample was analyzed using light microscopy to count and identify organisms within a certain area. Three water samples from two nearshore sites, the patch reef site, and the platform-margin site were studied. Microbes, pennate diatoms, and black “amorphous masses” were found at all sites, although they occurred in much greater abundance at the two sites located within the lagoon. There is very little relationship between the types or number of water taxa and water depth, density, or size of encrusting foraminifera at each locality, suggesting that food particles in water are just one of many variables that control the distribution of encrusting foraminifera. Sediment samples were collected from beneath cobbles using a plastic container. A 5% solution of formalin was added to buffer the samples, which were stained with Rose Bengal and preserved in 190 proof ethanol. Three 1-ml subsamples were analyzed using light microscopy to count and identify stained organisms at each site. Two sediment samples were taken at all sites, except for Goniolithon Shoal, for a total of 39 ml of sediment analyzed. Nearshore sites varied significantly in their counts of meiofauna, which included foraminifera, crustaceans, nematodes, and annelids. A strong correlation was found between counts of meiofauna and encrusting foraminiferal size, density and assemblage composition, which suggests that more potential food particles in sediment influences the distribution of encrusting foraminifera.