LARGE BENTHIC FORAMINIFERA IN SEA-GRASS HABITATS: STANDING CROP VERSUS SEDIMENT ASSEMBLAGES AT SAN SALVADOR, BAHAMAS

Shallow-water benthic foraminifera in tropical and semi-tropical environments have received much research attention because they are significant producers of calcium carbonate and because of their importance as paleoenvironmental indicators. The purpose of the present study is to investigate the relationship between large (> 1 mm) foraminifera living on vegetation and those found (live and dead) in the sea-floor sediment. One objective was to establish substrate preference for the dominant foraminiferal taxa. Furthermore, we hypothesized that, because most taxa are phytal, both the species composition and relative abundance of foraminifera in the sediment would correspond to the types and overall density of the vegetation.

Six localities representing a range of vegetation densities, water depths, and water-energy conditions were selected: three in Grahams Harbor and three in the Snow Bay area. SCUBA was used to record vegetation densities by counting individual algae and sea grass in 50x50 cm grids located at 10m intervals along transects. The top 1 cm of sediment was collected from each station, and one hundred individuals of each of the major genera of sea grasses and algae were collected for each locality. Samples were preserved in buffered formalin. In the laboratory, vegetation was examined with a binocular microscope, and foraminifera were picked from sediment fractions larger than 0.5 mm. Live specimens were recognized by the presence of colored protoplasm. Total foraminiferal density was recorded as the number of individuals per plant and per gram of sediment picked.

Preliminary results show that Thalassia and Halimeda are the favored substrates for phytal foraminifera. Cornuspiramia, Sorites, and Planorbulina are the dominant foraminifera on both of these, in proportions that vary according to locality. Archaias and Cyclorbiculina are rarely attached to vegetation but are the most plentiful taxa in the sediment, even as live individuals. Thus, the assemblages do not match. Cornuspiramia, Sorites, and Planorbulina are susceptible to breakage and therefore less likely to be preserved in the sediment, while Archaias is more durable. Foraminiferal density in the sediment corresponds to vegetation density to some degree, but other factors (possibly water energy and nutrient supply) also play a role.