Relationships between large benthic foraminifera and their seagrass habitats, San Salvador, Bahamas

Abstract

Vegetation beds are important in the ecology and depositional history of platform carbonates. The vegetation itself, frequently dominated by the seagrass Thalassia, has a low preservation potential, but plants and benthic algae are substrates for a variety of epibionts, including foraminifera, which have a significant fossil record. This actualistic study examines the potential of large benthic foraminifera to serve as proxies for the presence, taxonomic composition, density, and environmental variables of ancient vegetation beds. Six sites having a range of vegetation densities, water depths, and water energies were selected. Grass-bed densities were measured by counting individual sea grasses and algae in 50x50-cm grids. One hundred individuals of each of the major vegetation genera were collected from each site, as well as the top 1 cm of sediment. Foraminifera were picked from the collected vegetation and from the 2.0 to 0.5-mm sediment fractions. Live and dead foraminifera were counted from vegetation and sediment samples, and total foraminiferal density was recorded. Dead foraminifera were categorized by taphonomic condition. Cluster analysis was used to group sites according to foraminiferal data and by vegetation type. Sorites and Planorbulina were the dominant live foraminifera found on the favored substrates, Thalassia and Halimeda. Sediment assemblages from the same sites were dominated by (dead) Archaias, Quinqueloculina/ Triloculina, and Valvulina. Cluster analysis shows that the sediment foraminiferal assemblages do not separate according to vegetation type. The density of tests in the sediment corresponds to vegetation density to some degree, but factors such as nutrient supply may play major roles. Thus, the species composition and density of foraminifera alone do not accurately reflect the taxonomic composition and density of sea-grass beds. In contrast, taphonomic condition is a consistent indicator of the presence and density of sea-grass beds. Lush stands of Thalassia protect delicate tests on the seafloor. Here altered tests are frequently encrusted, whereas breakage and abrasion are more common in higher energy, sand-dominated sparse beds. For these reasons, taphonomic states should be included in carbonate-platform foraminiferal studies.