Analysis of foraminifera from the Tagus Estuary, Portugal

The distribution and microhabitat of living benthic fora- minifera (15 calcareous and 6 agglutinated) have been studied in two box cores from the Tagus Prodelta. Stable oxygen and carbon isotopes were analysed for eight different species from six surface samples from the Tagus Prodelta and Estuary. At the two box core stations, most of the living foraminifera were restricted to the oxygenated top cm of the sediment and generally show a shallow infaunal behavior. Those taxa are e.g. Rectuvigerina phlegeri, Stainforthia fusiformis and species of the genus Bolivina, which is the most abundant genus in the Tagus Prodelta. Infaunal species are found down to 10 cm depth, and some infaunal taxa, e.g. Bulimina marginata, Globobulimina auriculata and Nonionella turgida, inhabit the low oxic or anoxic sediments. The deep infaunal species are suggested to feed selectively, on refractory organic matter or on the bacterial stocks, while the opportunistic shallow infaunal species are believed to feed on fresh phytodetritus or labile organic matter. Our data show that there is a close connection between the concentration of foraminifera and the distribution of organic matter in the area. The highest abundance of living benthic foraminifera was found in sediments close to the Tagus river plume, where the sediments have relatively high organic carbon contents. The spatial distribution of the stable isotope values of different benthic foraminifera reflects the distribution of the low salinity and relatively high temperature water with high organic carbon fluxes within the Tagus Estuary.