Communities and microhabitats of living benthic foraminifera from the tropical East Atlantic

Living (Rose Bengal stained) benthic foraminifera were collected with a multicorer from six stations between 2°N and 12°S off West Africa. The foraminiferal communities in the investigated area reflect the direct influence of different productivity regimes, and are characterized by spatially and seasonally varying upwelling activity. At five stations, foraminiferal abundance coincides well with the gradient of surface productivity. However, at one station off the Congo River, the influence of strong fresh water discharge is documented. Although this station lies directly in the center of an upwelling area, foraminiferal standing stocks are surprisingly low. It is suggested that the Congo discharge may induce a fractionation of the organic matter into small and light particles of low nutritional content, by contrast to the relatively fast-sinking aggregates found in the centers of high productivity areas. Quality and quantity of the organic matter seem to influence the distribution of microhabitats as well. The flux of organic carbon to the sea-floor controls the sequence of degradation of organic matter in sediment and the position of different redox fronts. The vertical foraminiferal stratification within sediment closely parallels the distribution of oxygen and nitrate in porewater, and reflects different nutritive strategies and adaptation to different types of organic matter. The epifauna and shallow infauna colonize oxygenated sediments where labile organic matter is available. The intermediate infauna (M. barleeanum) is linked to the zone of nitrate reduction in sediments where epifaunal and shallow infaunal species are not competitive anymore, and must feed on bacterial biomass or on metabolizable nutritious particles produced by bacterial degradation of more refractory organic matter. The deep infauna shows its maximum distribution in anoxic sediments, where no easily metabolizable organic matter is available.

Sediment fraction analysis and population density of living larger foraminifera in the Persian Gulf (Table 1A, 1B)

Living Heterostegina depressa were found in the Persian Gulf on shallows and sides of islands in the Central Basin. Preliminary culture experiments furnished information on life span, salinity tolerances and population density of species. Reproduction processes (probably asexual) could be observed several times. A possible carbonate production of ca. 150 g/year/m**2 has been estimated.

Abundance of living coccolithophores in the northern Arabian Sea (Table 2)

The present study was conducted to provide information about living coccolithophores from the northern Arabian Sea as potential proxies in palaeoceanographic studies. In all, 71 plankton samples from 16 stations collected in September 1993 were analysed for their contents of living coccolithophores. Absolute abundances range from less than 400 coccospheres per litre in surface waters to 35 000 spheres per litre at intermediate water depths. From 49 identified taxa, nine species contribute significant cell numbers of more than 2000 coccospheres per litre and comprise more than 10% of the communities in at least one sample. Important species are (in approximate order of cell abundances): Gephyrocapsa oceanica, Florisphaera profunda, Oolithotus antillarum, Calciosolenia murrayi, Umbellosphaera irregularis, Emiliania huxleyi, Umbellosphaera tenuis, Calciopappus rigidus, and Algirosphaera robusta. At most profiles, a vertical succession of coccolithophore species was found. Calciosolenia murrayi and C. rigidus were restricted to surface waters, whereas high numbers of F. profunda and A. robusta occurred at depths below 40 m. The coccolithophore communities reflected the local oceanographic situation and seemed to be more dependent on mixed layer depth and nutrient availability than on temperature and salinity changes. Additionally, synecologic competition with diatoms in part controlled the species composition and generally reduced the abundance of coccolithophores. Synecological and ecological tolerances of species were discussed with the help of cluster analysis.

Distribution of living benthic foraminifera in sediment cores of cruise M77 Leg 1 and 2, 63-2000 µm fraction

Recent benthic foraminifera and their distribution in surface sediments were studied on a transect through the Peruvian oxygen minimum zone (OMZ) between 10 and 12°S. The OMZ with its steep gradients of oxygen concentrations allows to determine the oxygen-dependent changes of species compositions in a relatively small area. Our results from sediments of thirteen multicorer stations from 79 to 823 m water depth demonstrate that calcareous species, especially bolivinids dominate the assemblages throughout the OMZ. The depth distribution of several species matches distinct ranges of bottom water oxygen levels. The distribution pattern inferred a proxy which allows to estimate dissolved oxygen concentrations for reconstructing oxygen levels in the geological past.