Living benthic foraminiferal assemblages along a latitudinal transect at 1000m depth off the Portuguese margin

Abstract

The relationships between the quantity and biochemical composition of organic matter and the species diversity and community structure of living benthic foraminiferal assemblages have been investigated at four sites on the Portuguese margin (NE Atlantic). All of the sites are located at approximately 1000m depth. Two out of the four sites are located in the Nazaré and Cascais submarine canyons, while the other two are positioned on the adjacent open slope. The composition and vertical distribution in the sediment of the foraminiferal assemblages have been investigated in the topmost 10cm for the >150µm fraction and in the top cm for the 63-150µm size fraction. Foraminiferal abundance and species richness are related to the quantity and biochemical composition of the sedimentary organic matter, as well as to the stability of the sea floor. The open slope stations are characterised by a relatively low quantity and nutritional quality of organic matter. The faunas of the two open slope stations are much poorer than those found in Nazaré canyon. At both stations, there is a succession of shallow, intermediate and deep infaunal species, suggesting a fairly deep oxygen penetration into the sediment. At the northern open slope station, the faunal density is about two times lower than at the southern station, where the fauna is very largely concentrated in the uppermost half cm. This difference coincides with a lower nutritional quality of organic matter at the northern station. The faunas of the two canyons, where the quantity and nutritional quality of bio-available organic matter are higher, are very different. The rich fauna of Nazaré canyon is characterised by a strong dominance of intermediate and deep infaunal species (e.g. Melonis barleeanus and Chilostomella oolina) in superficial sediment layers, suggesting a low bottom-water oxygen concentration and a minimal oxygen penetration into the sediment. InCascais canyon themuch poorer faunas of the superficial sediment layers are characterised by the co-occurrence of shallow, intermediate and deep infaunal taxa, again suggesting a rather limited oxygen penetration into the sediment.We suggest that the relatively low densities in the Cascais canyon could reflect an early stage of ecosystem colonisation after a recent turbidite deposition.