BENTHIC FORAMINIFERAL ASSEMBLAGES IN THE CURRENT-DOMINATED STRAIT OF BONIFACIO (MEDITERRANEAN SEA)

摘要

This study explores relationships between benthic foraminiferal assemblages and environmental factors in the Strait of Bonifacio, which separates the islands of Corsica and Sardinia, west of Italy Peninsula. Benthic foraminifera were identified in 31 samples collected during an oceanographic cruise (‘‘Bocche 2003’’ – P.I.C Interreg III Project) that provided new geomorphologic and sedimentologic data on the western continental shelf of the strait. Biotic parameters (species diversity, density, Fisher a index, Shannon-Weaver index, and Dominance) were calculated and multivariate analyses (Cluster Analysis, Principal Component Analysis, and Canonical Correspondence Analysis) were performed to quantify foraminiferal assemblages in the context of environmental parameters (depth, current velocities, sediment texture, and organic matter). The most abundant benthic foraminifera in the surface sediments were Elphidium cf. E. advenum, E. complanatum, E. crispum, Lobatula lobatula, Miliolinella subrotunda, Planorbulina mediterranensis, Quinqueloculina laevigata, Rosalina vilardeboana, Spiroplectinella sagittula, and Textularia agglutinans. Their spatial distributions strongly correlated with bathymetry and hydrodynamic energy, as well as texture and organic matter content of sediments. Three group of stations were differentiated from environments that varied mainly by hydrodynamic energy. The first group included the shallowest stations, close to Corsica and Sardinia, where sediments were mainly silty sand. Three of these stations had the highest proportions of organic matter. The foraminiferal species in that group are divided into two assemblages. In the first assemblage most of the 20 species were present in homogenous proportions, while the second assemblage was dominated by R. vilardeboana and L. lobatula. The two other groups of stations were characterized by the influence of strong currents, and coarse sand and gravel substrates with the lowest proportions of organic matter. In the second group the five dominant species, E. crispum, E. concameratus and T. agglutinans appeared to be best adapted to this high-energy environment. In the third group of species, the presence of P. mediterranensis and S. sagittula was at its height. Other notable findings were: 1) strong current velocities supported the dominance of epibenthic foraminifera (e.g., L. lobatula and P. mediterranensis) permanently attached to seagrass and algae; 2) a strong affinity for substrates with more abundant organic carbon and higher C/N ratios was shown by M. subrotunda, Neoconorbina nitida, Quinqueloculina stelligera, Rosalina globularis, R. vilardeboana, and Sigmoilinita costata; and 3) the distribution of S. sagittula is closely linked with bathymetry. These results indicate that benthic foraminifera assemblages can be useful as hydrodynamic energy proxy for a valuable characterization of specific environments.