Geographical and bathymetric distribution of foraminiferal assemblages from the Alboran Sea (western Mediterranean)

A total of 176 surface sediment samples, recovered in the Alboran Sea by the oceanographic vessel Vizconde de Eza during the 2002 and 2003 campaigns, were analysed for their foraminiferal content. Planktonic and benthic foraminifera were picked up, studied and classified. In addition, a series of diversity indices were calculated. An attempt was made to correlate diversity (Fisher alpha index) with longitude, latitude and depth. Planktonic assemblages are strongly dominated by the species Globorotalia inflata (d'Orbigny) together with Globigerina bulloides d'Orbigny. Correlation analysis show only a weak relation between planktonic foraminiferal diversity and both longitude and distance to the coast, which in the Alboran Sea are strongly related with depth. Benthic assemblage diversity is correlated with longitude, thus reflecting a West to East diversity increase pattern, and with depth, latitude and distance to the coast, suggesting a diversity decrease with depth. No significant correlation has been found between average chlorophyll concentration and either planktonic or benthic foraminiferal diversities. A Q-mode Hierarchical Cluster Analysis (HCA) recognised four clusters with different bathymetric and geographical distribution: Cluster A, mostly in the easternmost part of the studied area, in the Almeria Gulf, is composed of shallow highly diverse samples, dominated by Cibicidoides lobatulus (Walker & Jacob) and Textularia agglutinans d'Orbigny, indicating current-influenced environments with close algal or vegetation-covered areas; Cluster B includes the deepest, less diverse assemblages, strongly dominated by the low-oxygen tolerant species Uvigerina mediterranea Hofker and Uvigerina peregrina Cushman; Cluster C groups many outer shelf-upper bathyal samples along the Malaga and Almeria coast with opportunistic species like U. peregrina, Cassidulina laevigata d'Orbigny, Bulimina marginata d'Orbigny and U. mediterranea that reflect abundant organic supply in an upwelling area and, finally, Cluster D, representing shallow relatively diverse samples, mainly located on the Alboran Ridge, and with predominating Dorothia pseudoturris (Cushman), Textularia pseudorugosa Lacroix, Elphidium crispum (Linné) and C. lobatulus related with coarse-grained current-influenced sediments and nearby vegetation-covered areas.