The impact of the Mediterranean Outflow Water (MOW) on benthic foraminiferal assemblages and surface sediments at the southern Portuguese continental margin

Benthic foraminifera and surface sediments were analyzed from 23 box-core and core-top samples from 250 to 3600 m depth at the southern Portuguese continental margin between 37 and 38 °N. This area is affected by the MOW which flows northwards along the slope between 600 and 1500 m waterdepth. Sediment structures, grain-size distribution and clay mineral assemblages of surface sediments indicate active winnowing in the upper part of the MOW, and deposition of the fine fraction near its lower boundary. R-mode cluster analyses of foraminiferal census data from the > 250 μm fraction revealed four associations. The ‘Shelf Edge Association’ occurs on glauconitic shelf and upper slope sands down to 268 m. The ‘Upper Slope Association’ is recognized between 498 and 1300 m where sand-silt-clays to clayey silts are encountered. The ‘Lower Slope Association’ is found between 1405 and 2985 m and differs markedly from assemblages below 3000 m (‘Deep Water Association’) where the near-surface sediments are soft hemipelagic clays. The ‘Boundary Layer Association’ shows no significant depth limits but abundance maxima both in the density interface above the MOW and in the interval of decreasing turbulence below it. The boundaries of the ‘Shelf Edge’ and ‘Upper Slope Association’ correspond to these hydrographic boundary layers whereas the boundary between ‘Lower Slope’ and ‘Deep Water Assemblage’ is related to threshold values of nutrient flux to the benthic community. Examination of hard-substrates reveals 27 different species of epibenthic foraminifera. Six of these, the ‘Epibenthos Group’, were found attached to elevated substrates only within the MOW. Above the MOW, elevated substrates were inhabited by different species, whilst below the MOW they were not used by any epibenthic foraminifers. Evidently, the hydrodynamic environment of the MOW current provides an ecological niche which is efficiently used by opportunistic suspension feeders. The ‘Epibenthos Group’ is more abundant in the middle part of the study area between 37 °20′ and 37 °40′N where the sand content of surface sediments is also higher. Both indicate an enhanced sediment-water interaction in this area which results from an obstruction of the MOW flow by the ‘Principes d'Avis’ basement spur.