Diatom assemblages as upwelling indicators in surface sediments off Portugal

Abstract

Diatom abundance and assemblage composition have been determined for 169 surface sediment samples from the Portuguese margin, an area where seasonal upwelling occurs each year from April to October. Absolute diatom abundance distribution patterns reflect the more intense and/or persistent and more homogeneous upwelling north of the Nazare Canyon, the position of the “inner” upwelling front south of Lisbon, and the eastern continuation of the coastal upwelling jet along the Algarve coast. Distribution patterns of species and of ecologically defined groups, meroplanktonic, oceanic, freshwater and marine benthic, support the hypothesis that diatom species and assemblages are good indicators of the changing character of the upwelling system. Small species of the genus Thalassiosira occur in areas of persistent upwelling where availability of nutrients is more constant and/or higher. Paralia sulcata (Ehr.) Cleve is an abundant species whose mean cell diameter increases as nutrient availability decreases. Resting spores, mainly of Chaetoceros Ehrenberg, record the position of the zone where higher variability occurs, the “inner” upwelling front.