Accumulation of Zn, Pb, Cu, Cr and Ni in Sediments Between Roots of the Tagus Estuary Salt Marshes, Portugal

Sediment cores of 60 cm length were collected from two Tagus estuary salt marshes. At each salt marsh, samples were taken from a non-vegetated zone and one from each of areas dominated by Halimione portulacoides, Spartina maritimaand Arthrocnemum fruticosum.Cores were sliced in situand, at each sediment layer, redox potential and pH were measured, and the organic matter content (LOI), grain size, and concentrations of Zn, Cu, Pb, Ni and Cr were determined. Sediment between roots and non-vegetated sediments of the same depth (5 –15 cm) were extracted with several acid solutions, and the metal concentrations were compared. Metal residues were determined in roots of vascular plants. Sediment between roots was more oxidative, more acidic and richer in organic matter than non-vegetated sediment. Profiles of Zn, Pb and Cu concentrations in vegetated sediments differed from those recorded in non-vegetated areas: at subsurface layers (where root density is higher), Zn, Pb and Cu were enriched. The percentages of Zn, Pb and Cu removed by acetic acid, nitric acid and DTPA extractions from sediment between roots were much lower than those from non-vegetated sediments, being preferentially linked to the residual fraction. Chromium and Ni behave differently no subsurface enrichment being found and their associations being similar in the two types of sediment. Furthermore, Ni concentrations in roots were much lower than in bulk sediments, while levels of Zn and Pb were similar and Cu values higher. These results point out that plants are an important feature for metal accumulation in salt marshes.