Distribution of Fe, Mn, Cu and Cd in Upper Sediments and Sediment-Trap Material of Ria Formosa (Portugal)

Abstract

In shallow highly productive coastal environments intense biogeochemical activity is concentrated in the upper sediment layers. However, pore water composition of permeable inter-tidal sediments change as water flows over the areas during ebb and flood tides. In order to examine metal post-mobilisation, inter-tidal sediment cores of 4 cm length and particles settled in sediment-trap s over several periods during 10 months were collected in Ria Formosa (SW Iberian Peninsula). Concentrations of Al, Fe, Mn, Cu and Cd, loss on ignition and the proportion of fine fraction were determined in sediment layers of 0.5 cm thickness and in sediment-trap material. Levels of Mn in sediments were similar to those in sediment-trap material, although enrichment in the topmost sediment layer (0.1 mm) was observed as MnO 2 is formed. Although Fe in sediments and sediment-trap material ranged within similar intervals, two relationships with Al were found showing that sediment-trap material are impoverished in Fe in comparison to sediments. This suggests that Fe is mainly cycled inside the sediment. Copper and Cd exhibited different patterns. Levels in sediment-trap material were not correlated to Al or LOI, probably due to local contamination or uptake by plankton. These values were lower than levels in the topmost sediment layer , suggesting that Cu and Cd that reach the sediment surface are rapidly released to the water column. Concentrations in sediments, including the topmost layer, exhibited two relationships to Al, for both Cu and Cd. One corresponding to background values and the other found in stations closer to the anthropogenic sources. In these cases Cu and Cd are incorporated into the sediments that is facilitated by AVS values close to the sediment surface. ADDITIONALINDEXWORDS: Surface sediments, heavy metals, geochemistr y, coastal lagoon, Portugal.