BIOCENOSES OF BENTHIC FORAMINIFERA OF THE AVEIRO CONTINENTAL SHELF (PORTUGAL): INFLUENCE OF THE UPWELLING EVENTS AND OTHER SHELF PROCESSES

This work aims to compare the dimensions and composition of benthic foraminiferal biocenoses (living specimens) during two summer sampling events. Forty-four sediment samples were collected on the Aveiro Continental Shelf (Center of Portugal) (latitude of 40º30'N-40º50'N, longitude of 8º46'W-9º20'W) for granulometry, total organic matter (TOM) and living foraminiferal analyses. The sediment samples were collected during summers of 1994 and 1995, on stations located along transects (east-west direction) and between 10-200 m depth. During the sampling campaigns, measurements of salinity, temperature and density data were recorded in the water column. The results showed that the living assemblages were mainly found in stations located between 20-80 m depth. The abundance of living foraminifera was generally reduced at depths <20 m in the so-called “coastal deposits”, where the sediments are frequently remobilized and transported by the littoral drift. Living benthic foraminiferal densities were also reduced in stations at 80-200 m depth, despite the high sedimentary TOM contents. Results obtained in this work indicate that, in this marine setting, the most determinant factors for the dimension and composition of living foraminifera are not the sediments’ granulometry and organic matter content. In fact, the coastal dynamics, sediment stability, availability of food quality, among other factors, such as the bottom salinity oscillations and their combination, should better explain the abundance of living foraminifera and the biocenoses composition

Early diagenesis and clay mineral adsorption as driving factors of metal pollution in sediments: the case of Aveiro Lagoon (Portugal)

This work aims to define the factors driving the accumulation of metals in the sediment of the lagoon of Aveiro (Portugal). The role of initial diagenetic processes in controlling trace metal retention in surface sediment is traced by mineralogy, magnetic susceptibility and geochemical analyses. Although several studies have focused on the metal distribution in this polihaline and anthropized coastal lagoon, most of them have been solely focused on the total metal concentrations. This study instead represents the first attempt to evaluate in a vast area of the Aveiro Lagoon the role of biogeochemical processes in metal availability and distribution in three extracted phases: exchangeable cations adsorbed by clay and elements co-precipitated with carbonates (S1),organic matter (S2) and amorphous Mn hydroxides (S3). According to the sediment guideline values, the sediment is polluted by, for instance, As and Hg in the inner area of theMurtosa Channel, Pb in the Espinheiro Channel, Aveiro City canals and Aveiro Harbour, and Zn in the northern area of the Ovar Channel. These sites are located near the source areas of pollutants and have the highest total available concentrations in each extracted phase. The total available concentrations of all toxic metals are however associated, firstly, with the production of amorphous Mn hydroxides in most of the areas and, secondly, with adsorption by organic compounds. The interplay of the different processes implies that not all of the sites near pollution sources have polluted surface sediment. The accumulation of metals depends on not only the pollution source but also the changing in the redox state of the sediments that may cause alterations in the sediment retention or releasing of redox-sensitive metals. Results of this work sugsuggest that the biogeochemical processes may play a significant role in the increase of the pollutants in the sediment of the Aveiro Lagoon