Resumen de Sediment dynamics and associated heavy metals in small river-and wave-dominated inner shelf (Barcelona city, Nw Mediterranean)
Wave storms and river inputs have been found to be the dominant forcing mechanism of sediment transport in continental shelves dominated by medium to large rivers around the world (e.g. Drake and Cacchione, 1985; Sherwood et al., 1994; Ogston and Stemberg, 1999; Traykovski et al., 2000; Wadman and McNinch, 2008) and in the northwestern Mediterranean Sea (Jiménez et al., 1999; Puig et al., 2001; Guillén et al., 2002, 2006; Palanques et al., 2002, 2011; Ferré et al., 2005; Roussiez et al., 2005; Ulses et al., 2008). However, the contribution of “small” Mediterranean river systems in fine-grained sediment dynamics is not well known. Flood and storm events redistribute not only sediment but also all the associated matter as well, such a heavy metals, which are indicators of the impact of industrial and urban activities. For these reasons, the present thesis project involves studying the dynamics of sediments and associated heavy metals across the inner continental shelf off the Besòs River (Barcelona), and in particular determining the effect of floods and storms in a littoral system affected by industrial and urban activities. To this end, several oceanographic surveys were carried out between 20 and 40 m water depths in the Barcelona continental shelf to obtain a variety of data: to register vertical hydrographical profiles (temperature, salinity and turbidity); to record time series of physical parameters (currents, salinity, temperature, turbidity and seabed variation) by deploying oceanographic equipment; and to take sediment samples (short cores and Van Veen grabs) and suspended and downward particulate matter from water samples and sediment traps for further sedimentological and geochemical analysis. The time period of the present study comprised three seasons, from autumn 2007 to spring 2008, which showed distinct features between them. The purpose of section 3 was to describe the main hydrographic and hydrodynamic conditions during the study period to support the interpretation of sediment transport processes. Three different periods in terms of storm and river activity matched the natural seasons of the year: The period comprised between end of September 2007 and December 2007 (autumn) was characterized by an intense frequency of northeastern-eastern storms mostly coupled with high but short river discharges and a convergence of the across-shelf current between 20 m and 30 m water depths; between January and early March 2008 (winter), the wave and river activity and frequency were reduced significantly with offshore currents dominating the across component; and finally, from March to mid June 2008 (spring), specially between May and June 2008, river discharges lasted more than one month and occurred under low wave energy conditions or in conjunction with S-SE and SW wave storms. The sediment response to the forcing conditions observed during the study period was analyzed in section 4. The grain size and porosity measurements taken in all the sediment samples collected during the experiment (sediment cores, sediment grabs and sediment traps) showed a high spatial and temporal variability of the bottom sediment properties, downward sediment fluxes and near-bottom sediment concentrations across the inner-shelf. Those changes were associated to periods of high wave shear stress, river and current energy and therefore, were more intense in autumn 2007 and spring 2008. However, an across shelf spatial variability was observed during this period which was associated to the sediment availability (i.e. critical shear stress gradients). As a result, the same storm produced larger nearbottom sediment concentration in deeper waters due to the limitation on the availability of resuspendable sediment in the near-shore. The resultant sediment dynamics across the inner shelf and results in sediment transport modeling during a resuspension event were then addressed in section 5. In the inner shelf off Barcelona, the general circulation of sediment transport was mainly directed towards the southwest (along-shelf) during the study period, however, the seaward component was considerably relevant and favored the segregation of coarse and fine sediment from the nearshore towards deeper areas. Nonetheless, noticeable differences in sediment transport patterns were observed across the inner shelf. Nearbottom sediment transport at 20 m water depth was mainly offshore, while in deeper parts of the inner shelf the along-shelf component dominated the sediment transport. The resulting sediment transport and its variability across the shelf, deposited riverine and storm-derived fine sediment in an along-shelf path towards the southwest between 20 m and 30 m water depth and only under the strong storms were transferred seaward toward the 40 m water depth site and to deeper areas. These differences had in turn a strong seasonal component related to the availability of fine sediments from river inputs and the energy of waves and currents. In this sense, autumn and spring registered events affected mostly at 20 m water depth and autumn and early winter months were more energetic at deeper waters reversing the sing of sediment transport gradients across the shelf. During this event, the sediment transport model captured adequately the transport rates across the inner shelf and allowed the estimation of the integrated vertical sediment flux in 10 meters of the water column above the seabed. Consequently with the observed and modelled pattern of sediment transport, the seabed variation was higher at 30 m than at 20 m water depth, with a total seabed erosion of about 10 cm and 4 cm, respectively. The implications of the sediment dynamics during the study period in the transference of heavy metals from the Besòs River across the shelf was analyzed in section 6. The Besòs River introduced heavy metal pollutants into the nearshore continuously associated with the regular regime and sporadically during increments in river discharge. Only during high wave and current energy the anthropogenic contamination, previously deposited in the shallow inner-shelf, reached deeper areas. As a result, the most contaminated sediment accumulated southwestward from the river and sewer mouths along the inner shelf decreasing offshore. The most affected area associated to the Besòs river influence was located around the tripod sites, especially between 20 m and 30 m water depths decreasing offshore.Finally, the main conclusions and proposals for future research of this thesis are discussed in section 7. In it, the importance of "small" Mediterranean river systems in the transfer of sediment across the continental shelf is highlighted from the sediment transport events observed during the study period.
