Thesis submitted to the Universidade Nova de Lisboa,Faculdade de Ciências e Tecnologia for the degree of Doctor of Philosophy in Environmental EngineeringEstuaries are areas of high sediment dynamics. Particles in suspension are an important vehicle of several biochemical substances and contaminants. Therefore, the knowledge on the processes ruling
sediment dynamics is needed to develop tools for estuarine water quality management. Presently, few numerical models for sediment dynamics incorporate biological interactions with sediment dynamics.
The aim of this study is to gain understanding of the macrobenthic influences on cohesive sediment dynamics. The research was focused on the effects of two species of macrobenthos; 1) the cockle Cerastoderma edule (densities of 280 to 1000 ind m-2). Work on this species focused on sediment transport and deposition, by studying the hydrodynamic effect on the sediment removal
activity (filtration) and its effects on topography and on the current velocities at the boundary layer 2)
The ragworm Nereis diversicolor (densities of 320 to 1200 ind m-2). This work focused on sediment erodability and consolidation by studying the bioturbation effect on changes in the sediment properties,and 3) the effect of contamination (3 nmol Cu g-1 dw) on the bioturbation activity of N. diversicolor
and on sediment dynamics.
The results from experiments performed in a racetrack and in an annular flume showed that
increasing density of C. edule is proportional to increasing sediment topography and related to reduced
current velocities near the bed and increased shear velocity (u*), hydrodynamic roughness length and turbulence kinetic energy (TKE). The higher TKE values were related to the presence of active
filtering C. edule, producing additional turbulence to the TKE produced by topography. The effect of filtration activity on turbulence is most pronounced at low velocities (u* < 1.5 cm s-1), in agreement with a unimodal response to increased velocities. Increasing densities of N. diversicolor are related to increased sediment shear strength (SS), increased biodiffusion coefficients (Db) and increased erosion rates (ER). This antagonistic effect of increase SS and ER is explained by erosion of bigger aggregates resulting from biological bound sediments.
This study provides some evidence that copper contaminated sediments are more stable, as a consequence of decrease in biological response to toxicity, observed in lower values of shear strength and erosion rates. In addition, new methodologies for the determination of some of the parametersinvolved with this research field are suggested.This thesis was financed by Fundação para a
Ciência e Tecnologia SFRH/BD/6188/200
