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Content archived on 2024-06-16

Influence of biological and physical processes on intertidal sediment dynamics and on the release of pollutants trapped in sediments and the toxicity of these pollutants

Final Activity Report Summary - BIO-ENGINEERS (Influence of biological and physical processes on intertidal sediment dynamics and on the release of pollutants trapped in sediments ...)

The aim of this study was to understand and quantify the influence of biological and physical processes on intertidal sediment dynamics and on the release of contaminants trapped in sediments. Since industrialisation in the late 1800s, the amount of contaminants and toxic substances entering estuaries has greatly increased. The sediment acts as a sink for many particle-adsorbed contaminants such as trace-metals (e.g. copper, cadmium and zinc) or organics (polycyclic aromatic hydrocarbons PAH). The sediment can also act as a subsequent contamination source, by either diffusion from the pore water or/and by resuspension of sediments. Sediments resuspension can be due to both physical (tidal cycles currents, storms) and biological disturbance. Organisms that have an impact on the sediment stability are called ecosystem engineers. They are either increasing the sediment stability (=bio-stabilisers; e.g. seagrass beds, macroalgae mats, mussel beds) or decreasing the sediment stability and increasing its resuspension (=bio-destabilisers or bioturbating animals which disturb the sediments by feeding and burrowing; e.g. bivalves or worms).

The present project has investigated the joint effects of simulated tidal-current cycles and bioturbation on sediment destabilisation and contaminant resuspension from natural sediments. For this purpose, two types of studies have been realised. First, the effects of bioturbation and simulated tidal cycles on near-bed hydrodynamics and sediment dynamics have been investigated. Secondly, the effects of bioturbation and simulated tidal cycles on sediment resuspension and the consequent release of contaminants from historically contaminated sediments have been analysed and quantified. All the experiments were performed in annular flumes using natural undisturbed sediment collected on the river Fal and on the river Tamar (South West of England). Continuous sinusoidal cycles of currents (3 to 18 cm/s) were generated in the flumes to mimic 6h tidal cycles. The common cockle Cerastoderma edule was used as the bioturbating/destabiliser organism. This bivalve disturbs the sediment with its range of shaking, vertical, and horizontal movements. The mussel Mytilus edulis was used as a bioindicator of the potential sub-lethal toxicity of contaminants released into the water column. At the end of each experiment, current speeds were increased from 5 to 50 cm/s in 11 steps to measure suspended sediment concentration, velocity profiles and turbulences profiles at each step.

The study showed that an increase in cockle density increased the bed roughness and decreased the near-bed velocity relative to the free-stream velocity. It also increased the near-bed turbulence and the bed shear stress. An increase in bed shear stress increased the sediment erodability and the sediment resuspension. The erosion velocity threshold (the velocity required to resuspend 1 g/m2 of sediment) decreased with increasing cockle density. This demonstrated that an increase in the density of bioturbator organisms modified both the near-bed hydrodynamics and the sediment stability, by increasing turbulence and physical disturbance of the surface sediments.

The combined effects of bioturbation and simulated tidal cycles resulted in the release of PAH from the sediments into the water column. The resuspension of sediment increased the release of high-molecular weight PAHs associated with sediment particles. Low-molecular weight PAHs, because of their higher water solubility, were less affected by an increase in sediment resuspension. In metal contaminated sediments only currents were responsible for increasing the release of metals (Cu, Cd, Zn) from the sediments. The measured feeding rates of mussels exposed to the resuspended contaminated sediments were surprisingly not affected by the various conditions, indicating that the contaminated sediments from the Tamar and Fal were not inducing a sublethal toxic effect.