Influence of cockle Cerastoderma edule bioturbation and tidal-current cycles on resuspension of sediment and polycyclic aromatic hydrocarbons

Experiments were performed to investigate the impact of cockle population density Cerastoderma edule on the resuspension of naturally contaminated sediments collected from the Tamar estuary (SW England). Annular flumes generated tidal-current cycles for 7 to 9 d. The suspended sediment concentration (SSC) at peak flow increased 5-fold with increasing cockle population density, although the 2 highest densities yielded similar resuspension. Polycyclic aromatic hydrocarbons (PAHs) in the sediment were analysed by gas chromatography/mass spectrometry (GC/MS) at the beginning of the experiment, and in the water column of each flume after 2 and 6 d at both the maximum and minimum current speeds. At the end of each experiment sediment erodability was measured as a function of a stepwise increase in current speed. Sediment erosion increased up to 10-fold with increasing cockle population density. However, at the highest density the SSC was lower than that observed for the medium density, probably due to binding resulting from higher mucus secretion and pseudofaeces production. Current-induced resuspension of contaminated sediment was dependent on the density of the cockles, The correlation between the suspended sediment concentrations and the concentrations of PAH was weak for low molecular weight PAHs (phenanthrene and anthracene) due to their higher water solubility. In contrast, higher molecular weight PAHs (fluoranthene, pyrene, benz(a)anthracene and chrysene) showed a strong correlation with suspended particulates as a result of their higher hydrophobicity.