Influence of organic enrichment on the partitioning and bioavailability of cadmium in a microcosm study

Interactive processes between organic enrichment and cadmium (Cd) were examined in a microcosm experiment over a 10 wk period. Microcosms were stocked with 4 benthic species: Amphiura filiformis, Amphiura chiajei, Abra alba and Nereis diversicolor. The partitioning of Cd between the sediment and pore water phases and the subsequent bioaccumulation of Cd in benthic macrofauna were investigated under different organic enrichment regimes. Additions of Cd without organic enrichment resulted in significant increases in both the sediment and pore water Cd concentrations; the magnitude was much greater in the pore water, with a 3-fold increase indicating that the pore water phase showed the greatest fractional changes in response to Cd input. The subsequent bioaccumulation of Cd to 3 benthic species, Amphiura filiformis, Amphiura chiajei and Abra alba, also increased with this Cd only treatment, as in the ambient sedimentary environment. Cd additions combined with different organic enrichment regimes of cellulose fibres, sewage and benthic algae, at low and high doses, significantly altered the Cd partitioning in the sedimentary matrix. Both algal treatments and high sewage levels significantly decreased the sediment redox potential, with a corresponding increase in sulfide activity. All organic types reduced pore water Cd concentrations, showing a highly significant organic dosage response. However, Cd bioaccumulation in the 3 benthic species examined showed significant variation between the different types or sources of organic material, with an increasing gradient of Cd bioaccumulation in the sequence: fibres < sewage < algae. The growth response of N. diversicolor represented a biological measure of benthic food 'quality' and followed this same increasing trend in response to organic enrichment. Therefore, direct concordance was shown between the effect of organic type on polychaete growth and Cd bioaccumulation, indicating that high quality, potential benthic 'food', such as algae, leads to increased Cd bioaccumulation. This study demonstrates that organic 'quality' or a related parameter forms a most significant factor in determining Cd bioaccumulation to the marine benthos, and that some sources of organic enrichment do indeed reduce metal bioaccumulation.