EFFECTS OF BIOTURBATION BY THE LUGWORM ARENICOLA-MARINA ON CADMIUM UPTAKE AND DISTRIBUTION IN SANDY SEDIMENTS

The effect of bioturbation by the lugworm Arenicola marina on uptake a nd distribution of cadmium in sediment was assessed using laboratory s ediment cores. Carrier-free Cd-109 was added to the water phase each d ay. Bioturbation (irrigation) was measured using bromide (Br-) as a tr acer for water movement. In cores without lugworms all Cd was found in the surface sediment where it continued to build up over 16 d of expo sure. In cores containing lugworms a distinct peak of Cd was found bot h at the sediment surface and, after a few days, at the feeding pocket of the worm (10 to 15 cm depth). During the 16 d of exposure this sub surface peak broadened and eventually Cd was found in all depths from top to feeding depth of the individual worm. Compared to sediment core s without worms, the presence of lugworms more than doubled the rate o f removal of Cd from water to sediment. This was attributed to an incr eased turnover of sediment (due to feeding activity), an increased sed iment surface area (due to fecal casts, head shaft, tube and irrigatio n of the whole burrow) and an increased contact of Cd-labelled water w ith potential binding sites in the sediment due to irrigation. Exposur e to 1 ppm Cd reduced the fractional rate at which lugworms removed Cd from the water (as % of Cd in the water). The total Cd flux to the se diment, however, was much greater due to the higher Cd concentrations in the water. Water fluxes estimated using Br- as a solute tracer reve aled a 10- to 20-fold increase in water exchange of the sediments when lugworms were present. This enhanced water flux was not affected by e xposure of lugworms to 1 ppm Cd. The results indicate that the presenc e of bioturbating infauna influences both the uptake rates of trace me tals in near-shore sediments and the distribution of those metals.