Sediment (60 mg dry weight in 5 ml 0.2 mm filtered seawater held in a centrifuge tube) was radiolableled with radiotracers. Radioisotope additions were 54 kBq l21 (corresponding to 6.6 nM) for 109Cd (in 0.1 N HCl), 92.7 kBq l21 (corresponding to 0.2 nM) for 51Cr (in 0.1 N HCl), and 54 kBq l21 (corresponding to 6.4 nM) for 65Zn (in 0.1
N HCl). Prior to the radioisotope additions, a microlitre quantity of 0.5 N NaOH was added to maintain the pH at 8. The radiotracers were radiolabeled (or equilibrated) onto sediment for 5 days (or 10 days in the oxic / anoxic experiment described below), and then centrifuged twice at 800 3 g for 20 min and the supernatant was discarded. The sediment pellet was resuspended in 5 ml 0.2 mm filtered seawater before being fed to the bivalves. Partitioning of the radiotracers onto the sediment was not determined in our study.