A ‘Bremerhaven caisson' was used to investigate the interactions of cadmium (Cd) and lead (Pb) with sea water, particulate matter (SPM), sediments, and biota in an enclosed (13m2) tidal mud flat area on the island of Borkum (FRG). Artificial contamination was accomplished by injecting Cd as a chloride and Pb as a nitrate into the caisson so as to maintain metal concentrations of 100 µg/1 in the inflowing water of each tidal cycle for 25 days. Each component of the system was sampled at selected intervals inside and outside the caisson. The results show that most (85%) of the injected Cd remained in the soluble phase whereas most (90%) of the Pb was transferred to the particulate phase. Cyclic changes in particulate Cd and Pb concentrations occur and indicate that Cd is progressively taken up by the SPM over each tidal cycle, but after an initial rapid uptake, particulate Pb concentrations remain relatively constant in the system. In the sediments, Cd concentrations decrease with depth but increase with time, with the highest increases occurring in the surface sediment layers. Lead concentrations initially remain constant with depth, but eventually Pb becomes enriched in the top 3 cm of the sediments. Biota show an exponential increase in Cd and Pb during the experiment. Lead is enriched relative to Cd by a factor of 2 in mussels despite its low level in the dissolved phase. Infaunal benthic species have lower metal concentrations than the filter feeders.

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