At two locations in southern Lake Huron (U.S.A.), twelve 35.5-cm diameter cores of fine-grained sediments were taken for comparison of the vertical distributions of 210pb and faUlout 137Cs with the distributions of benthic macroinvertebrates, mainly oligochaete worms (Tubificidae) and the amphipod, Pontoporeia affinis. Locations were selected on the basis of 210pb distributions measured a year earlier which indicated contrasting depths of mixing of surface sediments. At one location the activity of 21 opb is uniform down to about 6 cm and 95% of total invertebrates occur within this zone; at the other location the zone of constant activity is only 3 cm deep but 90% of the invertebrates occur within it. Comparison of published tubificid defecation rates with sediment accumulation rates based on 2 lOpb shows that oligochaetes alone can account for mixing in one case while the effects of amphipods may be required in the case of shallower mixing. If mixing is represented as a diffusional process, eddy diffusion coefficients are at least 5.8 and 3.3 cm 2 yr-1 at respective locations. In comparison with bioturbation, molecular diffusion is of minor importance in the post-depositional mobility of 137Cs" The necessity for introducing a diffusion coefficient varying continuously with depth is indicated by characteristics of the distribution of 137Cs" Biological reworking of near-surface sediments is an important process affecting radioactivity and chemical profiles in profundal deposits of this and probably other Great Lakes.

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