EFFECT OF DEPOSIT FEEDERS ON MIGRATION OF 13 7 Cs IN LAKE SEDIMENTS

IIlite clay particles with adsorbed 137Cs were added as a submillimeter layer to the surface of stir-clay sediments contained in rectangular Plexiglas ceils stored in a temperature-regulated aquarium, in order to tmee the effect of the ollgochaete, Tubifex tubifex, and the amphipod, Pontoporeia hoyi, on mass redistribution near the sedimentwater interface. A well-collimated NaI gamma detector scanned each sediment column (~10 cm deep) at daily or weekly intervals for six months, depicting the time evolution of radioactivity with and without added benthos. In a ceil with tubificids (~5 X 104 m-2), which feed below 3 cm and defecate on surface sediments, the labeled layer was buried at a rate of 0.052 ± 0.007 era/day (20°C). When labeled particles entered the feeding zone, 137Cs reappeared in surface sediments creating a bimodal activity prof'de. In time, the activity tended toward a uniform distribution over the upper 6 cm, decreasing exponentially below to undetectable levels by 9 cm. In a cell with amphipods (~1.6 × 104 m -2) uniform activity developed rapidly (-17 days) down to a well-defned depth (1.5 cm). The mixing of sediments by Pontoporeia is described by a simple quantitative model of eddy diffusive mixing of sediment solids. The value of the diffusion coefficient, 4.4 cm2/yr (7°C) was computed from a least squares fit of theoretical to observed profile broadening over time. In a cell without benthos, small but measurable migration of 137Cs indicated an effective molecuhr diffusion coefficient of 0.02 cm2/yr.