effect of Oligochaete worms on soulte transport. In each of two cells con- 

 taining natural sediment and lake water, we added a submillimeter layer of 

 cesium-labeled sediment and about 20 microCuries ».f sodium-22 as NaCl. To 

 one cell we then added worms to achieve a density of about 70,000 m-2. Fol- 

 lowing this treatment the two cells, control and worm, were scanned about 

 once a day for over 10 days. The results of the experiment are illustrated 

 in Figure 10. Profiles of cesium-137 and sodium-22 are shown after an 

 elapsed time of about 200 hours. In the control cell, there is no signifi- 

 cant displacement of the marked layer while in the worm cell, the layer has 

 moved downward by an amount corresponding to a rate of about 0.055 cm/day. 

 In the worm cell, the Na-22 has penetrated further into the sediments than 

 in the control. Note that the measurements in the worm cell were made about 

 40 hours earlier than in the control. Thus the downward movement of the 

 sodium-22 would be even more pronounced if the profiles could have been 

 taken at the same time. The solid curve is the expected distribution of 

 sodium-22 based on a solution to the diffusion equation with values of the 

 diffusion coefficient chosen to give the best least squares fit to the data. 

 In the control cell, the effective diffusion coefficient is 3.9 x 10"^ 

 cm^/sec while in the worm cell, the value is 13.1 x 10"^ cm^/sec. Thus, the 

 presence of tubificid worms at a density of about 70,000 m"^ enhances the 

 diffusion coefficient by over a factor of 3. In a separate experiment where 

 the sediments had been conditioned by allowing worms to create an equili- 

 brium system of burrows, but where there was no active reworking at the time 

 of adding radiotracers, the diffusion coefficient for Na-22 transport was 

 still enhanced (x2) over its value in a control cell having no conditioned 

 sediments. Therefore, it seems that the enhancement of pore water diffusion 

 by tubificid worms results from their loosening of the sediments through the 

 creation of a system of burrow channels rather than to their momentary life 

 activities. Thus, the short-term effect of reducing or terminating the bur- 

 rowing activity of worms through exposure to aquatic pollutants would seem 

 to be small but the long-term result would appear to be the collapse of the 

 burrow structure with an associated reduction in the ability of ions to 

 migrate through pore fluids. 



With proper experimental design, the radiotracer method could be used to 

 examine the effect of aquatic pollutants on benthos-mediated transport of 

 solutes. However, a more direct approach is to relate measured sediment- 

 water fluxes to the density of activities of benthos. 



NUTRIENT FLUXES FROM UNDISTURBED SEDIMENT CORES 



We have taken this approach in collecting a series of cores from various 

 locations in the Great Lakes (Remmert et aj^. 1977; Robbins et^ al_. 1976). 

 Undisturbed 7.5 cm diameter cores of fine-grained sediments from Lakes 

 Michigan, Huron, and Erie were stored in in situ temperatures ('v^50° C) in 

 their original plastic liners along with a^out 10 cm of overlying water. 

 Increases in the concentration of reactive dissolved silica over periods of 

 hours to days in stirred, oxygenated overlying water provided estimates of 

 the rate of exchange of dissolved silicon across the sediment water inter- 

 face. The increases in the concentration of silicon (ppm Si) versus time 

 is shown for a core from Saginaw Bay, Lake Huron in Figure 11. The release 



214 



