to indicate some preliminary results of exposing benthos-sediment microcosms 

 to toxic substances. 



STRATIGRAPHIC EFFECTS OF NATURAL POPULATIONS 



In early attempts to interpret radioactivity profiles in sediments of 

 the Great Lakes (Robbins and Edgington 1975) it became clear that signifi- 

 cant mixing of material occurred over the upper 10 cm of sediment. From 

 later work (Robbins et^ aj_. 1977) it was evident that the sediment mixing was 

 due to the presence of benthic organisms. At two locations in Lake Huron, 

 twelve cores of fine-grained sediment were taken for comparison of the 

 vertical distributions of the naturally occurring radionuclide, lead-210, 

 and fallout cesium-137 with the distributions of benthic macroinvertebrates. 

 In the absence of mixing, the activity of lead-210 should decrease exponen- 

 tially with sediment depth reflecting radioactive decay (T]/2 " 22.26 yr) on 

 burial. In actuality, the lead-210 activity was constant down to 6 cm in 

 cores at one location and 95% of the total invertebrates occurred within the 

 zone of constant activity. At the other location, the zone of constant 

 activity was only 3 cm deep but more than 90% of the benthos were confined 

 to it. In each case comparison of published tubificid reworking rates with 

 sediment accumulation rates showed that the activities of benthos were able 

 to account for the mixing of sediments. An example of the effect of sedi- 

 ment mixing on cesium-137 profiles is given in Figure 1 for a core from Lake 

 Erie where the sedimentation rate is exceptionally high. The observed al- 

 teration in the radioactivity profile over that expected in the absence of 

 steady-state mixing is consistent with the measured vertical distribution of 

 benthos which at this location consists primarily of mature and immature 

 Oligochaete worms. Studies of the distribution of natural and fallout 

 radionuclides in cores from Lake Erie (Edgington and Robbins 1979), Lake 

 Huron (Johansen and Robbins 1977) and Lake Michigan (Edgington and Robbins 

 1975) show that the mixing of surface sediments occurs widely in the Great 

 Lakes. 



It may thus be expected that altered patterns of sediment mixing result- 

 ing from exposure of benthos to aquatic pollutants could result in altered 

 and possibly uninterpretable radioactivity and heavy metal profiles. From 

 our studies (Robbins 1977) it is apparent that the time resolution with 

 which lake-wide pollution changes can be reconstructed from sedimentary re- 

 cords is limited by benthic reworking (bioturbation) . Increased benthos 

 mortality would be likely to improve the long-term resolution because of the 

 associated reduction in sediment mixing. 



LABORATORY STUDIES USING RADIOTRACERS 



To investigate the role of benthos in the transport of sediment parti- 

 cles in a controlled ans systematic way, experiments were set up in the lab- 

 oratory using a particle-bound radiotracer, cesium-137. Illite clay parti- 

 cles with adsorbed cesium-137 were added as a submi llimeter layer to the 

 surface of fine-grained sediments contained in plastic cells of a rectangu- 

 lar cross section stored in a temperature-regulated aquarium. A well-col li- 



202 



