to make quantitative observations of processes occurring within it. The no- 

 tion of using radiotracers to measure aspects of behavior in a noninterac- 

 tive way can be extended to include species other than benthos, other be- 

 haviors, and other aquatic microcosms. 



So far, we have not applied the method for carefully controlled assay of 

 toxic substances but only for several trivial cases where it was important 

 to demonstrate that the addition of major ions to water overlying cells 

 would not affect reworking rates. Still, general features of the experi- 

 ments are useful to consider. Either sulfate (SO4) or chloride (CI) ions 

 were added as Na2S04 or NaCl to a series of cells containing Oligochaete 

 worms and a surface-labeled layer of sediment. Prior to addition of the 

 ions, the reworking rate in each cell had been measured with the scanning 

 system. Results are shown in Figure 8 for addition of NaCl. Below 5000 

 micrograms Cl/ml (ppm) no change in reworking rate was observed while the 

 rate decreased abruptly following addition of NaCl at a concentration in 

 overlying water of 10,000 ppm. In this case, the reduction was probably not 

 a behavioral but rather a mortality effect. The results for sulfate are 

 given in Figure 9 for two species of Oligochaete worms, Tubifex tubifex de- 

 rived from Lake Michigan sediments and laboratory culture of Limnodrilus 

 hoffmeisteri . The ratio of final to initial reworking rate is shown versus 

 concentration. Again, significant decreases in reworking rates occur only 

 for the very high concentrations used in the experiment. These levels of 

 course far exceed any encountered in most lakes. There appear to be signi- 

 ficant differences in the response of the two Oligochaete populations to SO4 

 additions. More important experiments will involve additions of metals and 

 toxic organics to these microcosms. Such work would represent a continua- 

 tion of studies by others, notably Brkovic-Popovic and Popovic who have in- 

 vestigated the effect of heavy metals on survival (1977a) and on respiration 

 rate (1977b) of tubificid worms. Problems will arise in the interpretation 

 of the effects of nonconservative substances on the system, which were far 

 less significant in the case of conservative ions like sulfate and chloride. 

 Nonconservative materials may rapidly adsorb to sediment particles and 

 little meaning may be attached to the concentration in overlying water. As 

 sophistication develops in the use of such radiotracer behavioral assay 

 methods, it will be desirable to take the community approach as there is 

 considerably evidence for species interaction effects (Petr 1976). In re- 

 lated studies, it would be desirable to look at the relation between toxic 

 substance exposure and the ability of benthos to avoid predation (Hall et 

 al . 1979). A further effect which can be studied with relative ease with 

 this method is the response of benthos to chronic oxygen depletion. Under 

 conditions of oxygen depletion, feeding of tubificid worms is minimal. In 

 sediments of Lake Easthwaite, worms spend most time at the mud-water inter- 

 face (Stockner and Lund 1970) but resume feeding with the restoration of 

 aerobic conditions. 



The scanning method described above can also be used to investigate the 

 effects of benthos on interstital transport. By using both a particle-bound 

 radiotracer such as cesium-137 (Kj '^ 5000, Robbins et al. 1977) and a rela- 

 tively conservative gamma emitting isotope, sodium-22 Xl<(j ^ 2, Lerman and 

 Weiler 1970), reworking rates and molecular diffusion rates can be deter- 

 mined simultaneously. In a prototype experiment, we have investigated the 



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