mated sodium iodide crystal gaima detector scanned the length of each of 

 several cells at daily or weekly intervals over a period of several months 

 in order to determine how several benthic species transported labeled parti- 

 cles away from the sediment surface. The experimental set up with an ex- 

 posed aquarium containing several cells, a detector and a counting system 

 are shown in Figure 2. Details of the construction and operation of the 

 system are found in Robbins et al_. (1979). The actual and measured distri- 

 bution of activity from a submi llimeter line source is shown in Figure 3. 

 The nearly Gaussian profile of measured activity mainly reflects collimator 

 geometry. The limited broadening of the line source in the control cell 

 (with no benthos present) is due to molecular diffusion. 



When Oligochaete worms are added to surface-labeled sediments, the 

 radioactivity profile evolves over a six-month period as illustrated in Fi- 

 gure 4. The shaded areas represent the profile corrected for the effects 

 of finite detector resolution. The initial effect of the worms on the dis- 

 tribution is one of burial. This is, of course, consistent with the well- 

 known behavior of these organisms. They penetrate sediments to about 10 cm 

 depth to feed while at the same time holding their tails above the sediment 

 surface to defacate. This behavior has led Rhoads (1974) to describe such 

 organisms as "conveyor-belt" species. In time, the marked layer is buried 

 to the point where it encounters the zone of feeding and begins to reappear 

 at the sediment surface. During the initial burial period, the reworking 

 rate is essentially constant as can be seen in Figure 5 which shows the lo- 

 cation of the peak activity versus time. The burial rate is about 0.052 + 

 0.007 cm/day at 20 degree C. Error bars primarily reflect uncertainty in 

 locating the sediment-water interface due to irregular pile up of fecal 

 mounds. 



The interaction of the amphipod, Pontoporeia hoyi , with sediments 

 strongly contrasts with that of Oligochaete worms. As can be seen in Fi- 

 gure 6, the activity spreads downward from the surface under the action of 

 Pontoporeia without significant advection. This species burrows randomly 

 through the upper several centimeters of sediment and thus serves to move 

 sediment particles in a manner akin to eddy diffusion. Shown in Figure 7 

 are the corrected peak width versus time plus a theoretical relationship 

 based on the assumption that particle motion is truly eddy diffusional in 

 character. Details of the calculation are given in Robbins et aj_. 1979. 

 The diffusion coefficient implied by the data is 4.4 cm^/yr for an amphipod 

 density of 16,000 cm-2. 



While the two benthic species investigated have Mery different modes of 

 interaction with sediments, their effect on vertical particle movement can 

 in each case be quantitatively described and measured with a precision and 

 rapidity which suggests the radiotracer method as a useful behavioral bio- 

 assay technique. Mery precise reworking rates, expressed either in terms of 

 a sediment burial rate or eddy diffusion coefficient, can be determined 

 under realistic conditions in a matter of a few days. This radiotracer 

 method of observing a particular organism's behavior offers the special ad- 

 vantage of being noninteracti ve to a \jery high degree. The gamma radiation 

 passes readily through the cell walls and, once radionuclides have been 

 added to the system, no further interaction with the microcosm is required 



204 



