PLUTONIUM IN THE GREAT LAKES 679 



I- 



0.01 



0.1 



1.0 



0.01 



0.1 



1.0 



PARTICLE FLUX (J), mg cnn-^ day"'' 



Fig. 13 Seasonal variation in particle-settling-rate profflesfrom sediment-trap studies in 

 the southern basin of Lake Michigan (ANL-5, 1976). 



component (plankton remains) is low, the particle flux profile is very nearly exponential 

 from depths of 65 up to 10 m. This may indicate vertical mixing by eddy diffusion, at 

 least below the thermocline. The apparently high sedimentation rate (~365 mg cm~^ 

 yr~^) in traps near the bottom must reflect both the effect of resuspension and 

 deposition of transient particles, such as diatom frustules and calcite. The mass 

 sedimentation rate for consoHdated sediments in this region is only 30 mg cm~^ yr~^ 

 (Edgington and Robbins, 1976). In view of these results, resuspension during isothermal 

 mixing could be of some importance for the return of plutonium to the whole water 

 column at this inshore station. The effect of resuspension in shallower waters could also 

 influence plutonium levels in offshore areas, depending on the efficiency of horizontal 



