67 S TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Q. 

 LU 

 Q 



X Station 5 — 

 A Station 19 

 • Station 18 _ 

 ■ Station 16 



■ — x- 



-TTTTTTTJ 



kzzd \ \ \ L 



0.2 0.4 0.6 0.8 1.0 



PARTICLE FLUX (J), mg cm"2 day"'" 



1.2 



Fig. 12 Particle-settling-rate profiles from sediment-trap studies at several stations in the 

 southern basin of Lake Michigan (1976). 



(milligrams per square centimeter per year) is the downward particle flux at the 

 sediment— water interface. 



This scale length is confirmed by sediment-trap studies conducted at a series of 

 locations in Lake Michigan. Each of the particle-settling-rate profiles (J in milligrams per 

 square centimeter per day) shown in Fig. 12 has a "foot" that extends from the bottom 

 upward to about 10 to 15 m, The rate of decrease in J over the interval is very nearly 

 exponential and has a scale of about 5 m, which is essentially the same at each location 

 except at station 5. Therefore it is likely that the effects of resuspension are confined to 

 the bottom 10 to 15 m in deep waters (over 80 m). It is possible that Umited numbers of 

 smaller particles of freshly formed aggregates of lower densities in the benthic floe may 

 be resuspended to greater average heights above the bottom. 



However, the monthly series of settling-rate profiles from station ANL-5 (Fig. 13) 

 suggest that there is a nonconstant settling rate above 20 m that is probably due to 

 seasonal variations in particle production in the epilimnion. In addition, these profiles 

 suggest that resuspension may account for most of the settUng material at relatively 

 shallow stations similar to this.* During the period in August when the nonash 



*This presents a problem in that, although the water depth of ANL-5 is representative of the mean 

 depth of the lake (~84 m), it is close enough to shore (~12 km) to make it highly susceptible to 

 contributions from shoreline erosion and tributary inputs. 



