35 



was then measured using an ORTEC Intrinsic Germanium Detector 

 connected to a 4096-channel multichannel analyzer. The 

 unsupported activity was obtained by difference between total 

 residual 2l0pb activity and supported activity of each sample as 

 assessed from ^Mgj 



Bulk-sediment accumulation rates (SEDACCUM) were estimated 

 for samples from lakes in both survey sets using the following 

 formula: 



SEDACCUM = F210pbA-l 



where SEDACCUM = g dry sedimentcm-2 yr^ 



F210pb = flux of 210pb fallout (pCi cm-2 yr^) 



A = unsupported 2l0pb activity in sediment (pCi g-^. 



Annual diatom accumulation rates (D-ACCUM) were estimated as 



follows: 



D-ACCUM = (D-CONCXSEDACCUM) 

 where D-ACCUM = valves cm-2 yj-1 



The annual accumulation rates of periphytic valves (PERIACCM) and 

 euplanktonic valves (EUPLACCM) were obtained by multiplying the 

 periphyton and euplankton proportions of each sample by the total 

 diatom accumulation rates. 



Subsamples of 1.001 cm3 wet sediment were dried at 60 °C in a 

 drying oven. Dry weight per unit volume (rho) was then measured 

 for each sample by weighing the dry material on an Ainsworth 24N 

 analytical pan balance. Percent loss on ignition (% L.O.I.), a measure 

 of sedimentary organic matter content, was determined for each 

 sample by weighing the sediment samples before and after 



