178 R. T. Prentki et al. 



liter"' in snow and runoff water, but during the second half of the runoff 

 season the concentration dropped to 436 ^g- The yearly means ranged 

 from 266 to 769 Mg POC liter'' in the pond water. Most of the POC is 

 nonliving; algae and bacteria made up only 5 to 13%. POC is produced by 

 the growth and death of algae, of bacteria, and of zooplankton, from 

 zooplankton molts and fecal pellets, by resuspension of sediments, and by 

 precipitation of iron-humus floes. The concentrations in the pond are 

 similar to those in other moderately rich freshwaters. 



Because the concentration of POC was relatively stable throughout 

 the summer, the resuspension rate should equal the sedimentation rate. 

 This rate was measured with 12-cm-high cylindrical traps, 4.7 cm in 

 diameter. These underestimated the sedimentation rate compared with 

 measurements made on white Plexiglas sheets. The amounts of POC in the 

 water column averaged 90 to 200 mg POC m "^ day "' but the sedimenta- 

 tion rate averaged 1200 to 2400 mg C m"^ day"'. Thus, much of the 

 material that appeared in the traps must have been resuspended each day. 

 Sedimentation rate was significantly correlated with the maximum wind 

 speed, so the resuspension may only have occurred during brief periods of 

 each day. 



POC was lowest in ponds with the highest sedimentation rates. It is 

 possible that large particles may capture the smaller particles as they fall. 

 Another possible control of POC is grazing by zooplankton. These 

 animals are abundant and active; they filter all the pond water every 2 

 days. However, their effect on the POC is unknown. 



