Chemistry 149 



40 60 



Oxalate Fe, mg g"' 



FIGURE 4-28. Concentration of oxalate P 

 and oxalate Fe in the sediment of some 

 ponds of the IBP watershed. Oxalate Pi = 

 (6.5) oxalate Fe- 110. 



unless Pond J, with a "low" oxalate Pi to Fe ratio, was dropped from the 

 regression. When this was done, the data from the remaining four ponds 

 all fell close to the regression line: oxalate Pi (^g g ') = (6.5) oxalate 

 Fe (mg g ') —1 10, (r = 0.98**). Pond J was destructively sampled during 

 the IBP program and the low ratio of oxalate Pi to Fe could be due to 

 destruction of a surface layer that was built up over 3,000 years. 



Sediment sorption control of pondwater DRP concentrations across 

 the watershed can also be illustrated by an indirect bioassay approach 

 utilizing primary productivity rates (provided by M. C. Miller). In Figure 

 4-29 the log of midsummer phytoplankton primary production has been 

 plotted against phosphate sorption indices of the individual pond 

 sediments. All of the intensively studied ponds fall on or near a line 

 denoting an inverse, semi-logarithmic relationship between index and 

 phytoplankton production. This in turn suggests that there is a linear 

 correlation between the low, midsummer primary production and the 

 natural equilibrium phosphate concentration maintained by the sediment. 



Sediment sorption equilibria can set only a lower limit on 

 phytoplankton productivity. The rate of equilibration between water and 

 sediment is identical to the 0.8 day turnover time of pondwater DRP. In 

 spring, when runoff is high, and in late summer as both zooplankton 



