148 



R. T. Prentki et al. 



FIGURE 4-27. Concentrations (yig Fe g'^) of oxalate Fe in the top 

 3 cm of sediment of ponds in the IBP watershed. 



that they are becoming less phosphate-limited, for pond aging may also be 

 a factor in the accumulation of iron in the surface sediment. The young 

 ponds within the former lake basin are relatively low in surficial iron as 

 they average only 57 mg oxalate-extractable iron g ' (oxalate Fe), some 

 30% below the mean of watershed ponds (Figure 4-27). This oxalate- 

 extractable iron is measured by atomic absorption on oxalate Pi extracts. 



In these surficial pond sediments, unlike the sediments previously 

 presented in Tables 4-21 and 4-22, inorganic phosphorus and iron 

 parameters were not significantly correlated (r = 0.34). If, as suggested 

 above, neither sediment inorganic phosphorus nor iron concentrations are 

 at steady state but are instead slowly increasing, then ratios of phosphorus 

 to iron would be partially dependent upon pond age and any dependence of 

 the distribution of inorganic phosphorus on iron would be masked by the 

 aging effect. Sediment with a lower ratio of inorganic phosphorus to iron- 

 sorbent would also be expected to remove more phosphate from solution 

 and would bind it more strongly. Therefore, in ponds of similar origin and 

 age those ponds with higher iron content should also accumulate more 

 inorganic phosphorus. 



This hypothesis was tested by graphing concentrations of oxalate Pi 

 against oxalate Fe concentrations of those ponds in the watershed that 

 make up a single age class and that have a common origin (B, C, E, J, and 

 17, Figure 4-28). The resulting correlation was not significant (r = 0.70) 



