138 



R. T. Prentki et al. 



Shallower sediments are enriched relative to deeper sediments in both 

 iron and phosphorus (Figure 4-21). The weaker correlation and decrease in 

 slope of the resulting correlation when surface sediment data are included 

 indicate that the scatter in the upper sediments is caused by anomalous 

 enrichment in iron. In Pond B, to 10 cm sediments averaged 820 Mg P 

 g ' (S.E. = 77, n = 7) and 26 mg Fe g " ' (S.E.=4.7, n = 10) and to 1 cm 

 sediments 1080 Mg P g ' (S.E. = 23, n = 8) and 50 mg Fe g ^ (S.E.=2.5, 

 n= 12). These surface enrichments demonstrate a 1.9 concentration factor 

 for iron but only a 1.3 factor for phosphorus. Thus iron appears to be 

 differentially mobilized. The positive correlation within the water column 

 of dissolved iron with dissolved organic carbon, with humic color, and with 

 rainfall also suggests differential mobilization of iron, possibly through 

 release from vascular vegetation of either soluble iron or compounds that 

 complex iron. Such a mechanism would result in a net movement of iron 

 from the sediments of the plant zone into the vegetation-free central 

 basins. Only two sediment samples plotted in Figure 4-21 were collected 

 from vascular plant zones. These two were depleted in iron relative to their 

 phosphorus content (820 Mg P, 15 mg Fe g ' and 968 Mg P, 16.2 mg Fe 

 g^^); therefore, the data are consistent with the hypothesis that the plant 

 zone is the source of the iron that enriches the central basins. 



Very few of the sediment samples discussed above, and none of the 

 samples of deeper sediments for which the iron-phosphorus correlation 

 was even stronger, were further fractionated into organic and inorganic 

 phosphorus. Thus, there are no data on the mechanism behind the 

 relationship. 



Inorganic phosphorus and iron measurements indicate that sediment 

 inorganic phosphorus is fixed onto a single phosphate-sorbing complex by 



TABLE 4-21 Correlation Matrix for the Results of a Fractionation of Phos- 

 phorus (Chang and Jackson) ofOto 1 cm Pond C Sediments 



^Significant at the 95% level of probability 



