Chemistry 145 



DRP Variations in IBP Ponds 



Based on the relationships identified in the previous sections, it is 

 possible to explain the variations in the dissolved reactive phosphorus 

 (DRP) among the IBP watershed ponds. 



Pondwater DRP concentrations across the watershed are not 

 randomly distributed, but decrease downslope (Figure 4-23). Although this 

 gradient is in part related to geomorphology (high-centered polygons and 

 high-phosphorus trough ponds occur upslope and low-centered polygons 

 and low-phosphorus ponds occur downslope), the gradient should have an 

 underlying geochemical basis. The six ponds, A-B, C, E, F, J, and 17 (see 

 Figure 3-1 ), all lie within the youngest drained lake basin and therefore are 

 the watershed's youngest ponds; in addition, all fall within the lowest two 

 DRP contours. The immediate factor controlling DRP concentrations 

 across the watershed should be the phosphate sorption by sediments which 

 in turn is controlled by the concentration of exchangeable inorganic 

 phosphorus present in the sediments. Thus, a plot of DRP against the 

 amount of oxalate-extractable inorganic phosphorus in the sediment 

 (oxalate Pi) as a Temkin isotherm (Figure 4-24) results in significant linear 

 correlations for both: oxalate Pi (/xg P g ~ ' ) = ( 1 1 5 1 ) log DRP (^g P liter ' ) 

 -22 with r = 0.56*. Several important factors may account for the 69% of 

 the variance not explained by the regression. One factor is that the oxalate 

 reagents used extract the reductant soluble phosphorus (after Chang and 

 Jackson) in addition to phosphate in the two Chang and Jackson extracts 

 (NH4F-P plus NaOH-P) thought to constitute the exchangeable inorganic 

 pool. This overestimate is highly variable as the Chang and Jackson 

 extraction data for the four surficial Pond C sediments used to construct 

 Table 4-21 would indicate a potential overestimate of to 43%. Additional 

 scatter in Figure 4-24 would be expected due to the combination of 

 sediment heterogeneity and the extremely small samples used in obtaining 

 the oxalate Pi numbers (single to 3 cm section of 2.5 cm-diameter 

 cores). Scatter should also result from changes in the DRP of the water 



DRP, ^g liter" 



FIGURE 4-24. Measurements of 

 DRP and oxalate Pi in ponds of the 

 IBP watershed plotted as a Temkin 

 isotherm. 



