Major Findings 11 



0.001 and 0.002 mg P liter'. This is the form of phosphorus that is 

 available to algae and higher plants; their primary production can be 

 enhanced over several weeks by adding phosphorus to a pond. Even in the 

 fertilized ponds, however, the concentrations of inorganic phosphorus 

 rapidly decline to 0.001 to 0.002 mg P liter '. Why are these phosphorus 

 concentrations so low and why are there such small changes over the 

 summer? 



A part of the answer is that the dissolved reactive phosphorus (DRP) 

 cycles very rapidly in the ponds (Figure 1-7). For example, there is 0.14 mg 

 P m~^ in the water on the day illustrated in the figure while the bacteria 

 and algae take up 5.8 mg P m ^ day " \ At the same time, there is also a 

 transport of 0.73 mg P m ^ into the DRP pool of the interstitial water; the 

 DRP thus turns over 50 times per day in the ponds. During the rapid 

 turnover of the small amount of DRP in the water, the large quantities of 

 phosphorus in the sediment turn over very slowly and actually buffer the 

 whole system. 



The other part of the answer lies in the chemical properties of the 

 sediment. When DRP enters the pond, it quickly moves to the sediment 

 where much of it is sorbed onto a hydrous iron complex. The 

 concentration of DRP and the release rate of the sorbed phosphorus are 

 controlled by a chemical equilibrium; ponds with different amounts of iron 

 and inorganic phosphorus in the surface sediments will have different 



10.0 



FIGURE 1-8. Oxalate extractable 

 phosphorus in the sediments of 

 five tundra ponds of similar 

 origin plotted against the dis- 

 solved reactive phosphate (DRP) 

 of the overlying water column. 



400 500 



Phosphate Sorption Index 



600 



FIGURE 1-9. Algal photosyn- 

 thesis in the water column of a 

 series of tundra ponds plotted 

 against the phosphate sorption 

 index of the underlying sediments 

 (9 August 1973). (Data are from 

 Prentki 1976.) 



