136 R. T. Prentki et al. 



but still more rapid than any other rate. Benthic bacteria in the ponds cycle 

 large quantities of DRP through DOP (mostly XP) which lyses back to 

 DRP. However, neither uptake nor release of P by the sediment algae plus 

 bacteria compartment have been measured in situ. Uptake of DRP by 

 vascular plant roots occurs throughout the sediment active layer and the 

 0.39 mg P m " day ' represented in the flow diagram is for the entire 

 thaw zone rather than just for the 0-5 cm layer. 



CONTROL OF PHOSPHORUS 



Introduction and Methods 



Phosphorus availability in aquatic ecosystems is normally controlled 

 by both extrinsic and intrinsic factors. In tundra ponds the extrinsic factor, 

 phosphorus loading through runoff and precipitation, is unimportant 

 because imports of allochthonous phosphorus are so small and are 

 equalled or exceeded by pond exports (Table 4-19 and Figure 4-19). The 

 intrinsic factors, those controlling the equilibrium and cycling rates, are 

 most important. 



The dominant source of recycled phosphorus in the water of tundra 

 ponds is the vascular plants. These take up phosphorus from the sediment 

 into their roots and then secrete a large amount of the phosphorus into the 

 water while the plant is alive. The phosphorus is also leached from the 

 plant material within a few days after the plant dies. The phosphorus 

 secreted by the plants into the water is enough to drive the entire 

 phytoplankton production for the year. 



Thus, the supply of phosphorus to the water is adequate; the question 

 is, however, what happens to the phosphorus after it reaches the water? 

 Why are the phosphorus concentrations so low and why are there only 

 small changes in the concentrations throughout the summer? 



In this section, we will show that the phosphorus concentrations and 

 supply rates are controlled by the buffering of the sediments. Almost all 

 the phosphorus that enters the water column quickly moves to the 

 sediment. There it is either strongly sorbed to the sediment or is retained in 

 reductant-soluble (probably occluded within hydrous iron hydroxides) or 

 organic form. In these three cases, the phosphorus is unavailable to 

 planktonic organisms. The sorbed phosphorus will exchange with the 

 overlying water but its availability is limited by the low concentration of 

 DRP in equilibrium with the sediment and by the rate of sediment- 

 pondwater exchange. The cold climate affects phosphorus cycling in the 

 ponds most directly by slowing mineralization of organic phosphorus. 

 Mineralization of this phosphorus to inorganic form would significantly 



