174 R. T. Prentki et al. 



aluminum, or calcium minerals. The sorbed phosphorus is in dynamic 

 equilibrium with P in solution and can be mobilized. The amount of sorbed 

 P in the sediments was large, about 3,600 mg P m^^ in the top 10 cm of 

 Pond C while there was 25,000 mg total P m ^ In the same pond, the 

 interstitial water contained 2 mg P m "^ and the water column contained 5 

 mg P m ■^ (20 cm of water). 



The phosphorus enters the pond through winter snow, summer rain, 

 and spring runoff. It leaves the pond in spring runoff. The winter snow 

 contains about 4 Mg P liter ' (equal amounts of DRP, DUP, and PP) 

 while the summer rain contains 7.2 /xg DRP liter ' and 0.7 Mg DUP 

 liter"'. As the meltwater from the winter snow moves in sheets across the 

 tundra, phosphorus is leached rapidly from vegetation, fecal pellets, and 

 litter on the tundra. The water entering the pond has 2.4 Mg DRP liter"' 

 and 11.3 Mg DUP liter"'. In the pond, concentrations are somewhat 

 higher so the water leaving the ponds contains 2.5 Mg DRP liter"' and 13.1 

 Mg DUP liter '. The budget, in units of mg P m "^ yr"' is: winter snow, 

 + 0.23; spring runoff entering, +10.41; spring runoff leaving, —11.86; 

 summer precipitation, +0.51. The net balance is —0.70 or a slight loss 

 from the ponds. The ponds are in equilibrium, really, as all the terms in the 

 budget will change slightly from year to year. When the large amount 

 (25,000 mg P m ^) of P in the sediment is considered, it appears that much 

 of the phosphorus in the sediments was there before the pond was formed. 

 The amount of P lost, less than 1 mg P m~^ yr"', is extremely small 

 compared to the total amount of P circulating in the ponds. For example, 

 if the primary productivity is around 40 g C m "' yr"', then this represents 

 approximately 500 mg P m"^ yr"' circulating in the algae and rooted 

 plants. 



The phosphorus cycled very rapidly through the plankton; in fact, the 

 rate of uptake was about 200 times greater than the amount needed for 

 algal growth. The isotope studies showed that DRP is taken up by bacteria 

 and algae and most is immediately released again. The released P is 

 mostly dissolved organic phosphorus. The organic P released by the 

 phytoplankton has a low molecular weight (XP) and can either break 

 down again to DRP or combine with colloids to form colloidal P (high 

 molecular weight). The XP and colloidal P pool is as large or larger than 

 the DRP pool. Phosphorus cycles through these organic fractions as much 

 as 50 times a day. Uptake rates of DRP into plankton were 13 to 320 Mg P 

 liter"' day"' where the DRP was only around 2 Mg P liter"'. Obviously 

 the algae have actually 2 to 4 times the amount of P in the DRP available 

 for growth. There were also changes in the 70% of the DUP which is not in 

 the XP or colloidal P form but this pool is resistant to biological 

 breakdown. 



Zooplankton excrete phosphorus mainly as DRP. In the pond, the 

 zooplankton excreted about 0.13 Mg P liter"' which is enough to account 

 for all of the P needed for algal productivity. 



