126 R. T. Prentki et al. 



shallow water depth and rapid water movement over the tundra surface, 

 the residence time of water is much less than in the ponds (3 hours vs. 36 

 hours) and the contact with the soil surface is great. As a consequence, 

 phosphorus sorption is more rapid over the tundra surface than in the 

 ponds (Prentki 1976) and the concentrations in the meltwater are lower 

 than those in the ponds by about 0.13 ^lg of DRP and about 1.8 Mg of DUP 

 liter ~\ As meltwater moves across the watershed, phosphorus is thus 

 removed from the ponds and resorbed by the terrestrial soils; the extent of 

 removal depends on both the difference in phosphorus concentration 

 between ponds and tundra and on the volume of water flowing through the 

 ponds. With an area of watershed above Pond B of 14,000 m^ and a 5.8 cm 

 potential runoff (Table 3-6) the estimated volume flowing through the 

 pond during breakup is 800 m^ and the loss of DRP and DUP is 0.1 and 

 1.4 mg P m ', respectively. 



Summer precipitation at Barrow (Table 4-19), averages 7.2 Mg DRP 

 liter "^ and 0.7 ^g DUP liter '. With a mean of 6.5 cm of precipitation in 

 summer, the input into the ponds from this source amounts to 0.47 mg 

 m"' DRP and 0.05 mg m"' DUP (Table 4-20). Unlike the winter 

 snowpack data, these figures do not include dry fallout between periods of 

 precipitation, and so the possibility exists that an additional amount of 

 phosphorus enters the tundra system as dust. However, the very low winter 

 bulk P concentrations suggest that dry fall must be very low. 



Another possible input exists. As discussed earlier (Chapter 3), soil- 

 water could enter the ponds during dry periods when the water level falls 

 drastically. This was not considered important for the nitrogen budget, 

 and is likely not important for the phosphorus budget either. It is an area 

 of some uncertainty, however, so calculations are presented below of the 

 quantities involved. There are four measurements of interstitial water in 

 the upper layers of soil adjacent to the ponds; the averages are 4.4 ^g DRP 

 liter ' and 26.3 Mg DUP liter"'. If, in a moderately dry summer such as 

 1971, 6 cm of water enters the pond, this would be added inputs of 0.26 mg 

 DRP m"^ and 1.58 mg DUP m"". The size of this is about equal to the 

 annual budget (Table 4-20) but is still very low when the large amounts of 

 P in the sediments are considered. 



Annually, the ponds gain 0.5 mg DRP while losing 1 .3 mg DUP m "^ 

 (Table 4-20), suggesting that phosphate retention mechanisms of the 

 ponds are extremely effective but that dissolved organic phosphorus 

 cannot be as efficiently retained. The annual loss of phosphorus calculated 

 here, 0.7 mg, is only 0.003% of the 25 g P m~^ present in the top 10 cm of 

 Pond B sediments, suggesting that the pond system is very close to a 

 steady state condition in respect to phosphorus. The loss is especially small 

 when compared to the P cycling each year in the biota. In the primary 

 producers alone, at least 500 mg P m Ms fixed each year (40 g C m~^). A 

 similar conservation of P was found in Hubbard Brook watershed where 

 less than 1 mg P m~' also was lost each year despite the hundreds of 

 milligrams of P circulating (Hobbie and Likens 1973). In Char Lake, 18 



