116 R. T. Prentki et al. 



hydrolysis of dissolved organic phosphorus. Samples collected prior to 23 

 June 1970, samples with high DRP concentrations, and those otherwise so 

 noted were not extracted prior to spectrophotometric determination. Both 

 dissolved unreactive phosphorus and particulate phosphorus retained on 

 membrane filters were oxidized with persulfate (Menzel and Corwin 

 1965), after which analyses were made by the single solution method. 

 Zooplankton were refluxed with perchloric acid and P was determined as 

 phosphomolybdic acid (Strickland and Parsons 1965). 



Dissolved reactive phosphorus determinations usually overestimate 

 actual phosphate concentrations, often by orders of magnitude. 

 Hydrolysis of dissolved organic phosphorus compounds or arsenic 

 interference are the most commonly accepted causes (Rigler 1966, 1968, 

 Downes and Paerl 1978, Chamberlain and Shapiro 1969). In the tundra 

 ponds, however, extensive tests showed that the DRP measurement 

 included only dissolved phosphate. First, arsenic should not interfere in 

 extraction phosphomolybdate analyses of unpolluted waters such as the 

 Barrow tundra ponds. Next, Prentki (1976) has demonstrated that 

 extracted DRP in tundra ponds does not include colloidal P or XP. In 

 addition, application of Rigler's (1966) fadiobioassay test to pond samples 

 did not result in apparently lower phosphate uptake velocities at higher 

 added phosphate concentrations. Finally, other ^^P kinetic experiments 

 that assumed the DRP equal to phosphate gave internally consistent mass 

 balances. 



Dissolved reactive phosphorus concentrations are always low in the 

 ponds, usually between 1 and 2 ^g P liter '; monthly averages over the 2 

 years of intensive sampling show only modest seasonal or inter-pond 

 differences (Table 4-16). In addition, diel variations in dissolved DRP 

 concentrations are of the same magnitude as seasonal variations, thereby 

 masking any seasonal trends. The only distinguishable seasonal features 

 occur as a result of the thaw in June (Figure 4-1 6a). At this time, DRP 

 concentrations are generally high; they are maintained by phosphorus 

 entering in runoff and leaching from standing vascular vegetation. Next, 

 the rate of phosphorus supply decreases in late June as both runoff and 

 leaching taper off. Thus, as the phosphorus demand increases with the 

 onset of the phytoplankton bloom, the DRP concentrations are depressed 

 to below 1 ng P liter '. 



Dissolved unreactive phosphorus (DUP) is initially re-introduced into 

 the ponds each spring through litter decomposition, sediment leaching, 

 and runoff. Concentrations early in the thaw season in 1970 to 1972 rank 

 in the same order as snow pack depth for those years, with 1971 having the 

 greatest snow depth and highest average June DUP concentration and 

 1970 having the least and lowest. This is very evident in Figure 4- 16b, 

 where DUP concentrations in Pond B in 1971 are almost twice those in 

 1970. In part, this DUP may be resorbed by the sediment and soil surface 

 during runoff, and this sorption would be favored by greater sediment- 



