132 



R. T. Prentki et al. 



About 70% of the dissolved unreactive phosphorus (DUP) is resistant 

 to biological uptake and breakdown. As noted (Table 4-16), DUP is up to 

 10 times more abundant than DRP. Prentki (1976) reported that 70% of 

 the DUP was resistant to breakdown by the phosphatases in the water and 

 by sunlight (photo-oxidation). Yet, there are changes in DUP in a pond 

 over a 24-hour period and these changes do not occur if the water is 

 isolated from the sediment in a large carboy. Evidently the DUP is 

 released from the sediments at lower temperatures (Figure 4-20). 



The measurements of total particulate phosphorus (PP) do not 

 include the zooplankton large enough to be picked off filters. Most of this 

 PP is sestonic detritus (Figure 4-19) that has only a low rate of exchange of 

 phosphorus with the DRP pool. No patterns in PP concentrations were 

 found over the season or on a single day. 



Our work with zooplankton has been limited to measuring 

 phosphorus excretion of Daphnia middendorffiana, Lepidurus arcticus, 

 and fairyshrimp. For these experiments, adult Daphnia from Pond B, 

 containing an average of 1 .9 //g P each, were captured in late August 1 97 1 

 and maintained at in situ concentration outdoors in a carboy. They were 

 fed ^^P labeled plankton until the *"P incorporation into body phosphorus 

 approached an asymptote (about 2 days). Several Daphnia were then 

 rinsed with unlabeled filtered pond water, and placed into a bottle 

 containing unlabeled plankton spiked with 1 mg PO4-P to minimize 



18 



'^ 16 



a. 



a 14 

 a 



12 



10 12 



Water Temperature, °C 



14 



FIGURE 4-20. Dissolved unreactive 

 phosphorus concentrations and water tem- 

 peratures during a diel cycle. Pond D, 23-24 

 July 1971. The lines at three points are the 

 range of duplicate analyses. 



