134 R. T. Prentki et al. 



inorganic phosphorus that quickly appears constitute a metabolic pool in 

 the bacteria which equilibrates with extracellular phosphate many times 

 faster than the whole organism does. Phosphorus in the XP-like 

 phosphorus pool is then either excreted as XP or is incorporated into a 

 bound phosphorus pool. 



The results of the experiment with protozoan grazers and bacteria 

 (Barsdate et al. 1974) showed that the grazers induced a higher rate of P 

 turnover in the bacteria. For example, in the experiments, the uptake rate 

 of bacteria cultures alone was 7.3 ng P liter " ' hr ' (this is 2.5x 10 ' Mg P 

 cell ~ ' hr " '). In cultures with bacteria plus a ciliate, the uptake was 9.3 jUg 

 P liter ~ ' hr ' (but the number of bacteria was smaller so the uptake was 

 equal to 1 6.6 x 10 ' jug P cell ' hr '). In the grazed system, the ciliates 

 could ingest (and presumably excrete) 0.403 ng P liter ' hr '. Thus P 

 excretion was not responsible for the increased bacterial activity. This 

 activity was very high indeed, and is greater than the 0.6 to 1 .0 x 10 ' ^g P 

 celP' hr~^ reported for bacteria by Fuhs et al. (1972). The difference is 

 presumably a result of physiological differences between rapidly dividing 

 bacteria in grazed systems and relatively static bacteria of ungrazed 

 systems. 



In aquatic systems with macrophytes, the rooted plants may move 

 otherwise unavailable nutrients from sediments into water either 

 indirectly, by decomposition of litter (Pomeroy et al. 1972), or directly by 

 live plant excretion into the water column (McRoy and Barsdate 1970; 

 McRoyetal. 1972). 



A Carex enclosure in Pond C, isolated from runoff by walls and from 

 sediment by bottom ice, peaked at 122 ^lg DRP liter ' during snowmelt. 

 This concentration was 50-fold higher than that of pondwater or Carex 

 exclosures, and corresponded to at least a 3.1 mg P m~^ release from 

 Carex litter during snowmelt. Since this release of litter P takes place in 

 part during spring runoff, an appreciable portion of the phosphorus 

 mineralized is flushed out of the ponds and lost from the system, as 

 described in the P budget. Phosphorus release from green-harvested Carex 

 aquatilis litter in a pond was also rapid in summer (34 ^lg P (g dry wt)^ 

 day"'), and more than 60% of the initial plant phosphorus was released 

 during the first month of immersion. Release apparently is a leaching 

 process, rather than microbial decomposition, as controls in sealed, 

 mercury-poisoned containers in the pond lost phosphorus at a similar rate. 

 Both the magnitude and time span of the phosphorus release observed here 

 are in good agreement with field measurements made on aquatic 

 vegetation in the Kiev Reservoir (Korelyakova 1968). 



Excretion of DRP into pond water by the leaves of live Carex plants 

 was demonstrated in a series of laboratory and in situ experiments; the 

 average rate (0.1 Mg P (g dry wt) " ' day ~ ' ) was much less than the loss rate 

 from dead Carex (see above). A somewhat greater amount (up to 1 Mg P (g 

 plant)"' day ') appears on the subaerial portions of the leaves. Some of 

 this P release may be through guttation (Chapin and Bloom 1976). We 



