Primary Producers 241 



0.32 



c 

 o 



o. 0.24 



E 



2 



E 



-^ 0.16 - 



0) 



o 



Q. 



3 



o 

 o 



q: 



> 0.08 - 



o 



3 



E 

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16 

 Time, hr 



24 



32 



FIGURE 5-37. Cumulative uptake of NHy-N by 

 Carex roots. Experiment 1 = 2.0ixg at N liter'\ 2 

 = 3.3, 3 ^ 4.7,4 = 6.0, 5 = 11.3, 6 = 15.3, 7 

 = 30.0 and 8 = 40. 7. All experiments were run 

 at 10°C in constant light. (After McRoy and 

 Alexander 1975.) 



ammonia concentrations at a very low level while adjacent areas without 

 plants have high concentrations. 



In fact, the concentrations of ammonia in the sediments are so low 

 that uptake rates are severely restricted (McRoy and Alexander 1975). 

 These rates were measured with "'N and with techniques described in 

 Chapter 4. Whole plants were placed in partitioned containers such that 

 the water surrounding the roots was isolated from that surrounding the 

 leaves. The uptake by Carex roots was somewhat erratic but in general 

 was linear over the first hours of the experiment (Figure 5-37). From data 

 like these, the uptake rates at various concentrations could be plotted 

 (Figure 5-38) and kinetic parameters calculated. The V^ax for root uptake 

 was 0.0275 Mg N (^g plant N) " ' hr ' and the K (half-saturation constant) 

 was between 117.6 and 175.0 Mg NH3-N liter '. The half-saturation 

 values are 2 to 3 times greater than the concentrations of ammonia- 

 nitrogen found in the sediments (53 to 75 Mg N liter ', Chapter 4). In this 

 region of the uptake curve in Figure 5-38 (4 to 6 Mg-at liter '), the uptake 

 is directly proportional to the concentration. Whether or not these low 

 concentrations are at all limiting to photosynthesis or growth will depend 

 upon the rate of supply of ammonia. 



