240 



V. Alexander et al. 



nutrients are more abundant in this environment. The relatively high K for 

 P uptake (a half-saturation constant) also is an indication of the relative 

 richness of the pond sediment environment (Table 5-10); a low K is an 

 indication of an uptake system that is effective at low concentrations 

 (Wright and Hobbie 1966). 



The Qio for phosphate absorption is quite low, usually between 1.3 

 and 1.6. For Carex aquatilis Chapin and Bloom (1976) report 12 values 

 whose average was 1.5. The temperature optimum for uptake was 30°C in 

 this plant so adaptation does not include a decrease in this parameter. 

 More important is the linearity between uptake of P and temperature 

 which continues down to 1°C. Thus, roots are capable of absorbing 

 phosphate as soon as the soil thaws. In contrast, Sutton's (1969) review 

 shows that temperate plants are unable to absorb phosphate at low 

 temperatures. 



Nitrogen 



There are no experimental data that prove a nitrogen limitation on 

 the photosynthesis or growth of vascular plants in either the terrestrial or 

 the aquatic environments. However, this does not mean that there is no 

 limitation in the ponds. The only data we have which indicate that nitrogen 

 is important are the very low concentrations of ammonia found in the 

 stands of emergent plants (Figure 5-36). Obviously the plants keep the 



O 0) 



o ~ 



3.0 



2.0 



g 



I? 



< ty 







Q. 

 (U 



Q E 

 o 





 20 



Sediment 

 Somple Depth 



0-8 cm 



^ 



8-16 cm 





No Vosculor Plants Arctophila No 



fulvo Vascular 

 Plants 



Carex aquotilis 



FIGURE 5-36. Concentrations of interstitial ammonia (^g at N liter'') in 

 the sediments of Pond J, J 973. The center of the pond, a region with no 

 vascular plants, is at the left and the shore is at the right. (After Alex- 

 ander and Barsdate 1975.) 



