238 



V. Alexander et al. 



can only have effect after the plants have had time to change their 

 photosynthetic capacity. The studies on terrestrial plants at Barrow 

 showed that in the normal situation, the plants have a constant pool of 

 carbohydrates. When nutrients are added, this pool is depleted slightly to 

 produce more leaf tissue and the plants come to a new steady-state 

 relationship with the environment. In the terrestrial system this increased 

 growth is accompanied by increased standing dead and increased litter; the 

 insulating effect of both of these may change soil temperatures, depth of 

 thaw, etc. 



The implication for the aquatic plants from these terrestrial findings 

 is that high plant production in ponds is primarily a result of better 

 nutrient conditions for the plants in the ponds than in the terrestrial sites. 

 Yet, measurements of nutrients across transects indicate that 

 concentrations of phosphorus are actually lower in the aquatic sediments 

 than in terrestrial soils (Shaver et al. 1979). It is probable that there is 

 increased availability of nutrients or a more rapid rate of supply in aquatic 

 sediments than in soils. If this is true, then the low concentrations of 

 nutrients in the sediments are caused by the uptake by the roots. 



It is important to note that there may be differences between the 

 interactions of phosphorus with terrestrial soils and with pond sediments. 

 In ponds, the productivity of planktonic algae was related to the binding 



0.5 



FIGURE 5-35. Rate of P uptake by Carex 

 aquatilis from different sites at four different 

 concentrations of phosphate. Rates were meas- 

 ured at JO°C and each point is the average. 

 (After Shaver et al. 1979.) 



