Chemistry 115 



The importance of the sediments in supplying nitrogen is illustrated in 

 an experiment carried out in two plastic swimming pools (each 6.4 m ') 

 filled with lake water but containing no sediments (Table 4- 15). On 17 July 

 1971, 109 ng PO4-P (liter of pond)"' were added to Pond 8 while Pond 7 

 was maintained as a control. Although a slight increase in productivity and 

 chlorophyll content occurred in Pond 8, this was very minor until 500 ^g 

 NH3-N (liter of pond) " ' was added on 7 August. Following this, there was 

 a striking increase in primary productivity compared with the control, 

 although there was also a slight increase in the control. The ratio of the 

 removal rates of N:P based on chemical data was 72:1 in Pond 7 when the 

 phosphorus was limiting photosynthesis. After fertilization with ammonia 

 in Pond 8 the ratio fell to 14:1 and then to 10:1 after 1 week. This low ratio 

 may indicate that nitrogen was becoming limiting. We conclude that 

 phosphorus alone does not produce a great fertilizing effect unless either 

 sediments are present or nitrogen is supplied. 



It appears that the sediments are controlling the nitrogen 

 concentrations in the pond waters. This control acts through 

 decomposition of the abundant organic matter in the sediment. Some of 

 the ammonia released during decomposition is taken up by grasses and 

 sedges (Table 4-9), some is taken up by epipelic algae, and some diffuses 

 into the water column. The regulation of these processes is unknown. 



PHOSPHORUS * 



Concentrations in Water 



The tundra has low phosphate concentrations in both aquatic and 

 terrestrial environments. Thus, we thought that phosphorus could be 

 limiting to plants in the ponds and that phosphorus cycling should be 

 intensively studied. The first 2 years of the study concentrated on 

 descriptions of phosphorus compartments and seasonal cycles in the water 

 column; the later 2 years emphasized sediment chemistry and phosphorus 

 pathways. 



Concentrations of dissolved reactive phosphorus (DRP), dissolved 

 total phosphorus (DTP), and particulate phosphorus (PP), were measured. 

 Dissolved unreactive phosphorus (DUP) was calculated by subtraction of 

 DRP from DTP. 



Dissolved reactive phosphorus in samples was analyzed by the single 

 solution phosphomolybdate technique (Strickland and Parsons 1965) 

 followed by extraction into isoamyl alcohol. Reagent contact time prior to 

 color extraction was strictly limited to 5-10 minutes to minimize 



*R.T. Prentki 



