Chemistry 111 



increase in photosynthesis or phytoplankton biomass by several days 

 (Figure 4-13). Inorganic nitrogen uptake, measured with '""N techniques, 

 was depressed for 24 hr after the initial fertilization and then increased 

 rapidly. The implication is that the additional nitrogen required for this 

 uptake and the accumulation of nitrogen in the particulate phase probably 

 came from the sediments. No nitrogen fixation was detected in the water 

 or in the sediments during this period, nor was there any great change in 

 inorganic nitrogen concentration in the pond water. 



The response of nitrogen uptake to a steady infusion of phosphorus 

 into a pond was tested in subponds (250 liters) of Ponds B and C in 1970. 

 Phosphate was supplied daily at 2 and 10 Mg P per liter of pond, and the 

 effects on the phytoplankton nitrogen regime were compared with control 

 subponds. Here again, there was a positive response of nitrogen uptake to 

 phosphorus addition; the maximum response occurred at the lower 

 phosphorus infusion rate (Figure 4-14). This increase is, of course, 

 independent of biomass as the results are expressed as N uptake per 

 microgram of N in the total particulate matter in the water. Thus, nitrogen 

 uptake responded to increased phosphorus availability and allowed an 

 increase in phytoplankton productivity and biomass. 



400 



- 300 - 



o> 



:l 



c' 

 « 



o 



200 ■ 



9) 



♦— 

 O 



O 



O 



Q. 



100 ■ 



24 26 28 



Jul 



H 4 



'> _ 



3 o V 



"S »- 



O 0) 



2 tt •:= 



>' a 

 6 



FIGURE 4-13. Particulate nitrogen, rate of nitrogen up- 

 take, and primary productivity in the plankton of Pond D, 

 1970. On 25 July, 0.31 mg P per liter of pond volume were 

 added and on 28 July an additional 1.5 mg P per liter of 

 pond volume were added. 



