172 R. T. Prentki et al. 



and compares well with the terrestrial soil value of 48 mg N m"^ yr"'. 

 However, exchangeable inorganic N in the top 20 cm of sediments was 

 high: 810 mg N m"^ in the weedbed and 1400 mg N m"^ in the central 

 basin. Another input of nitrogen to the system is rainfall. During the 

 summer, the concentrations were 255 ^g NH 3-N liter " ' and 25 ^g NO3-N 

 liter "^; in contrast, rainfall at Hubbard Brook, N.H. contains more NO3 

 than NH3. The total input of dissolved inorganic nitrogen (DIN) at 

 Barrow was 11.5 mg N m~^ yr"' in rainfall. Seepage from the drainage 

 basin was not important but an important amount of DON was retained in 

 the ponds during the runoff of the meltwater (39 mg m^). The DIN 

 retained was only 1.7 mg m ^ Denitrification also occurred in the 

 sediments but the daily rate, 0.18 /xg NO 3 liter"', would not have affected 

 the NO3 pool (1 1 Mg NO3-N liter " ') very much. 



The total budget for the input and output of nitrogen from the ponds 

 was calculated from the fixation, denitrification, rainfall, and runoff 

 values. There was a net input of 10.5 mg DIN m"^ and of 69.6 mg DON 

 m"^ The largest inputs were summer rainfall, nitrogen fixation, and 

 spring runoff, while runoff was the biggest export. The amounts retained 

 (80 mg N m^^) were small compared with the total amounts in the pond 

 (219 g N m"^) and its sediments but the total amount added each year, 

 about 80 mg N m"", was enough to supply 66% of the DIN needed for 

 plankton photosynthesis in the pond. 



Ammonia is the preferred nitrogen source for the plankton. The mean 

 uptake rate was 0.16 Mg NH3-N liter"' hr"' which replaces all the NHsin 

 150 hours. Nitrate was taken up much more slowly (0.004 Mg NO 3-N 

 liter " ' hr " ', turnover time more than 3000 hours). The measured nitrogen 

 uptake rate agrees well with the calculated rate based on the 

 photosynthesis. If the C:N ratio is 8:1, then the plankton productivity of 1 

 g C m"^ yr"' gave an averge uptake rate of 0.17 Mg N liter ' hr ', very 

 close to the measured value. 



Similar calculations for the benthic algae give an average uptake rate 



-2 u_-i 1 r„_ti t„j _!„_* :..„„ -7 en „ tvt „ 2 u_i 



of 500 Mg N m "" hr " and for the rooted plants gives 750 Mg N m hr 

 There is adequate NH3 for the benthic algae (1400 mg NH3-N m "^ in the 

 top 20 cm) but barely enough NH3 for the rooted plants (only 18 mg NH3- 

 N m~^ in the top 20 cm of the plant beds). Obviously, ammonia is being 

 rapidly regenerated. This regeneration or supply rate for the water column 

 was measured with isotope dilution experiments ( ""N). The average supply 

 rate was 1 .9 Mg NH 3-N liter " ' hr " ' which is more than adequate to supply 

 the plankton. There are no data for the sediments. 



Nitrification likely occurs in the ponds but the evidence is indirect. 

 Nitrifying bacteria are present in the Barrow soils and high nitrate levels 

 occur early in the year. This process is certainly a slow one compared to 

 ammonification. 



When phosphorus was added to a natural pond, there was an 

 immediate increase in the uptake of nitrogen by the plankton; later, there 

 was an increase in the photosynthesis rate. In experimental ponds with 



