108 R. T. Prentki et al. 



filter after incubation in situ, and the nitrogen converted to a gas for mass 

 spectrometer isotope ratio determination. 



Ammonia was the preferred nitrogen source for the plankton (Table 

 4-13), with a mean 1971 uptake rate of 0.157 ng N liter^ hr"' and a 

 maximum of 0.41 ng N liter"'. At these rates, the ammonia available in 

 the water seems adequate for the phytoplankton production; assuming 

 steady state conditions and a mean ammonia concentration of 21 Mg N 

 liter ', the turnover time is 150 hours. The mean nitrate uptake rate was 

 0.01 Mg liter ~ ' hr ~ ' which implies that the turnover time was thousands of 

 hours (the mean concentration was 13 Mg N liter " '). 



The uptake of these nutrients in Ikroavik Lake was 2 to 3 times 

 higher than in the ponds (a mean of 0.48 ng NHs-N and of 0.01 ^g NO3-N 

 liter ' hr '). Compared with other systems, the uptake rates in the ponds 

 are quite low. For example, the northwest Atlantic data of Dugdale and 

 Goering (1967) had ranges of NO3 uptake of 0.002 to 5.6 Mg N liter"' 

 hr"' while the NH3-N uptake rates ranged from 0.14 to 2.3. In Sanctuary 

 Lake, Pennsylvania, peak ammonia uptake rates as high as 4.1 Mg N 

 liter " ' hr " ' were measured while peak nitrate rates were 0.40 ^g N liter ' 

 hr " ' (Dugdale and Dugdale 1 965). 



Inorganic nitrogen is also taken up by algae on the surface of the 

 sediment and by the roots of the sedge and grass growing in the pond. The 

 primary productivity of the sediment algae and rooted plants is almost 2 

 orders of magnitude above that of the plankton algae and their uptake of 

 nitrogen is proportionately higher. 



The rate of uptake of nitrogen may be calculated from the primary 

 productivity because the ratio of C:N is approximately constant. In the 

 phytoplankton of Pond B, for example, the annual primary production is 

 about 1 g C m ' ^ This is equal to 1 20 mg N m " ^ taken up or 1 .2 mg m " ^ 

 day " ' for a 100-day growing season. If the average depth of the pond is 30 

 cm, then this is 300 liters m"^ of pond or an uptake of 4 ^g N liter"' 

 day " ' or 0. 1 7 Mg N liter " ' hr " ' . This is very close to the average uptake 

 measured with '"^N of 0.156 (Table 4-13). Thus, there is agreement 

 between the '^N uptake data and the N uptake calculated from the '^C 

 productivity. 



If we assume that benthic algae behave the same way as plankton 

 algae with respect to nitrogen uptake, then the primary productivity of 

 about 10 g C m"^ yr"' is equal to an uptake of 1.2 g N m"^ yr "' or 12 mg 

 N m"^ day"'. The large plants (Carex) have a total production of about 

 450 g dry wt m " ^ yr ' (roots and shoots) in the plant beds or about 1 80 g 

 C m"^ yr"'. Only 2% of that dry weight is nitrogen and the plants 

 translocate and conserve about half of the nitrogen from senescent tissue 

 (Chapin et al. 1975). Therefore, the total net uptake is 1.8 g N yr "' or 18 

 mg m " ^ day ' . This compares with 1 2 mg N m " ^ day " ' for the sediment 

 algae and 1 .2 mg N m " ^ day " ' for the plankton. 



There is abundant inorganic nitrogen in the interstitial water except in 

 the plant beds (Table 4-9). As a first approximation, we can assume that 



