Primary Producers 211 



temperature experiments showed a subsequent die! rhythm in rate with a 

 peak at about 1800 and an amplitude of ±25% of the mean rate. The 

 rhythm was suppressed, however, in samples taken directly from the pond 

 or held in carboys exposed to normal oscillations in light intensity and 

 temperature. 



While endogenous rhythms in photosynthesis and other algal 

 processes such as nutrient uptake (Chisholm 1974) may-Well occur in the 

 Barrow phytoplankton and may be important in determining species 

 composition (Stross et al. 1973, Stross and Pemrick 1973), their direct 

 contribution to the low production levels seen in the Barrow ponds is 

 probably negligible. Before such a rhythm could cause a large decrease in 

 productivity, its amplitude must be large in comparison to light- and 

 temperature-induced cycles in photosynthesis and be out of phase with 

 them. A species having this rhythm, it would seem, would be quickly 

 eliminated from the plankton assemblage. Furthermore, the usual absence 

 of the rhythm in samples exposed to normal fluctuations in light and 

 temperature and the frequent reversal of this suppression by addition of 

 phosphate suggests that at most times the rhythm is insignificant because 

 of nutrient limitation. 



Nutrients 



Pond studies at Barrow prior to the IBP program as well as 

 preliminary chemical analyses early in this study suggested that nutrient 

 limitation was probably an important control of photosynthetic rates. 

 Kalff (1971), using 48- hour bottle bioassays, demonstrated both 

 phosphorus limitation at times of peak phytoplankton productivity and 

 ammonia limitation early in the season in water overlying unmelted ice 

 and later during a period of very high photosynthesis. In addition, both N 

 and P concentrations are low in tundra ponds (Figures 4-12, 4-16) in 

 comparison with temperate ponds. 



Our nitrogen data, viewed as a whole, indicate that the rates of supply 

 of nitrogen in both the water column and sediments of the ponds usually 

 exceed the average rates of nitrogen uptake by the algae. The mean rate of 

 uptake of ammonia, the preferred form of N for these phytoplankton, was 

 0.29 Mg N liter ' ' hr ~ ' in 1 970. This is about 1 % of the average ammonia 

 concentration in the pond water (32.8 ng N liter '). The rate of recycling 

 of ammonia via biological ammonification in the water column was 

 estimated from ^''N dilution procedures to range from 0.4 to 3.3 Mg N 

 liter' hr"' in 1970 and 1971. Hence, ammonification processes alone 

 could probably maintain a slightly higher rate of N uptake than is usually 

 seen in the ponds. In addition, indirect evidence suggests that ammonia in 

 the water column is also resupplied at a high rate from the sediments. For 

 example, the addition of ammonia to bioassay bottles did not increase 



