216 



V. Alexander et al. 



0.5 



0.4 



o 0.2 



a. 



0.1 - 



Pond B-4 ^ 

 HI PO4 



Phytoplankton 



Pond B 



20 50 100 



500 



Pond B-4 

 HI PO4" 



20 



50 100 



I 



500 



^g P liter added 



FIGURE 5-25. Photosynthesis of planktonic 

 and benthic algae from Pond B and from a 

 phosphorus-enriched subpond (B-4) meas- 

 ured at different phosphate additions. The 

 experiment was run on 16 August 1972 from 

 1300 to 1900 hours (Alaska Daylight Time). 



The contradictory results from the long- and short-term nutrient 

 bioassays should be noted. Lack of response to nutrient addition in short- 

 term bottle bioassays is not sufficient evidence, in itself, of lack of 

 photosynthetic limitation by nutrients. The short-term ^''C assay does 

 appear to have been used in this way in earlier studies (Ryther and 

 Guillard 1959, Goldman 1963). 



Although we attempted to carry out a detailed experimental analysis 

 of the biologically important phosphorus fractions and their interactions 

 in the ponds (Chapter 4, Prentki 1976), apparent contradictions remain 

 and the specific mechanisms of phosphorus control on photosynthesis 

 remain unidentified. For example, the phytoplankton at Barrow appear to 

 take up phosphate into a functionally distinct "soluble" pool at a rate 

 approximately 200 times that needed to sustain growth, despite the fact 

 that they are P-limited. Only a very small fraction of the assimilated P is 

 then transferred into the "bound," structural phosphorus pool and the 



