210 



V. Alexander et al. 



Our data suggest that temperature is indirectly responsible for the 

 high degree of photosynthetic photoinhibition of arctic phytoplankton. 

 Light levels during the Barrow summer (Chapter 3) are not appreciably 

 higher than at lower latitudes nor, as mentioned above, are the /o 5 values 

 for the phytoplankton. What is important, then, is the inverse relationship 

 between temperature and the minimum intensity for photoinhibition (the 

 limiting optimum light intensity, Hutchinson 1957). A similar argument 

 has been given by Sorokin and Konovalova (1973) for data on winter 

 diatoms in the Japan Sea. 



There is some evidence that the phytoplankton of these ponds are able 

 to increase their photosynthetic efficiency in response to the declining light 

 during a single day. This is similar to the seasonal response described 

 earlier and to the "sun-shade" adaptations in marine plankton (Steemann 

 Nielsen and Hansen 1961). The evidence is from Kalffs (1969) finding of a 

 three-fold increase in I max and a five-fold increase in P^ax between noon 

 and late evening in Barrow ponds. Our attempt to repeat Kalffs 

 experiment for the phytoplankton on 10 July 1973 failed completely 

 because a fog bank suddenly moved in and reduced the light to nearly zero 

 (from 2100 to 0600). Data for the epipelic algae were better but no 

 significant diurnal changes in Pmax or in /o 5 were seen (Figure 5-21). 



The results of measurements of planktonic photosynthesis under 

 constant temperature and light conditions raise the possibility of an 

 endogenous rhythm in the planktonic Pmax, despite the general observation 

 (Doty 1959) that such rhythms decrease in amplitude towards the poles. 

 Phytoplankton samples exposed to dim light or enriched with phosphate in 

 carboys for 4 to 6 hours prior to the beginning of these constant light and 



3.2 



V 2.4 



^ 1.6 



E 



0.8 







-I 1 1 1 1 1 1 1 1 r 



Pmox (8°C) 



1200 1600 2000 

 10 Ju 



0400 0800 



I I Jul 



0.20 

 0.16 



0.12 E 



0.08 



0.04 



1200 



FIGURE 5-21. Amount of photosynthetically 

 available light (half of the incident radiation), rate of 

 ^maxf^'' ^^^ epipelic algae, and \q j on 10 and 11 July 

 1973, Pond B. 



