206 



V. Alexander et al. 



0.08 



0.06 - 



c 

 E 



-2" 0.04 - 



IT) 



d 



0.02 - 



1 < 1 1 1 ' 1 r 



•Total Daily Radiation 



20 



Jun 



Plonktonic 



_J I I i_ 



10 20 



Jul 



10 20 

 Aug 



800 



400 ^ 



FIGURE 5-17. Iqj at 8°C during the summer of 1973. 

 The dotted line is the total daily radiation of the pond 

 surface. 



The /o.s values for both the epipelic algae and phytoplankton (Figure 

 5-17), although they vary over the season, are not unusual when compared 

 with other aquatic systems. The tundra pond values are slightly lower, for 

 example, than the /o 5 values of 0.04 to 2.0 ly min"' calculated for a 

 natural Asterionella population (Tailing 1957). Photosynthesis in the pond 

 populations became light saturated at 0. 10 to 0.20 ly min " ', placing them 

 in the middle of a wide range of literature values summarized in Strickland 

 (1960). For this reason, unusually high /o 5 values would not appear to be 

 contributing to the low seasonal productivity of the ponds. 



The epipelic /05 was higher than the planktonic /05 at all 

 experimental temperatures throughout most of 1973 (e.g., at 8°C, Figure 5- 

 17), suggesting that the sediment algae are better adapted to high light 

 intensities than are the phytoplankton. Similar results come from 

 intertidal areas (Burkholder et al. 1965, Gargas 1971). However, it is our 

 view that such comparisons may be invalid because of the necessary 

 presence of sediment particles in the epipelic samples used for our ^''C 

 measurements (see Stanley 1976a). At all times, some of the algae will be 

 shaded by the particles. Thus, if shading effects by the sediment particles 

 were significant, the average light intensities at the epipeUc cell surface 

 would be much lower than the light intensities incident on the whole 

 sample and the calculated /0.5 values would be too high. Attempts to 

 separate the epipelic cells from the sediment were unsuccessful. 



Both epipelic and planktonic /o 5 values are highest in early summer 

 and lowest in late August. Epipelic values declined from 0.06 ly min " ^ on 

 25 June to about 0.02 ly min "' by 27 August (at 8°C); planktonic values 

 declined from 0.04 ly min ' to 0.01 ly min^^ between the same dates 

 (Figure 5-17). A roughly parallel decline also occurs in the total incident 

 illumination following the summer solstice; this suggests a positive 

 adaptive response of the algae to declining light as discussed above. 



