208 V. Alexander etal. 



2.8 



2.4 



2.0 



OD 



E 



a. 



1.6 



1.2 



0.8 ■ 



0.4 



-1 1 1 1 1 1 1 1 1 1 r- 



10 20 

 Jun 



10 20 



Jul 



10 20 

 Aug 



FIGURE 5-19. Ratio o/P^^^ to B (both in 

 mg C) for phytoplankton at 8 °C in Pond 

 B, 1971. 



with no apparent July peak (Figure 5-10) as occurred for the 1973 Pmax 

 values (Figure 5-18). This suggests that the ratio of the epipelic Pmax to B 

 declines in late summer. However, no measurements could be made of the 

 epipelic biomass actually photosynthesizing in the top several millimeters 

 of sediment because it is not possible to section the coarse, unconsolidated 

 sediments. If, as we think likely, these "surface" algae undergo cumulative 

 burial into the dark underlying sediments so that the ratio of "surface" to 

 "buried" algae decreases over the summer, then an apparent but incorrect 

 decline in the ratio o{ Pmax to B would result. In fact, in simulation runs of 

 the benthic carbon-flow sub-model (see Chapter 10), the seasonal curve of 

 "surface" algal biomass is identical in form to the Pmax curve of Figure 5- 

 18 (see Stanley 1976b). 



The tundra pond phytoplankton are frequently inhibited by 

 supraoptimal light intensities while even on the brightest of days the 

 epipelic algae are undersaturated. This is illustrated in Figure 5-15, which 

 shows the photosynthesis versus light curves, and in Figure 5-20, which 

 gives the ratio of observed photosynthetic rates {P) at ambient light 

 intensity to the light-saturated rate {Pmax). A ratio of 1.0 in Figure 5-20 is 

 equivalent to light-saturated photosynthesis; values less than 1.0 represent 

 the degree of light undersaturation or oversaturation (inhibition) of 

 photosynthesis. The epipelic algae were never inhibited and their 

 photosynthesis was as much as 50% below saturation at 20°C, while at the 

 same temperature the phytoplankton were around 80% light-saturated. At 

 lower temperatures, however, light inhibition was common in the pelagic 

 algae with the strongest inhibition (50% or greater) occurring at the lowest 



