EPPLEY: PHYTOPLANKTON AND TEMPERATURE 



THE VARIATION OF 



ASSIMILATION NUMBER WITH 



TEMPERATURE IN THE SEA 



The maximum expected values of /u, at dif- 

 ferent temperatures, from Equation (1), can be 

 used to predict maximum assimilation numbers 

 to be expected in the sea (as grams carbon/gram 

 chlorophyll a per time). Combining Equations 

 (1) and (5) gives rise to Figures 8 and 9 to 

 show assimilation numbers per day and per hour 

 for different C/Chl. a ratios in the phytoplank- 

 ton. Actual rates would be lower than those 

 shown for the reasons already discussed and 

 would require the growth of small-celled phy- 

 toplankters with light essentially saturating for 

 photosynthesis and with adequate nutrient con- 

 centrations. Aruga (1965b) presents graphs of 

 assimilation numbers vs. temperature, with var- 

 ious light levels, for Sce7iedesmus sp. grown at 

 20°C. His curves resemble these in form. 



The question of the influence of daylength 

 upon /ji is ignored in Figure 8 and needs further 

 investigation before generalities may be drawn. 

 In Figure 9 it was assumed that /jl in natural 

 phytoplankton assemblages will be one-half the 

 value calculated from Equation (1) since that 

 function assumes continuous light rather than 

 natural illumination of, on the average, 12 hr 

 light and 12 hr dark. 



There are several reasons why the dramatic 

 potential effects of temperature on assimilation 

 number are not often observed in oceanic studies 

 and why so little variation in assimilation num- 

 bers has been observed (cf. Ryther and Yentsch, 

 1958; Curl and Small, 1965). One of these is 

 that growth at different temperatures results in 

 shifts in the chemical composition of phyto- 

 plankton. Increased C/Chl. a ratios at low tem- 

 perature would tend to increase assimilation 

 numbers in cold waters over those predicted by 

 Figures 8 and 9 and a constant C/Chl. ft ratio 

 cannot be assumed. 



Steemann Nielsen and Jorgensen (1968a, b) 

 point out that while the lowering of the tem- 

 perature of a culture of Skeletonema costatum 

 reduced the growth rate (by an amount to be 

 expected from Figure 1 and Equation (1)), the 



A C/CHL = 30 



Q C/CHL = 60 



X C/CHL = 90 



X C/CHL = 120 



10 20 

 TEMPERflTURE IN DEGREES C. 



Figure 8. — The variation in maximum expected rate of 

 photosynthesis (assimilation number) with temperature. 

 Rates were computeci by combining Equations (1) and 

 (5) and are expressed as milligrams carbon/milligram 

 chlorophyll a/day. Continuous light was assumed. 



i C/CHL = 30 



a C/CHL = 60 



X C/CHL = 90 



E C/CHL = 120 



10 20 



TEMPERRTUBE IN DEGREES C. 



Figure 9. — Maximum expected photosynthetic rate (as- 

 similation number) from Equations (1) and (5) with 

 the assumption that the growth rate will be one-half 

 the value predicted by Equation (1) to adjust for natural 

 daylength averaging 12 hr light/day. Photosynthetic 

 rates are expressed as milligrams carbon assimilated/ 

 milligram chlorophylll a/hour. This figure gives values 

 more in line with ocean measurements than does Fig- 

 ure 8. 



1075 



