FISHERY BULLETIN: VOL. 70, NO. 4 



photosynthetic rate at light saturation was de- 

 creased by a lesser amount. Assimilation num- 

 bers for S. costatum at 2° or 8°C were higher 

 than would be expected from Figure 9, if it were 

 assumed that a constant C/Chl. a ratio was main- 

 tained at all temperatures. They observed that 

 cells at low temperature contained greater 

 amounts of photosynthetic enzymes and of or- 

 ganic matter than at higher temperatures. For 

 example, S. costatum assimilated 10.2 picogram 

 (pg) carbon/cell in one generation at 20°C, but 

 19.5 pg at 7°C (Jorgensen, 1968). The carbon 

 content of a cell nearly doubled between 20° and 

 7°C. Dunaliella tertiolecta cells were likewise 

 larger at low temperature than at high temper- 

 atures as were cells of Ditybmi brightivellii 

 (Table 5). This phenomenon seems to be gen- 

 eral for mesothermal marine phytoplankton, but 

 data for cold water species are not available. 

 Fluctuations in C/cell and in the C/Chl. a are 

 about twofold over the 10°-15°C range studied 

 (Table 5). Steele and Baird (1962) reported 

 high C/Chl. ft ratios in winter in Loch Nevis 

 and suggested that they resulted from low light 

 "etiolation." One wonders if low winter tem- 

 peratures might also play a role in this. 



We have seen that low temperature reduces 

 the assimilation number and promotes increased 

 carbon/chlorophyll a ratios. Similar effects re- 

 sult from nutrient deficiency and were well doc- 

 umented by McAllister, et al. (1964). An in- 

 fluence of nutrient deficiency on fx, was shown 

 also in Figure 5 for the North Pacific and was 

 noted in the eastern tropical Pacific (Thomas, 

 1970b). Low assimilation numbers for phyto- 

 plankton photosynthesis in nutrient-impover- 

 ished waters are well known (Curl and Small, 

 1965) and are clearly shown by Ichimura (1967; 

 see his graph of assimilation number vs. phos- 

 phate concentration in the waters of Tokyo 

 Bay). Caperon, Cattell, and Krasnick (1971) 

 reported 10 year increases in assimilation num- 

 bers in Kaneohe Bay, Oahu, Hawaii (from ap- 

 proximately 6-8 to 11-13 between 1960 and 1970) 

 which attended increased rates of waste dis- 

 charge into the bay. Hepher (1962) found as- 

 similation numbers of about 4 in unfertilized 

 fish ponds while values in fertilized ponds av- 

 eraged about 7.6. Furthermore, there are many 

 examples of enhanced "C-assimilation rates in 

 shipboard enrichment experiments in response 

 to nutrient additions. A recent report is that 



Table 5. — Carbon content of a cell and carbon/chlorophyll a ratios in 

 phytoplankton cultured at different temperatures. 



' D. brightwellii was cultured with irradiance 0.05 cal/cm^/min with periodic illumination 

 12L : 12D by Checkley (1972, see footnote 2 below). Values are for samples at the begin- 



ning of the light period 



2 Checkley, D. 1972. 



carbon to chlorophyll a 



Resour., La Jolla, Calif 



^ D. terlioUcta was 



The effect of the variation of growth temperature on the ratio of 

 1 a laboratory culture of Ditylum brightwellii. Univ. Calif., Inst. Mar. 

 (Unpubl. manuscr.) 

 cultured under continuous light with irradiance 0.07 cal/cm2/min. 



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