FISHERY BULLETIN: VOL. 70, NO, 4 



(C/Chl. a), and /jl. Figures 6 and 7 show this 

 relationship graphically where the calculated as- 



CO CJ 

 t- O 



& C/CHL = 30 



Q C/CHL = 60 



X C/CHL = 90 



X C/CHL = 120 



1 2 



SPECIFIC GROWTH HATE 

 (DOUBLINGS/DRY) 



Figure 6. — Photosynthetic rate (assimilation number/ 

 day) vs. the specific growth rate of the phytoplankton 

 computed from Equation (5). Photosynthetic rate is 

 expressed as milligrams carbon assimilated per day per 

 milligram chlorophyll a and is shown for several values 

 of the ratio carbon/chlorophyll a in the phytoplankton 

 crop (30, 60, 90, and 120 g/g). 



t— o 



A C/CHL = 30 



CD C/CHL = 60 



X C/CHL = 90 



X C/CHL = 120 



1 2 



SPECIFIC GROWTH RRTE 



(D0U6LINGS/DRT1 



similation number per day (Figure 6) or per 

 hour (Figure 7) is graphed as a function of fi 

 for different carbon/chlorophyll a ratios in the 

 crop. Carbon/chlorophyll ratios of Figures 6 

 and 7 are typical of the Peru upwelling region 

 (C/Chl. a 30-40) (Lorenzen, 1968; Strickland, 

 Eppley, and Rojas de Mendiola, 1969; Beers 

 et al., 1971) and the Western Arabian Sea 

 (Rjrther and Menzel, 1965b), or low^nutrient 

 surface waters off southern California (90-100) 

 (Eppley, 1968; Strickland, 1970); and of sur- 

 face waters in the North Pacific central gyre 

 (120-150) (Institute of Marine Resources, un- 

 published data) . The marked dependence of the 

 assimilation number upon the carbon/chloro- 

 phyll a ratio of the phytoplankton is noteworthy, 

 although little discussed in the literature. It is 

 interesting that assimilation numbers greater 

 than about 15 per hour (see Figure 7) are rarely 

 reported in the literature and one wonders 

 whether this is because of disbelief in the va- 

 lidity of the data or because high fx and high 

 C/Chl. a are somehow mutually exclusive in 

 nature. The latter is most likely since such high 

 assimilation rates and high fx would place ex- 

 treme demands for nutrients, such as N and P, 

 on the environment and could not long be sus- 

 tained without massive nutrient input. Even 

 at southern California sewage outfalls where 

 high rates of nutrient input prevail we found 

 low values for /x. These low values apparently 

 result from the buildup of high phytoplankton 

 crops which maintain low-nutrient levels in the 

 surface waters such that growth rate is nitrogen- 

 limited (Institute of Marine Resources).' Fur- 

 thermore, high C/Chl. a ratios seem to be typical 

 of nutrient depleted cells which grow slowly. 

 For example, carbon/chlorophyll a ratios in- 

 creased from 30 to over 150 with increasing 

 nitrogen limitation of growth in N-limited 

 chemostat cultures of marine phytoplankton 

 (Thomas and Dodson, in press; Institute of 

 Marine Resources'). 



Figure 7. — Same as Figure 6, but photosynthetic rates 

 (assimilation numbers) were calculated per hour, rather 

 than per day, assuming 12 hr light per day (i.e., values 

 of Figure 6 were divided by 12). 



° Institute of Marine Resources. 1972. Eutrophica- 

 tion in coastal waters: nitrogen as a controlling factor. 

 Final Rep. U.S. Environ. Prot. Agency, Proj. #16010 

 EHC. Inst. Mar. Resour., Univ. Calif., San Diego. 67 p. 



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