EPPLEY: PHYTOPLANKTON AND TEMPERATURE 



This can also be expressed as 



fX = 



1 



Ai 



log2 ( ]^^^^ ) (4) 



where Ni is the initial cell concentration, No the 

 cell concentration after an interval of time, M, 

 and AN" is N2 — Nu To determine jx from 

 analogous carbon units one needs the initial 

 carbon content of the phytoplankton (Ci) and 

 either the carbon content after a time interval 

 A^, i.e., C2, or a measure of carbon assimilation 

 by the phytoplankton during the time interval, 

 i.e., AC. It will be assumed that the "C method 

 of measuring phytoplankton photosynthesis 

 (Steemann Nielsen, 1952) in fact measures AC, 

 the net increase in particulate carbon in the phy- 

 toplankton. This is indicated by several studies 

 with laboratory cultures which include two or 

 more independent measures of the rate of carbon 

 assimilation by the phytoplankton cells (Antia 

 et al., 1963; McAllister et al., 1964; Eppley and 

 Sloan, 1965; Ryther and Menzel, 1965a; Strick- 

 land, Holm-Hansen, Eppley, and Linn, 1969). 

 Then fi can be calculated from carbon data from 

 Equation (2). The evaluation of fi requires a 

 measurement of photosynthetic rate as carbon 

 and an estimate of the carbon content of the 

 phytoplankton at the initiation of the measure- 

 ment. Direct methods for the latter are not usu- 

 ally suitable because of detrital carbon in na- 



tural waters and indirect methods must often 

 serve (see earlier discussion of methods of 

 measuring fi) . A convenient way of expressing 

 photosynthetic rate per unit phytoplankton 

 standing stock is the "assimilation number," i.e., 

 the rate of photosynthetic carbon assimilation 

 per weight of chlorophyll ft. The terms "assim- 

 ilation ratio" and "photosynthetic index" are 

 common synonyms for assimilation number. If 

 the carbon/chlorophyll a ratio in the phytoplank- 

 ton is known, its carbon content can, of course, be 

 calculated from chlorophyll measurements. Usu- 

 ally this is not the case and considerable effort 

 has been expended to derive such estimates (see, 

 for example, Harris and Riley, 1956; Cushing, 

 1958; Wright, 1959; Steele and Baird, 1961, 

 1962; Lorenzen, 1968; Eppley, 1968; Zeitzschel, 

 1970; Hobson, 1971). An equation expressing 

 fx (as doublings of cell carbon /day) in terms of 

 the assimilation number per day and the carbon/ 

 chlorophyll ration of the phytoplankton can be 

 derived from Equation (2) as 



H- 



1 



M 



, C/Chl. a + AC/Chl. a . ,_, 



^°^^ ^ OTChTi )• ^^) 



This equation is useful in that it directly relates 

 the assimilation number, i.e., the photosynthetic 

 rate per weight of chlorophyll (AC/Chl. a), the 

 carbon/chlorophyll a ratio of the phytoplankton 



Table 4. — Phytoplankton growth rates in lakes and ponds. 



1073 



