EPPLEY: PHYTOPLANKTON AND TEMPERATURE 



the euphotic zone judged from the rough pro- 

 portionality of ATP to chlorophyll. Estimates 

 derived from ATP appear to agree well with 

 those given by the first method (Holm-Hansen, 

 1969) and the ratio C/ATP approximates 250. 

 In determining an average fi for the phyto- 

 plankton the carbon content, as measured above, 

 is taken at the beginning of the photosynthesis 

 measurement to give phytoplankton carbon at 

 time zero (Co). The measured daily rate of 

 photosynthetic carbon assimilation, assumed to 

 represent net carbon assimilation (AC), is then 

 added to the carbon content after a day's growth. 

 The specific growth rate is then calculated as: 



for Pyrocystis species in situ (E. Swift, Uni- 

 versity of Rhode Island, personal communica- 

 tion). Changes in cell morphology related to 

 cell division probably give the least ambiguous 

 estimates of /u where advection and sinking are 

 not serious problems and when a parcel of water 

 can be followed over time. The time course of 

 change in valve diameter in diatoms seems to be 

 out of favor for estimating /a since valve di- 

 ameter in cultures may not decrease in a regular 

 way or always be proportional to the number of 

 cell divisions. Methods of measuring microbial 

 growth rates were recently reviewed by Brock 

 (1971). 



1 , ,Co + AC .n\ 



^ = T ^°^^ ^ C^ ) ^2) 



to give fx in doublings of cell carbon per day. 



It should be straightforward to compute /u, 

 using ATP determined initially and after 24-hr 

 incubation, and this has been done at least once 

 (Sutcliffe, Sheldon, and Prakash, 1970). We 

 have used chlorophyll a values, before and after 

 24- or 48-hr incubation, to compute /a but the 

 results were poor due to the plasticity of cell 

 chlorophyll a content and the difficulty of pro- 

 viding incubation conditions sufficiently close to 

 those in nature to maintain constant cell chlor- 

 ophyll a per cell or per weight of carbon (Eppley, 

 1968). 



Increase in the total volume of particulate 

 matter, determined with an electronic particle 

 counting and sizing machine, can also be used 

 to compute fx (Parsons, 1965; Cushing and Nich- 

 olson, 1966; Sheldon and Parsons, 1967). This 

 method holds much promise when changes are 

 large enough to be significant/)ver background 

 levels of particulates. The cost of the machines 

 is a serious drawback to wider use, and the prob- 

 lems in proper incubation of the sample to mimic 

 conditions in the sea are as serious here as in 

 the other incubation methods. 



Sweeney and Hastings (1958) used the per- 

 centage of paired dinoflagellate cells in cultures 

 as a measure of the time of day of cell division 

 and this has been used at sea (R. Doyle, Duke 

 University, personal communication). A vari- 

 ation on this theme has allowed estimates of /u. 



RESULTS OF GROWTH RATE 



MEASUREMENTS IN THE 



NATURAL PHYTOPLANKTON AT 



DIFFERENT TEMPERATURES 



In their classic paper of 1949, Riley, Stommel, 

 and Bumpus expressed photosynthetic rate as 

 the daily carbon assimilation per unit plant car- 

 bon, a measure readily calculated as fx in dou- 

 blings/day. They used Baly's equation as a 

 model. This equation includes temperature as 

 a variable influencing photosynthetic rate. The 

 constants in the equation were computed from 

 Baly's compilation of data on Chlorella cultures 

 and detached leaves, and from Jenkin's 1937 data 

 for a culture of Coscinodiscus incubated at var- 

 ious depths in the sea. I have calculated ex- 

 pected values of ^ using their Equation 6 for 

 different levels of total incident radiation (Fig- 

 ure 4). It is seen that the Baly equation is rel- 

 atively insensitive to temperature, in comparison 

 to Figure 1, and gives values inconsistent with 

 the results from laboratory cultures. 



Bunt and Lee (1970) provide a unique set of 

 data on the photosynthetic rates of Antarctic 

 phytoplankton which grow under the ice layer, 

 an environment with low ambient light and with 

 temperature approximately — 2°C. They also 

 provide seasonal values of the particulate carbon 

 and chlorophyll a concentration. A maximum, 

 midsummer, value of /a was less than 0.5 dou- 

 blings of cell carbon/day. 



Most of the data which allow estimates of ^ 

 are from nutrient-poor waters, such as are found 



1069 



