TEMPERATURE AND PHYTOPLANKTON GROWTH IN THE SEA 



Richard W. Eppley^ 



ABSTRACT 



The variation in growth rate with temperature of unicellular algae suggests that an 

 equation can be written to describe the maximum expected growth rate for temperatures 

 less than 40 °C. Measured rates of phytoplankton growth in the sea and in lakes are 

 reviewed and compared with maximum expected rates. The assimilation number (i.e., 

 rate of photosynthetic carbon assimilation per weight of chlorophyll a) for phytoplankton 

 photosjTithesis is related to the growth rate and the carbon/chlorophyll a ratio in the 

 phytoplankton. Since maximum expected growth rate can be estimated from tempera- 

 ture, the maximum expected assimilation number can also be estimated if the carbon/ 

 chlorophyll a ratio in the phytoplankton crop is known. 



Many investigations of phytoplankton photosynthesis in the ocean have included 

 measures of the assimilation number, while fewer data are available on growth rate. 

 Assimilation numbers for Antarctic seas are low as would be expected from the low 

 ambient temperatures. Tropical seas and temperate waters in summer often show low 

 assimilation numbers as a result of low ambient nutrient concentrations. However, 

 coastal estuaries with rapid nutrient regeneration processes show seasonal variations 

 in the assimilation number with temperature which agree well with expectation. 



The variation in maximum expected growth rate with temperature seems a logical 

 starting point for modeling phytoplankton growth and photosynthesis in the sea. 



Temperature does not seem to be very important 

 in the production of phytoplankton in the sea. 

 For example, Steemann Nielsen (1960) has 

 written, "Recent investigations have shown, 

 however, that the direct influence of tempera- 

 ture on organic production in the sea is fairly 

 insignificant." Other reviewers of photosyn- 

 thesis in the sea likewise give little or no con- 

 sideration to a role of temperature and Steemann 

 Nielsen's statements find widespread endorse- 

 ment in the published data on geographic and 

 seasonal variation in marine phytoplankton 

 photosynthesis. 



In response to this, the reader may ask at 

 least two questions: (1) Why is temperature 

 of so little importance and (2) why would any- 

 body write a review on temperature and phy- 

 toplankton growth in the oceans? Several an- 

 swers to the first question have appeared in 

 the literature and some of these will be discussed 



^ Institute of Marine Resources, University of Cal- 

 ifornia, San Diego, P.O. Box 109, La Jolla, CA 92037. 



in this account. I have two answers for the 

 second question. The purpose of this review is 



(1) to suggest maximum growth and photo- 

 synthetic rates which might be reasonably ex- 

 pected for natural marine phytoplankton and 



(2) to point out the interrelationship among 

 growth rate, photosynthetic assimilation num- 

 ber (i.e., rate/chlorophyll), and carbon/chlor- 

 ophyll a ratios in the phytoplankton. 



What follows is an attempt to show that tem- 

 perature sets an upper limit on phytoplankton 

 growth rate and on the rate of photosynthesis 

 per weight of chlorophyll, and that this upper 

 limit can be predicted from a knowledge of tem- 

 perature and the carbon and chlorophyll content 

 of the plants. 



It can perhaps be inferred, from above, that 

 phytoplankton growth in the oceans seldom ap- 

 proaches the upper limits imposed by the tem- 

 perature of the water and that temperature does 

 not figure prominently in simulation models for 

 primary production in the marine environment. 

 Other factors effect reduced rates of growth and 



Manuscript accepted May 1972. 



FISHERY BULLETIN: VOL. 70, NO. 4, 1972. 



1063 



