EPPLEY: PHYTOPLANKTON AND TEMPERATURE 



Rates of growth given by Equation (1) are 

 much higher than those which permit the op- 

 eration of mass cultures at maximum efficiency 

 of light utilization or nutrient removal. Max- 

 imum production will be achieved when the 

 product of /x and standing stock is a maximum, 

 and light is likely to be limiting growth at some 

 depth in the culture under these conditions (see, 

 for example, Ketchum, Lillick, and Redfield, 

 1949; Myers and Graham, 1959). 



The data of Figures 1 and 2 apply to cultures 

 grown with continuous illumination (or with 

 optimum daylength for those in which /^ passes 

 through a maximum at intermediate daylength 

 [Castenholz, 1964; Paasche, 1968]). This les- 

 sens the utility of the data for predictive pur- 

 poses with natural phytoplankton exposed to 

 seasonally varying daylength since the daylength 

 for maximum fx varies among species (Table 1). 

 Efforts to generalize on the influence of day- 



M Na CI 



25 



30 



35 

 TEMPERATURE 



40 



45 



Figure 3. — Growth rate vs. temperature curves for 

 Dunaliella tertiolecta measured in culture media con- 

 taining different salt concentrations (R. W. Eppley and 

 F. M. Macias, unpublished data). 



Table 1. — Daylength resulting in maximum growth rate 

 for some algae which show depressed growth rate in 

 continuous light. Some species which showed maximum 

 fi in 24 hr light/day are shown for comparison. 



Units are doublings/day. 



length on /x have not been very successful since 

 the daylength allowing maximum /x at a given 

 temperature seems to vary with the intensity of 

 illuinination (Tamiya et al. 1955; Terborgh and 

 Thimann, 1964). A proportion between fi and 

 the number of hours of light/24 hr is often as- 

 sumed but this can be only a first approximation. 



Use of Figure 1 and Equation (1) for insight 

 on the behavior of natural phytoplankton re- 

 quires the further assumption that the organ- 

 isms present are reasonably adapted to ambient 

 temperatures and are, preferably, at a temper- 

 ature somewhat less than optimum. Aruga 

 (1965a) has shown this to be so for the phjrto- 

 plankton of a pond on the University of Tokyo 

 campus. Smayda (1969) has discussed his own 

 and earlier observations on the distribution of 

 phytoplankton in nature where temperature 

 optima for growth in laboratory cultures were 

 invariably higher by several degrees than the 

 water temperature in which the species flourish. 



Figure 2 suggests that /jl for suboptimal tem- 

 peratures will be only slightly lower than would 

 be predicted from the maximum n for the species 

 given a temperature coefficient (Qio) for growth 

 of about 2. However, some organisms show a 

 critical lower temperature, above the freezing 

 point of water, below which no growth occurs. 

 Ukeles (1961) has listed such lower critical 

 temperatures for several species, and see Smayda 

 (1969) for another example. Temperatures in 

 excess of the optimum for growth result in a 

 much steeper decline in /jl with increasing tem- 

 perature than do suboptimal temperatures; 

 growth in this thermal region would be risky 



1067 



