FISHERY BULLETIN: VOL. 70, NO. 4 



Table 3. — Some comparison of the average growth rate of phytoplankton in the euphotic 

 zone in southern California coastal waters using different methods of estimation. 



1 Methods: (a) /i from photosynthetic rate and ATP X 250 = standing stock as carbon. 



(b) A from photosynthetic rate and standing stock carbon computed from cell numbers and cell 

 volumes. 



(c) II computed from assimilation rate of nitrate -\- ammonium + urea per unit particulate nitrogen. 

 Data for method (c) from McCarthy (1971) and Institute of Marine Resources (1972, see text 

 footnote 2). Other data are unpublished values from this laboratory. Surface water temper- 

 atures were 18°-20°C. Maximum expected growth rates would be about 1.5 doublings/day. 



tends to be lower than those from "C assimila- 

 tion rate because no correction was made for the 

 detrital nitrogen in the particulate matter, while 

 detrital carbon is not a complication in the other 

 methods. Low growth rates in these samples 

 resulted from nitrogen limitation. 



Rates of nitrogen assimilation per weight of 

 particulate N were measured in the Sargasso 

 Sea and Peru upwelling regions (Dugdale and 

 Goering, 1967; Dugdale and Maclsaac, 1971), 

 and in the eastern tropical Pacific Ocean (Goer- 

 ing, Wallen, and Nauman, 1970) which allow 

 estimates of yu,. 



As is readily seen from the above discussion 

 and the values of Tables 2 and 3 we have very 

 little data at hand to properly evaluate the role 

 of temperature in determining maximum rates 

 of phytoplankton growth in the sea, and whether 

 Figures 1 and 2 and Equation (1) are useful 

 guides for field work. It is hoped that this lack 

 will stimulate more effort to make growth rate 

 measurements. Most needed are /x values for 

 cold waters and warm, nutrient-rich waters. 



Meantime let us turn to lakes and ponds. Ad- 

 ditional growth rate data are available and the 

 influence of temperature on growth rate is often 

 apparent. Since growth rates seem comparable 

 in laboratory cultures for freshwater and ma- 

 rine unicellular algae, as noted earlier, /x vs. 

 temperature in lakes should be of equal interest 

 to limnology and oceanography. Some data are 

 given in Table 4 which confirm low jx values in 

 cold water and a variation in ^ with temperature 

 in outdoor ponds. 



The phytoplankton growth rates in lakes 

 which show a variation in ^ with temperature 

 were usually measured in the spring as the 

 waters were gradually warming but before nu- 

 trients were depleted to levels limiting to the 

 rate of phytoplankton growth (cf. Cannon, 

 Lund, and Sieminska, 1961). Presumably simi- 

 lar data could be gathered for nutrient-rich 

 estuaries or for temperate, coastal sea areas 

 where sufficient warming occurs to obtain a 

 reasonable range of temperatures before strati- 

 fication and nutrient depletion become severe. 

 The seasonal succession of phytoplankton in 

 coastal ocean waters has been much studied, and 

 the change in the phytoplankton crop from pre- 

 dominantly diatoms to flagellates, with the on- 

 set of nutrient depletion, would be accompanied 

 by a marked decrease in growth rate. One may 

 judge the magnitude of change from the com- 

 parison of /i in the Peru Current with /x in the 

 North Pacific central gyre (Figure 5). 



INTERRELATION BETWEEN SPECIFIC 



GROWTH RATE OF PHYTOPLANKTON 



AND ASSIMILATION NUMBER 



The specific growth rate of phytoplankton in 

 laboratory cultures is often measured from the 

 rate of increase in the concentration of cells in 

 the culture when cell counts are determined over 

 a time interval, i.e., 



H- = 



1 



M 



logs 





(3) 



1072 



