EPPLEY: PHVTOPLANKTON AND TEMPERATURE 



co-workers (University of Washington, 1970) 

 in tiie Peru upwelling region. 



The maximum values of /jl observed in depth 

 profiles off Peru approached those expected from 

 Figure 1 if the effect of daylength is considered 

 (Figure 5) but were lower as a result of low 

 insolation brought about by continuous cloud 

 cover. Depth profiles of (jl roughly parallel those 

 for photosynthetic rate per weight of chlorophyll 

 a and both show diminished rates with depth as 

 a result of decreasing light. 



Figure 5 also shows a depth profile of /a for 

 the North Central Pacific where /u. was depressed 

 because of low ambient nutrient concentrations. 

 Enrichment experiments suggested that growth 

 rate was limited at two stations by low concen- 

 trations of both nitrogen and phosphorus and at 

 a third station by nitrogen alone (Perry, in 

 press) . 



Thomas (1970b) and Thomas and Owen 

 (1971) reported values of fju for 10 m depths in 

 the eastern tropical Pacific Ocean. In situ /u, 

 was estimated to be about 0.2 doubling/day re- 

 sulting from low ambient nitrogen concentra- 

 tion. Shipboard cultures were enriched with 

 various concentrations of nitrogen (nitrate and 

 ammonium), and the variation of fx with nitro- 

 gen concentration was determined (Thomas, 

 1970b), Maximum values of /x were 1.1-1.5 

 doublings/day. 



In many cases nutrient limitation (in the up- 

 per surface waters) or light limitation (in deep- 

 er waters and in nonstratified water where ver- 

 tical mixing may reduce the effective light level 

 to which the phytoplankton are exposed) ap- 

 pears to decrease /x. The values expected from 

 Figure 1 are not realized under such conditions 

 and fi shows little or no temperature-dependence. 



Table 3 presents growth rates measured by 

 three different methods (i.e., from the velocity 

 of nitrogen assimilation per unit particulate ni- 

 trogen, from the photosynthetic carbon assim- 

 ilation rate per unit phytoplankton carbon, 

 where the carbon content of the phytoplankton 

 crop was determined from ATP, and from cell 

 concentration and cell volume). Growth rates 

 from the three methods usually agree within a 

 factor of two, but more precise methods are de- 

 sirable. The value from ^''N assimilation rate 



100 



GROWTH RATE (doublings/day) 

 0.2 0.4 0.6 0.8 1.0 



50 



1.2 1.4 



~i 1 T 1 r- 



I 



10 



NO 

 Me 



Figure 5. — Variation in growth rate of natural marine 

 phytoplankton with depth in the Peru Current, June 

 1969, and in the subtropical North Pacific central gyre, 

 November 1971 (this laboratory, unpublished). The 

 "light depth" of the ordinate was calculated as the ra- 

 diant energy at depth as a percentage of that at the 

 surface so that data from the two regions, with euphotic 

 zone depths of about 30 and about 150 m, could be com- 

 pared. The calculated line is based on Equation (1) 

 for 19 °C with the assumption that light limits growth 

 rate below the surface. The iJ.„^a\ from Equation (1) 

 was multiplied by (7/2.5 + H where / is the radiant 

 energy at depth as percent of surface. The half-satura- 

 tion constant of 2.59c is low (see Rodhe, 1965) and sug- 

 gests that the Peru Current phytoplankton were "shade 

 adapted." Hence, measured /x would be less than ex- 

 pected from Equation (1), in spite of abundant nu- 

 trients. In the North Pacific study enrichment exper- 

 iments and other data suggested limitation of phyto- 

 plankton growth rate by both nitrogen and phosphorus 

 concentration (Perry, Renger, Eppley, and Venrick, un- 

 published data). There the temperature in the mixed 

 layer was 22 °C and the maximum expected value would 

 be slightly greater than shown for the Peru Current. 



1071 



