measurements of successive links in the 

 energy chain. This is justified: (a) because no 

 better data exist, nor can they be obtained 

 without prohibitive expense until moored sta- 

 tions are very highly developed; and (b) be- 

 cause we may at present assume a steady 

 state m the tropical ocean, as other investi- 

 gators have done, rejecting only data from the 

 Gulf of Tehuantepec where it is probable that 

 such a state does not exist (see above). 



Throughout this investigation it has been 

 realized that noon station data show rather 

 consistently a significant positive linear rela- 

 tionship between phytoplankton standing crop, 

 as measured by chlorophyll a, and each of the 

 following simultaneously measured variables: 

 primary production (fig. 11) and standing crop 

 of zooplankton (see fig. 13, p. 37; also Holmes, 

 1958). This has strengthened the belief in the 

 existence of an approximately steady state. 



Variables related to primary production 

 (productivity).-- With all available stations 

 (n = 40) for the following variables: 



Xg log 1000 chlorophyll a (mg./m.^) 



X5 incident radiation (g.cal./cni.2/day) 



X4 mean mixed layer phosphate (/ig.-at./l.) 



X3 mixed layer depth (m.) 



X2 mean mixed layer oxygen (ml,/l,) 



X 2 mean mixed layer temperature (° C.) 



Y surface in situ productivity (mg.C/m,''/day) 



the multiple linear regression equation is 



Y = -36.5 + 36.3" Xg - O.O388X5 - 22.4 X4 - 0.143 X3 +15.16 Xj - 2,40 Xj 



This is significant (F = 5.79**) with r2 = 0.513. 

 With only X£, and X5 it is still significant but 

 with R'' lower (0.357) and standard error of 

 estimate slightly higher. The only variable 

 with a significant coefficient in either case is 

 X^, but correlations between the "independent" 

 variables probably obscure the effects of some 

 of them in the equation. For instance there 

 are significant correlations between X]^ and 

 X4, X3 and X5, and X5 and X(,. 



The 40 stations may be divided into a 

 "winter" group (October-February, various 

 cruises, within 200 miles of the coast) and a 

 "summer" group (April- June, TO-58-1 (SCOT), 

 up to 500 miles off the coast). For the winter 

 stations (n = 21) the corresponding equation is 



Y =-115.8 + 82.3" Xg +0.001X5 -26,5X4 -0.331X3 + 10.5X2 -2.98X1 



with F = 6.48**, and R^ = 0.735. For the sum- 

 mer stations (n = 19) we have 



Y = -70.55 + 7.56»X6 + 0.005 X5 + 46.0**X4+ 0.015X3 +3.03X2+0.888X2 



with F = 3.51* and R^ = 0.637. These results 

 suggest a stronger relationship between pro- 



04 06 0.1 2 04 06 



SURFACE CHLOROPHYLL A (MG/M^) 



Figure 11. --Surface chlorophyll a and productivity, plotted for noon 

 stations of six cruises. 



ductivity and phosphate in summer (when both 

 are, on the average, low) than in winter (when 

 they are higher). Phosphate may be limiting 

 for plant growth in suinnfier and not in winter. 

 There is a close relationship between produc- 

 tivity and chlorophyll a in both seasons. 



Variables related to zooplankton standing 

 crop . --Special interest attaches to the under- 

 standing and predictability of zooplankton 

 standing crop because it is close to tuna in 

 the energy chain. 



The purpose of the first series of analyses 

 was to seek relationships predictive for zoo- 

 plankton from nonbiological variables that 

 might be measured without too much trouble 

 from anchored stations. 



With the following variables: 



X5 mean mixed layer light attenuation coefficient (k) 



X4 mean mixed layer oxygen 



X3 mean mixed layer depth 



X2 mean mixed layer temperature 



X| incident radiation 



Y log 10 zooplankton in upper 250 or 300 m. (ml./l,000 m.3) 



the equations, for all stations (n = 40), winter 



34 



