COLEBROOK: FU'(Tl'ATIONS IN HIOMASS OK ZOOI'l.ANKTON 



1955 



1956 



1957 



1958 



1959 



<I5° 

 1 5°- 1 7° 

 >I7° 



FIGURE 7. — Top) A contoured diagram 

 of monthly vertical temperature 

 profiles for the upper 50 m for the years 

 1955-59 for the southern California 

 offshore zone (see Figure 1). CalCOFI 

 survey data. Bottom) A graph of the dif- 

 ference in sea level between Honolulu 

 and San Francisco at monthly intervals 

 for the years 1955-59 ( plotted from Saur 

 1972). 



proportion of the superficial wind-driven water 

 that is swept southward out of the North Pacific 

 subarctic circulation. 



There seems little doubt that the change in 

 temperature in 1957 and its persistence through 

 1958 and 1959 is related to the relative reduction 

 in biomass of the zooplankton associated with the 

 first principal components of all zones and most of 

 the taxonomic categories. The data presented by 

 Wickett showed a marked reduction in southward 

 transport at lat. 50°N, long. 140°W during 1958 

 and 1959 and this, coupled with the reduction in 

 the flow of the California Current in 1957 and 

 1958 (Figure 7), would appear to support Wick- 

 ett's suggestion of a direct influence by water 

 movements. The relationship between the north 

 to south geographical gradient (Figure 3) and the 

 first principal components is also entirely con- 

 sistent with this hypothesis. 



An examination of the remaining components 

 for each of the zones indicated the existence of a 

 second pattern of fluctuation common to most of 

 the zones. In Figure 8 are given graphs of a 

 component, other than the first, for each zone 

 selected to give the best approximation to a form 

 common to all the zones. In 8 of the 14 zones it is 

 the second component; in the remaining zones it is 

 either the third or the fourth component. Given 

 the quantity and the quality of the original data 

 and considering the large proportion of the 

 variability of the data associated with the first 

 components, the lack of consistency in the position 



of the common pattern among the components is 

 perhaps not surprising. Figure 9 shows the same 

 for each taxonomic category; again the majority 

 are second components and only one, for Radio- 

 laria, is the fourth component. The main features 

 of the pattern are a low in 1957 and highs in 1956 

 and 1958; 1955 and 1959 tend to be low but their 

 positions vary somewhat within both the zones 

 and the taxonomic categories. 



Coastal upwelling is a feature of the California 

 Current region, and Bakun (1973) has produced 

 estimates of relative fluctuations in upwelling at a 

 number of positions along the west coast of North 

 America. They are based on estimates of the 

 offshore component of the Ekman transport which 

 is in turn estimated from atmospheric pressure 

 fields. 



Monthly means of the upwelling index for five 

 positions off the coast at latitude and longitude 

 36°N, 122°W; 33°N, 119°W; 30°N, 116°W; 27°N, 

 116 C W; and 24°N, 113°W, for the period 1955-58 

 were extracted from Bakun's report. Uncertain- 

 ties about the differences in absolute terms 

 between the estimates at different positions 

 particularly off southern California, discussed by 

 Bakun, suggested that principal components 

 might provide a good method of summarizing the 

 data from this set of positions. For each calendar 

 month, analyses were carried out on the index 

 estimates for the five positions and the 5 yr. 

 Examination of the components showed that a 

 pattern common to the first 7 mo of the year was 



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