COLEBROOK: FLUCTUATIONS IN BIOMASS OF ZOOPLANKTON 



JAN 



FEB 



MAR 



55 56 57 58 59 55 56 57 58 59 55 56 57 58 59 



YEAR 



FIGURE 10. — Graphs of principal components of upwelling index 

 for the CalCOFI survey area for each month for the years 1955- 

 59. See text and Bakun 1 19731. 



off the coast of Oregon and the catch of the 

 Dungeness crab, Cancer magister, with a time lag 

 of about 18 mo. He attributed this to an increased 

 food supply in years with pronounced upwelling, 

 implying a relationship between upwelling and 

 plankton similar in sign to that found further 

 south in the California Current. 



CONCLUSIONS 



At least during the period 1955-59, a consider- 

 able proportion of the variability from year to year 

 in the biomass of zooplankton, as represented by 

 estimates for the taxa listed in Table 1, can be 

 associated with hydrographic events, variations 

 in the strength of the California Current, and 

 variations in the intensity of coastal upwelling. 



The precise mechanisms involved are not clear, 

 but in relation to the California Current there is a 

 similarity in the relationships within the taxa 

 with respect to both geographical distribution and 

 annual fluctuations in abundance which suggests 

 that advection of stocks may be involved to a 

 considerable extent. The influence of upwelling on 

 primary production through effects on tempera- 

 ture stratification and the supply of nutrients 

 probably accounts for the relationship with the 

 zooplankton. 



The only data that have been produced rou- 

 tinely from the whole series of CalCOFI cruises, 

 which relate to plankton other than fish eggs and 

 larvae, are in the form of displacement volumes of 

 unsorted samples (Smith 1971). The marked 

 coherence between the various taxonomic cate- 

 gories suggests that these data can be expected to 

 produce estimates of long-term variations which 

 indicate real changes in the abundance of the 

 zooplankton. Such data cannot, however, reflect 

 the geographical differentiation within the zoo- 

 plankton, and this imposes a limit, to the extent to 

 which they can be used, to provide the basis for the 

 examination of the influences of a complex of 

 environmental factors of the kind suggested by 

 this study as playing an important role in 

 determining the year-to-year fluctuations in the 

 plankton. 



The taxonomic categories used in this study 

 were selected by Isaacs et al. (1969) to represent 

 the plankton as food for fish. I have used them to 

 represent fluctuations in the zooplankton as such 

 for the 1955-59 period. 



For future studies the only definitive method of 

 selecting taxa to represent year-to-year changes 

 in the zooplankton is by trial and error: there are, 

 moreover, numerous possibilities, and the labor 

 involved would be prohibitive if some compromise 

 is not made. It is indicated above that there is a 

 tendency for taxa which have similar geograph- 

 ical distributions also to show similar year-to- 

 year fluctuations in abundance. As a first approxi- 

 mation, this fact might be used as a guide to the 

 selection of representative categories. It is implicit 

 that each selected category should be geographi- 

 cally homogeneous, and the set of categories should 

 cover the full range of geographical distributions. 



It is probable that the species is the highest 

 taxon for which geographical homogeneity can be 

 assumed, and even here there may be some species 

 which have geographically differentiated races. 

 Isaacs et al. (1969) gave an estimate of about 550 



367 



