COLEBROOK: FLUCTUATIONS IN BIOMASS OF ZOOPLANKTON 



CHAETOGNATHA 



CLADOCERA 



LARVACEA 



DECAPODA 



ing to the mean + 1 SD, the mean, and the mean - 1 SD. The keys to the contour levels for each category give the arithmetic values, as 

 grams per 1,000 m 3 , corresponding to these levels. 



chosen for the production of biomass data to cover 

 a period of marked change in physical conditions 

 and in the distribution of many species in the 

 CalCOFI area. The main features of these changes 

 have been described in the proceedings of a special 

 symposium (Sette and Isaacs 1960). The most 

 striking feature was a considerable warming of 

 the surface waters which started in the south in 

 1956 and spread through the area during 1957 

 (see, e.g., Longhurst 1967). 



The general form of the change can be typified 

 by the variation in temperature in the top 50 m in 

 the southern California offshore area shown in 



Figure 7. Favorite and McLain (1973) showed that 

 this is part of a widespread change in surface 

 temperature affecting almost the whole of the 

 North Pacific Ocean. The reasons for the change 

 are not yet completely clear. The initial warming 

 in 1957 appears to be associated with a reduction 

 in the flow of the California Current which 

 occurred between the late summer of 1957 and 

 midsummer 1958. As an index of the flow of the 

 California Current, Saur (1972) used the differ- 

 ence in sea level between Honolulu and San 

 Francisco. A plot of monthly means (with a linear 

 trend removed and adjusted to normal atmo- 



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