FISHERY BULLETIN: VOL. 84, NO. 1 



Lipid content of female C. pacificus, expressed as 

 percent dry weight or percent wet weight, was lowest 

 in the southern offshore region, but was quite similar 

 between the other three regions (Table 7). Lipid con- 

 tent (percent dry or wet weight) of stage V copepo- 

 dites from the northern nearshore region was higher 

 than the other regions. This stage showed the lowest 

 lipid content in the southern offshore region (Table 

 7). 



DISCUSSION 



Upwelling was taking place along the California 

 coast during April 1981. The resulting coastal low 

 surface water temperatures were most evident in the 

 northern part of the sampling grid, especially just 

 north of Point Conception. An upwelling index calcu- 

 lated for this region during mid-April was higher 

 than the 20-yr mean (Howe et al. 1981). The cold- 

 water plume extending into the Southern Califor- 

 nia Bight (Fig. 2) is a common phenomenon that 

 occurs when cold, upwelled water from the Point 

 Conception region becomes entrained into the south- 

 ward flowing California Current (Reid et al. 1958; 

 Bernstein et al. 1977; Lasker et al. 1981). The distri- 

 bution of phy toplankton biomass (estimated by sur- 

 face chlorophyll a) was the most obvious biological 

 feature associated with coastal upwelling. Phyto- 

 plankton patchiness in turn influenced zooplankton 

 biomass and nutritional parameters. The following 

 discusses 1) the relationships between various biol- 

 ogical properties influenced by upwelling and 2) 

 the persistence and consequences of biological meso- 

 scale patchiness within the California Current 

 System. 



The distributions and abundances of both euphau- 

 siid species were similar to previous reports (Brin- 

 ton 1962, 1967b, 1976, 1981; Brinton and Wyllie 

 1976; Youngbluth 1976). Euphausia pacifica is gen- 

 erally more abundant than Nematoscelis difficilis, 

 and the center of its distribution is located closer 

 to the coast. The abundance of E. pacifica within 

 the sampling grid was positively correlated with phy- 

 toplankton biomass, as has been noted by Young- 

 bluth (1976). Other herbivorous euphausiids (eg., 

 Thysanoessa raschii and T. inermis) also show this 

 same relationship (Sameoto 1976). 



The distribution and abundance of Calanus paci- 

 ficus stages were also similar to previous reports 

 (Fleminger 1964; Longhurst 1967). Both females and 

 stage V copepodites were most abundant close to the 

 coast near upwelling regions. In contrast to E. paci- 

 fica, abundances of the two C. pacificus stages show- 

 ed rather poor (but significant at 95% level) correla- 



tions with phytoplankton biomass (r values of 0.24 

 and 0.31). This result was surprising since both 

 species are considered herbivores. The weak corre- 

 lations between C. pacificus abundance and phyto- 

 plankton standing crop probably resulted from 

 small-scale heterogeneity and poor mobility of the 

 C. pacificus population. Populations of C. pacificus 

 along the California coast show a great deal of small- 

 scale patchiness on the order of 10's to 100's of 

 meters (Mullin and Brooks 1976; Star and Mullin 

 1981; Cox et al. 1982). Grazing by copepods within 

 these patches can greatly reduce the local phyto- 

 plankton standing crop. When samples are taken on 

 scales of 1 km or less, a poor or inverse correlation 

 between phytoplankton and zooplankton biomass 

 results (Mackas and Boyd 1979; Star and Mullin 

 1981). Zooplankton samples in this study were col- 

 lected from net tows that covered distances of about 

 1 km or less. Thus, the poor correlations in the 

 present study confirm results of previous studies and 

 can be explained on the basis of the sampling 

 procedure 



Laminarinase activity (LA) of C. pacificus and E. 

 pacifica was positively related to phytoplankton 

 standing crop. However, a strong relationship be- 

 tween these variables did not exist for either species 

 (correlation coefficients between 0.53 and 0.62). 

 These results were expected because, although most 

 studies agree that zooplankton digestive enzyme ac- 

 tivity and feeding rates are closely linked, enzyme 

 levels do not always represent instantaneous inges- 

 tion rates nor are they always related to the food en- 

 vironment at the time of collection (Head and Con- 

 over 1983; Hassett and Landry 1983; Head et al. 

 1984; Willason and Cox in press). 



We propose three, non-exclusive explanations for 

 the observed weak correlations between LA and phy- 

 toplankton biomass. First, time lags of 1 to 7 d in 

 the response of zooplankton digestive enzymes to 

 changing food concentrations (Mayzaud and Poulet 

 1978; Cox and Willason 1981; Willason 1983) can in- 

 fluence the association between enzyme levels and 

 the food environment. Because the standing stock 

 of phytoplankton is often very patchy and can change 

 rapidly, especially in upwelling regions, zooplankters 

 are probably continually acclimating to new condi- 

 tions and an equilibrium may seldom be reached 

 between enzyme activity, feeding rates, and food 

 concentration. 



Second, phytoplankton concentration may occa- 

 sionally be high in terms of chlorophyll a, but poor 

 in quality resulting in low consumption rates and low 

 digestive enzyme activity. Herbivorous zooplankton 

 feeding rates have been shown to be greatly de- 



172 



