plankton standing stock is in the mesozooplank- 

 ton fraction in this sector of the California 

 Current system. Considerably higher contribu- 

 tions occur in some regions and on some occa- 

 sions. It is difficult to estimate the contribution 

 of this standing stock to heterotrophic processes 

 such as grazing, oxidative metabolism, and fecal 

 pellet fluxes without further information on the 

 species composition of the two size fractions. 

 However, because physiological processes 

 depend on body mass (Banse and Mosher 1980), 

 the mesozooplankton would contribute dispro- 

 portionately to these water column processes. 



Consider, for example, Paracalanus parvus, a 

 relatively small-bodied copepod representative 

 of the mesozooplankton retained by a 202 |xm 

 mesh net, and the larger copepod Calanus 

 pacificus and euphausiid Euphaiisia pacifica, 

 the latter two representative of the macrozoo- 

 plankton captured by a 505 |xm mesh net. The 

 maximum daily specific ingestion rate of P. 

 parvus females is 1.0-2.2 d~^ (varying on a 

 carbon and a nitrogen specific basis, Checkley 

 1980), while that of Calanus pacificus is ca. 0.4 

 d"^ (Frost 1972) and that of adult Euphausia 

 pacifica is ca. 0.08 d"^ (Ohman 1984). Based on 

 these mass-specific rates, if 30.2% of the zoo- 

 plankton standing stock were P. parvus-like 

 organisms and the remainder were evenly split 

 between Ca/oHHS-like and Euplmusia-hke or- 

 ganisms, the mesozooplankton could account for 

 79% of the grazi'^g pressure. The presence of 

 larvaceans and salps would alter this estimate 

 because these gelatinous organisms are rela- 

 tively large but have higher specific rates of 

 metabolism than most crustacean zooplankton 

 (Alldredge and Madin 1982). Furthermore, the 

 estimate ignores the presence of omnivores and 

 predators and assumes, unrealistically, food- 

 satiated ingestion rates for all taxa. Neverthe- 

 less, the mesozooplankton is doubtless a signifi- 

 cant contributor to heterotrophic processes. 



The increased contribution of the mesozoo- 

 plankton in the most inshore domain may reflect 

 the presence of early developmental stages of 

 zooplankton species that show relatively high 

 rates of reproduction inshore (Brinton 1976; 

 Checkley 1980; Smith et al. 1986). The possible 

 trend toward a larger contribution of mesozoo- 

 plankton offshore along line 90 requires corro- 

 boration. Much further to the west, in the 

 oligotrophic central North Pacific, 20-40% of the 

 zooplankton standing stock collected with a 183 

 (xm net was <500 |a.m (Rodriguez and Mullin 

 1986). 



The relation between displacement volume 

 and ash-free dry mass for both size fractions 

 combined is quaUtatively similar to that reported 

 in Wiebe et al. (1975, fig. 4a); however, their 

 regression includes several size categories and 

 their dry mass values apparently included ash, 

 so that the two studies are not strictly compar- 

 able. 



Whether the relative importance of mesozoo- 

 plankton varies through time, such as during El 

 Nino-Southern Oscillation (ENSO) conditions, 

 is unknown. Smith (1985) documented an 

 appreciable compositional change within the 

 macrozooplankton during the strong ENSO of 

 1957-59. For example, comparing 1956 and 

 1958, the average thaliacean (primarily salp, 

 larvacean, doliolid) biomass decreased 27-fold 

 while the total copepod biomass decreased by 

 only 1/2 during this ENSO (Smith 1985), sug- 

 gesting differential responses to ENSO events 

 by different members of the pelagic food 

 web. 



How such compositional changes within the 

 macrozooplankton might relate to changes in 

 the relative importance of mesozooplankton or- 

 ganisms is not obvious. Since phytoplankton 

 concentrations can decrease markedly during 

 ENSO conditions (Fiedler 1984), the argument 

 could be made that smaller zooplankton, which 

 generally have lower food requirements to 

 sustain gi'owth and reproduction (Huntley and 

 Boyd 1984), might increase in relative abun- 

 dance. However, the lack of correlation be- 

 tween the fraction of zooplankton <505 jim and 

 Chla suggests that chlorophyll alone is too sim- 

 ple a measure of food availability. Microzoo- 

 plankton may also be an important prey source. 

 And it should be noted that a model based on 

 energetic considerations makes a contradictory 

 prediction, namely increased body size in oligo- 

 trophic regions (Gerritsen and Kou 1985). Addi- 

 tional information beyond food requirements 

 must also be considered, including life history 

 traits and species composition of the constituent 

 zooplankton, as well as mesoscale circulation 

 effects on species distributions (Haury et al. 

 1986). 



The present evidence for the significance of 

 the mesozooplankton in the Cahfornia Current 

 system is based upon a bulk measure of zoo- 

 plankton standing stock. Future efforts directed 

 toward understanding the mechanisms of re- 

 sponse of planktonic organisms to environmental 

 change should take account of species-specific 

 i-esponses of the mesozooplankton. 



974 



