WILLASON ET AL.: ZOOPLANKTON IN CALIFORNIA CURRENT 



pressed by the presence of unpalatable or toxic 

 phytoplankton (Fielder 1982). 



Third, recent evidence indicates that zooplankton 

 digestive enzymes do not show a substrate-specific 

 response. Head and Conover (1983) found that LA 

 in C. hyperboreus was induced in animals which were 

 fed an algae that did not contain laminarin. Willa- 

 son (1983) found that levels of laminarinase in E. 

 pacifica increased when animals consumed small, 

 nonreactive charcoal particles. This increase in ac- 

 tivity, however, was less than that of animals given 

 phytoplankton as a food source Hence, some types 

 of nonphytoplankton food, such as detrital particles 

 or fecal pellets, may also elicit a positive digestive 

 enzyme response. However, since E. pacifica and C. 

 pacificus are primarily herbivorous and are found 

 close to the coast where phytoplankton is abundant, 

 LA of these zooplankters is probably, for the most 

 part, controlled by phytoplankton consumption. 



Because of large-scale patchiness within the sam- 

 pling grid, relationships between the various biol- 

 ogical properties are much clearer when stations 

 were grouped and regions or mesoscale features 

 compared. Mesoscale patches (100 to 200 km) of C. 

 pacificus and E. pacifica with high LA values were 

 clearly associated with areas of highest phytoplank- 

 ton standing crop: south of San Francisco Bay and 

 particularly in the area adjacent to and just south 

 of Point Conception. Although laminarinase levels 

 may not always accurately represent the feeding con- 

 ditions at a single station (because of the reasons 

 stated above), large-scale comparisons indicate that 

 digestive enzyme levels of herbivorous zooplankton 

 are stongly influenced by overall food concentration 

 within an area. This suggests that animals near the 

 coastal upwelling regions were feeding at higher 

 rates than animals from other areas of the sampling 

 grid. 



In contrast to E. pacifica, neither the abundance 

 nor the LA of N. difficilis were correlated with phy- 

 toplankton standing crop. These differences between 

 the two euphausiid species are due most likely to dif- 

 ferent feeding modes or different food preferences. 

 Nematoscelis difficilis, unlike E. pacifica and C. paci- 

 ficus, is probably not a herbivore Nemoto (1967) con- 

 cluded that its mouthparts were very different from 

 those of most herbivorous euphausiids, and Willason 

 and Cox (in press) found that phytoplankton was only 

 a small part of the diet of N. difficilis. What is 

 puzzling, however, are the high levels of LA we some- 

 times found in N. difficilis, a range of values similar 

 to those of E. pacifica. Laminarinase levels in N. dif- 

 ficilis are apparently controlled by consumption of 

 a food source other than phytoplankton. Since we 



did not examine the gut contents of AT. difficilis nor 

 quantify potential food other than phytoplankton, 

 the type of food eaten by N. difficilis could not be 

 determined. 



Based on the weight and biochemical composition 

 of C. pacificus, the areas of high feeding activity 

 along the California coast appear to have been per- 

 sistent for periods of at least 1 to 2 wk. Calanus 

 pacificus from the northern nearshore region and 

 from the area near Point Conception were heavier, 

 had a lower water content, and a higher lipid con- 

 tent than copepods from other areas. This indicates 

 that these copepods have had prolonged exposure to 

 better feeding conditions. The use of zooplankton 

 biochemical composition and weight as indices of 

 relative "physiological" or "nutritional" state has 

 been documented in laboratory experiments. Vidal 

 (1980) showed a direct relationship between food con- 

 centration and weight of adult and stage V C. paci- 

 ficus. Since C. pacificus completes a life cycle in 

 about 30 d (Vidal 1980; Huntley and Brooks 1982) 

 and has a fixed number of molts to maturity, 1 or 

 2 wk at higher food concentrations can have a large 

 impact on adult size The lipid content of a zooplank- 

 ton species represents an energy reserve and is an 

 excellent indicator of nutritional state Lipid content 

 increases in well-fed animals and decreases in 

 starved animals (Lee et al. 1970, 1971; Mayzaud 

 1976; Hakanson 1984). During periods of starvation, 

 crustaceans in the laboratory also show an increase 

 in water content (Hiller-Adams and Childress 

 1983). 



Two field studies have shown that changes in food 

 quality and quantity can cause physiological or nutri- 

 tional changes in zooplankton populations (Omori 

 1970; Boyd et al. 1978). In both of these cases, zoo- 

 plankters were displaced from their optimal habitat 

 to areas of lower food concentration by currents or 

 eddies. The displaced zooplankters showed a lower 

 lipid content and a higher water content presumably 

 due to suboptimal nutrition. This may be what hap- 

 pened to individuals of C. pacificus in the offshore 

 areas of the California Current. These copepods 

 weighed less and were in poorer physiological con- 

 dition (high water content and low lipid content) than 

 C. pacificus located close to the upwelling regions. 

 Although the origins of these copepods are not 

 known, physical processes within the California Cur- 

 rent System such as eddy extensions (Bernstein et 

 al. 1977; Pelaez and Guan 1982; Haury 1984) or off- 

 shore surface transport mechanisms (Parrish et al. 

 1981) could displace zooplankters such as C. paci- 

 ficus to the food-poor offshore waters. 



Because euphausiids were captured at only about 



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