MULLIN ET AL.: VERTICAL STRUCTURE OF PLANKTON OFF CALIFORNIA 



would give the same result. Given the naupliar life 

 span of a few days, one would expect that, if repro- 

 duction were responsible, the ratio of nauplii to 

 adults would continue to increase for the 3 d fol- 

 lowing the storm (though such a finding would not 

 rule out advection). This is most readily tested for 

 Acartia (nauplii/female) and "Paracalanus" 

 (nauplii/adults and copepodites), since the older 

 stages remained within the 0-50 m water column 

 day and night (ANOVA classification 1 not sig- 

 nificant). The ratio, nauplii/female, for Acartia 

 increased dramatically; indeed, so much so that 

 published values of maximal fecundity (50 eggs/ 

 female per day, Landry 1978; Uye 1981) are barely 

 sufficient over the 5-d period including the storm 

 (1-5 April) to account for the observed ratio on 6 

 April, even if no death or metamorphosis of 

 nauplii occurred. This is because female Acartia, 

 though unusually abundant immediately after the 

 storm on 3 April, declined dramatically from that 

 time until 6 April (Fig. 2). For "Paracalanus" , 

 which has approximately the same maximal 

 fecundity as, Acartia (Checkley 1980a), the ratio of 

 nauplii to copepodites plus adults was slightly 

 greater on 6 April than during the preceding 2.5 d. 

 Evidence that the capacity of the environment to 

 sustain reproduction of "Paracalanus" increased 

 after the storm is presented in Section C below. 

 Hence, the results are qualitatively consistent 

 with the hypothesis that the storm stimulated 

 reproduction, especially in Acartia; but other 

 explanations cannot be ruled out. It is noteworthy, 

 however, that no "exotic" taxa appeared after the 

 storm. 



Significance of ANOVA interaction 1x2 indi- 

 cated that for two taxa (euphausiid furcilia and 

 CV Calanus), the tendency to be more abundant in 

 the upper 50 m at night than by day was more 

 pronounced before the storm. Another migratory 

 taxon — cyphonautes larvae — showed both a 

 change in average depth distribution within the 

 upper 50 m and a change in pattern of diel migra- 

 tion (significance in ANOVA interactions 2x3 

 and 1x2x3). The nighttime distributions of 

 cyphonautes larvae were similar before and after 

 the storm, but the daytime distribution was 

 shifted to shallower water after the storm; adult 

 Corycaeus showed the same (but nonsignificant) 

 tendency, though they did not have a significant 

 diel migration over the whole period (cf., evidence 

 for a reverse migration by this species off Southern 

 California in Fiedler 1983). Pleuromamma was 

 virtually absent from the upper 50 m during the 

 day both before and after the storm, but at night 



tended to occur shallower within this layer after 

 the storm. 



Diel migration was not detected in Calanus CII 

 and cm copepodites (ANOVA interaction 1x3 

 not significant), nor did they change significantly 

 in total abundance after the storm (ANOVA clas- 

 sification 2 not significant). There was, however, a 

 shoaling of the distributions of both stages both 

 day and night after the storm (ANOVA interaction 

 2x3 significant). Larval fish were also shallower 

 in our samples by day after the storm. 



The vertical distributions of these three taxa 

 were still stratified after the storm but were 

 shifted with respect to depth. Another potential 

 effect of the storm, which could also result in sig- 

 nificance of ANOVA interaction 2x3, is 

 homogenization of strongly stratified distribu- 

 tions into more nearly uniform ones. Reduction of 

 the temperature gradient (Fig. 1) reinforces this 

 possibility. The reverse process — an increase in 

 stratification — is possible as a result of biological 

 responses to the physical disturbance. Test 4 is a 

 simple way to examine this question, though it is 

 insufficient to detect some possible complex redis- 

 tributions. The results of this test were contrary to 

 expectation; only 2 taxa, Labidocera nauplii and 

 copepodites, had greater ranges of abundance in 

 the water column before the storm, while 13 taxa 

 had greater ranges after the storm. Included in the 

 latter group were Acartia and "other" nauplii, 

 both of which increased in overall abundance after 

 the storm, and all five juvenile copepodid stages of 

 Calanus. Thus, as far as the zooplankton is con- 

 cerned, poststorm stratification was generally 

 more marked than that prestorm. It may be of 

 significance, however, that the two taxa whose 

 prestorm abundances were more strongly 

 stratified than in the poststorm condition were 

 taxa with strong neustonic (nearsurface) affinities 

 (Barnett 1974; Appendix). 



Inspection of the data (see Appendix) revealed 

 several other taxa which appeared to have dis- 

 tributional changes of the sorts described above, 

 though these were not significant by the criteria 

 used in the statistical tests. This means that other 

 sources of variability in abundance — notably, 

 horizontal patchiness on the scales of a few kilo- 

 meters, or vertical internal motions creating high 

 variability at a fixed depth from day to day as 

 water passed the sampling location — were more 

 important than were those patterns of change the 

 statistical tests were chosen to detect. 



Finally, we can examine the overall similarities 

 in the zooplanktonic community of the upper 50 m 



157 



