FISHERY BULLETIN: VOL. 73, NO. 1 



Table 4. — Mean vertical distribution as cells/liter and as equivalent carbon ( ^ig C/liter) of the >20- jim non-setose size fraction at all 

 stations between 30 March and 6 April 1973 in MESCAL 11 survey area (n = 12 (0 m), n = 13 (10-50 m), n = 9 (75 m)) 



/Jig c/liter, or 86% of the mean, and 3,426 cells/ 

 liter (72%). 



Vertical Distribution 



The mean vertical distribution of the species 

 numerically and as carbon is given in Table 4. 

 Selected examples of the types of vertical distribu- 

 tion characterizing certain species are given in 

 Figure 2. 



The standing stock declined sharply between 20 

 and 30 m; a uniform abundance characterized the 

 upper 20 m. Both numerically and as biomass, the 

 mean population at 30 m was about 45% of that at 

 20 m (Table 4). Expressed as carbon, and relative 

 to the populations at 20 m, the mean populations 

 at greater depths were only 25% (40 m), 6% (50 m) 

 and 4.5% (75 m). About 62% of the mean carbon 

 content of 533.5 mg C/m^ in the upper 50 m oc- 

 curred in the upper 20 m, where a mean of 16.42 /Jg 

 C/liter is calculated. This pattern in vertical dis- 

 tribution is consistent with the mean compensa- 

 tion depth of about 23 m determined from Secchi 



disc measurements at 17 stations during this 

 cruise leg. 



The mean vertical carbon distribution of certain 

 species (Figure 2) illustrates that peak abundance 

 usually occurred in the upper 20 m. The photo- 

 taxic ceratians are most concentrated in the upper 

 10 m, with a rapid decrease (as percent of mean 

 maximum abundance) with depth. The possibility 

 that the "working distance" vertically within the 

 water column varies between species is suggested 

 by the representative distributions illustrated in 

 Figure 2. The depth at which 50% of the mean 

 maximum abundance occurred ranged from about 

 20 to 35 m between species, and from 25 to 55 m for 

 the 25% level. Differences in light requirements, 

 particularly that of growth at low intensities, 

 might account for the observed distributions, if a 

 physiological explanation can be applied. How- 

 ever, such distributions can also reflect differences 

 in sinking rates, differential grazing, etc. Thus, 

 while the underlying reasons are obscure, it is 

 evident that the biomass distribution within and 



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