O'CONNELL: VARIABILITY OF NEAR-SURFACE PLANKTON 



each successive cruise in 1961, and the median 

 for the last cruise in this year was more than 

 three times as high as any of the others. Median 

 densities of the large copepods were two to four 

 times as high for the last two cruises in 1961 

 as for the other cruises. Euphausiid medians 

 were about twice as high for the first two cruises 

 of 1961 than for the other cruises. The chaeto- 

 gnath median for the last cruise of 1962 was sig- 

 nificantly lower than for the first cruise of that 

 year, but no other difl'erences can be distin- 

 guished. 



In summary, the three crustacean groups 

 showed real differences in density near the 

 surface at night for the area as a whole. All 

 showed differences of three to four times be- 

 tween the 2 successive years. The euphausiids 

 and small copepods also showed real differences 

 of about two and three times within the first 

 of the 2 vears. 



VARIATION WITHIN THE AREA 



The 1.6-km samples were taken in block 

 clusters so that small-scale variability could be 

 described in respect to variability over the entire 

 area. The possibilities of comparison are lim- 

 ited, however, because parametric analyses were 

 avoided. The necessary assumptions about fre- 

 quency distribution of sample estimates could 

 not be satisfied for the present data. Frequency 

 distribution of block medians (or means) is var- 

 iable between cruises. 



FREQUENCY DISTRIBUTION OF 

 BLOCK MEDIANS 



Table 3 shows the frequency distribution of 

 block medians for the day and night periods of 

 each cruise. The distributions for each species 

 group vary noticeably, but there are similarities 



Table 3. — Frequency distribution of block medians by cruise and day (D) or night (N) period. 



687 



