O'CONNELL: \AR1 ABILITY OF NEAR-SURFACE PLANKTON 



cruise sets has slopes similar to the line. Varia- 

 tion is wide, but there are only four serious dis- 

 crepancies. Whatever the reasons for these 

 discrepancies, the four blocks were excluded 

 from calculation of the relative contributions of 

 species groups to dry weight concentrations for 

 day and night cruise periods. 



Table 9 shows the averages of dry weights 

 calculated for each species group for each cruise 

 period, and for all cruise periods pooled by day 

 and night. "Calculated" sample concentrations, 

 obtained by summing the values for the four spe- 

 cies groups, are compared with the average 

 measured concentrations for the cruise periods 

 in the last two columns. Though the calculated 

 concentrations are lower than the measured con- 

 centrations for all day periods, and higher than 

 the measured concentrations for three of the 

 night periods, the two sets show reasonably good 

 agreement, as do the day and night averages 

 for all cruises together. 



The measured sample concentrations for all 

 cruises pooled suggest that, on the average, dry 

 weight was 31 '~f higher at night than during 

 the day. The calculated concentrations suggest 

 that it was 48 '^r higher. Calculated values for 

 the species groups show that the dry weight 

 increase at night is largely attributable to in- 

 creases in the euphausiid group, with lesser in- 

 creases in the large copepods and also the small 

 copepods. Euphausiids were responsible for 

 more than one-third of dry weight concentration 



at night, on the average, and small copepods for 

 less than half of it. During the day, on the 

 other hand, small copepods accounted for about 

 three-fourths of dry weight concentration, with 

 most of the remainder divided between large 

 copepods and chaetognaths. 



VARIATION WITH TEMPERATURE 

 AND DISTANCE FROM LAND 



The data for each of the four species groups, 

 and for dry weight, were examined for possible 

 relationships with the independent variables, 

 temperature, miles from nearest land (including 

 islands), and miles from nearest point on the 

 mainland. Regressions were in the form log 

 Y = a + bX, where X — the independent var- 

 iable and Y = the dependent variable. Small 

 copepods, large copepods, and dry weight showed 

 significant trends with temperature, and chaeto- 

 gnaths showed significant trends with distance 

 from land (Table 10). For euphausiids, night 

 values alone, as well as day and night values 

 together, were tested but neither demonstrated 

 significant trends. 



The two copepod groups and dry weight all 

 show an inverse relation with temperature, but 

 in all cases the trends are largely attributable 

 to changes occurring in 1961, as a comparison 

 of nighttime cruise medians with average cruise 

 temperature shows (Table 11). The decline in 



Table 9. — Calculated average dry weight fractions (mg/m^) and percentages for species groups by cruise and day 



(D) or night (N) period. 



693 



