3° 



DISCOVERY REPORTS 



I alidity of the N 70 V and N 70 H analysis methods 



As a rule, after the larger and rarer organisms had been picked out, only one fraction 

 per sample was examined unless, as already explained, the nature of the sample — e.g. one 

 containing large quantities of both big and little organisms — made the use of two frac- 

 tions necessary. 



It was felt that the closer approach to accuracy in estimation obtained by analysing 

 several fractions from each sample and then taking their average was not justified, on 

 account of the unavoidable error involved in our methods in the field. When we re- 

 member that we are comparing stations in a survey one part of which took a week to 

 complete, and the remaining part of which was only completed after an interval of three 

 weeks, and when we know that the ocean currents and wind at the surface are constantly 

 moving the plankton, we must realize that the values obtained at different stations 

 represent no more than a very general approximation to that which they would have if 

 all the stations compared had been taken simultaneously. In considering any survey 

 we must imagine that we are comparing results of one part with those of another at the 

 same point in time. 



We know too that many organisms are very " patchy" in their occurrence instead of 

 being evenly distributed for many miles together. At St. 41 a series of vertical hauls 

 with the N 70 V closing net were made from the bottom to the top (265-150 m., 150- 

 100 m., 100-50 m., and 50 m. to the surface), and this series was repeated five times 

 during a period of eight hours at 1300, 1540, 1700, 1910 and 2105 o'clock respectively. 

 Whilst some organisms showed a variation over this period of time no greater than the 

 variation due to error in laboratory methods, the Copepods Ctenocalanus vanus, Dre- 

 panopus pectinatus, Oncea curvata and Oithona frigida, showed a range of variation 

 greatly exceeding the error in laboratory methods. These results will be further 

 discussed in the Zooplankton section on p. 263. We may here tabulate the results 

 for the four species just named, expressed as numbers per 50 m. haul for each of the 

 five series of hauls or their average. 



It will be seen that had we taken only one series of vertical hauls at this station, as was 

 the case throughout the rest of the survey, we should have recorded 1445 Ctenocalanus 

 vanus per 50 m. haul at 1300 o'clock, and only 260 if we had arrived on the station 

 eight hours later, or again 651 Drepanopus pectinatus at 1300 o'clock and 2832 if we had 

 arrived on the station only four hours later. These differences, since the hauls are taken 

 from just above the bottom to the surface, are independent of changes due to vertical 



