So6 DISCOVERY REPORTS 



During this cruise a series of consecutive hauls was made with the Plankton 

 Indicator, a small instrument which is fitted with planes and towed like the Recorder 

 but has, in place of the moving bandings, a simple gauze disc for the collection of 

 the plankton. 1 The discs are taken out and changed after each haul. It was towed at 

 about three metres below the surface and after each haul of just under one mile by 

 the ship's log, it was reloaded and immediately shot again. The process was repeated 

 in exactly the same way until twenty-six consecutive hauls had been obtained. The 

 analysis of the samples was made by the second author (N.E.). The primary object of 

 the experiment was to show whether or not the types of fluctuation in the numbers of 

 plankton organisms shown by the continuous Plankton Recorder really represented fluctu- 

 ations in the sea and were not due to variations in the mechanical working of the machine ; 

 each separate indicator disc sample corresponding to a small section of a recorder roll 



The series of consecutive samples was taken on April 8th, 1929, on a straight line 

 between 52 37' N, 3 30' W and 52 28' N, 4 10' W, beginning at 1822 o'clock and 

 ending at 2304 o'clock. Of the twenty-six samples two unfortunately (numbers 14 and 

 18) were lost before analysis owing to tube breakage in the transit of material. The 

 results of the analysis of the remainder are shown as histograms in Figs. 28-30. For the 

 first eight miles the phytoplankton (Fig. 28) was abundant but varied greatly in quantity ; 

 it was then taken in negligible quantities until the last two miles when an increase of three 

 species occurred. The zooplankton (Fig. 29) on the other hand was negligible for the 

 first seven miles but present in varying quantities for the remainder of the line. Whilst 

 there are differences in the spatial fluctuations of the four predominant species of 

 Copepoda they all show on the whole a remarkable similarity in the main outline of their 

 distribution; each shows a lesser zone of abundance about sample 10 and a larger one 

 towards the end of the series. Such an occurrence, if found on the recorder roll, might 

 give rise to doubts as to whether it might not be due to some variation in the working 

 of the mechanism. Here each sample was taken independently in exactly the same way. 

 Such a close correspondence in the fluctuations of several different species of Copepoda 

 is, on the evidence of a large number of subsequent continuous records, a comparatively 

 rare occurrence ; it was fortunate that it occurred on the occasion of this test series of 

 indicator samples so that its reality could be verified. The total phytoplankton and the 

 total copepoda are charted together in Fig. 30 ; the possible significance of this distribu- 

 tion will be referred to later. 



The reality of the marked variations in the abundance of the plankton from mile to 

 mile in the sea need not be laboured. The results of this consecutive series of indicator 

 samples, together with those taken with larger tow-nets at South Georgia in 1927 

 (Hardy and Gunther, 1935, p. 255) already referred to in the main paper and a similar 

 consecutive series in the South Atlantic (Hardy, 1936) show that the marked variations 

 in numbers typically revealed by the Continuous Recorder may be regarded as real 

 variations occurring in the sea. 



1 A description of this instrument is now in the press (Hardy, 1936, Journal of the Marine Biological 

 Association); it is an improvement upon that previously described (Hardy, 1926, Fishery Investigations 

 (England), Ser. II, Vol. VIII, No. 7). 



