i 4 o DISCOVERY REPORTS 



steep. The quantitative differences are so marked, as between the coastal current and the oceanic 

 surface-water to the west, that even such crude measures as settlement volumes can be shown to be 

 statistically significant. 



The numerically estimated totals of Protophyta, and of dominant groups or species, when plotted 

 logarithmically, showed distributional patterns in good agreement with hydrological features inde- 

 pendently assessed. Moreover a 50% alteration, in either direction, of the values assigned to the 

 contour lines made little difference to their position on the chart. Crude as the method undoubtedly 

 is, errors of this magnitude are very unlikely and the counts are therefore believed quite fairly to 

 represent the broad outlines of the distribution of such organisms as were retained by the finest grade 

 of bolting-silk. All the counting was carried out ashore, while working at the Plymouth Laboratory. 



The great variety of the Benguela current plankton is shown by the fact that although only diatoms 

 and Ceratia were identified down to species, the raw counts include some 200 categories of organisms. 

 This unfortunately makes it impracticable to publish them in full, but the tables of derived values, 

 dealing with group totals and relative abundance of more important categories, show the number of 

 categories observed during each estimation, in addition to volumes and fractions of samples examined. 

 This should give some idea of the qualitative richness of the microplankton of the area in addition to 

 its great quantity. 



For the study of relative importance of the various categories these counts should be quite satis- 

 factory since several hundred individuals were involved in most of them. More than 300 individuals 

 were counted at 79 % of the stations, and at more than half of these (42 % of the total) over 600 indivi- 

 duals. Only at the very poor stations, where concentration (as opposed to dilution) might greatly 

 have increased the manipulative errors, did the numbers fall below 300 per station. These poor 

 samples constituted 21 % of the total and the numbers of individuals counted in them ranged from 

 90 to 288 with an average of 209. 



Changes in relative abundance are important when it can be shown that species or groups have 

 'indicator value'; that their distributions are mainly restricted to water-masses that can be dis- 

 tinguished by their physical attributes. When this can be established, plankton distributions may 

 provide evidence of water-movement and of areas of mixing, of real value to the physical oceano- 

 grapher and to fishery research. 



It has proved impossible to complete group-sorting of the zooplankton collections in time to con- 

 sider them fully in this report. The vertical net series for the first survey have been almost completely 

 sorted, and individual zoologists able to work up some of the groups have come forward. In this 

 general description of the plankton conditions, we have mentioned only such important (and often 

 elementary) features as became apparent at the sorting stage, adding specific identifications confirmed 

 or established by specialists whenever it has been possible to do so. Some reports on single groups 

 have already appeared and acknowledgements to those who are aiding the work in this way will be 

 found in the zooplankton section. Examination of second survey material has only been completed 

 for pilchard eggs, larvae and post-larvae, the importance of which became apparent at an early stage. 

 A preliminary account of their distribution has already appeared (Hart and Marshall, 1951). 



Complete sorting of all groups, except Protozoa, Coelenterata, Copepoda and small nauplii (mainly 

 of Copepoda), has been attempted. For the latter subsampling proved the only practicable procedure, 

 and at a few very rich stations subsampling had to be adopted for some of the other groups also. 

 Nearly all the work has to be carried out under a binocular dissecting microscope, since many of the 

 animals were much smaller than their relatives in cold seas. We are very grateful to Dr M. V. Lebour, 

 and others working at Plymouth who have occasionally corrected or confirmed provisional identifica- 

 tions for us. 



