206 DISCOVERY REPORTS 



cycle in the sea, by comparison and contrast between the diatom populations found living in the area 

 (the biocoenosis) and the assemblage of forms found in the deposit (the thanatocoenosis). The problems 

 that this type of work may help to solve may even include evidence as to the speed of subsurface 

 currents, as may be seen from the brilliant exposition by Kolbe (1957) working on more specialized 

 samples from very deep waters (cores taken during the Albatross Expedition). 



Preliminary examination of our samples from the Walvis Bay line already suggests striking differences 

 between the bottom deposits and the plankton. Of the forms remaining recognizable in the deposits 

 a vastly greater proportion belong to the Discoidae than have yet been recorded in any of the numerous 

 plankton samples from the area of which analyses are now available. That this may be due to much 

 more rapid re-solution of the less strongly silicified forms has been suggested previously on the basis 

 of work in other diatom-rich waters (Hart, 1934, 1942). 



MICROPLANKTON 



Terminology and presentation of data 

 The term ' microplankton ' is used here to describe the approximate size-limits of the organisms, plant 

 and animal, captured in routine vertical hauls of the N 50 V. These nets were constructed as near to 

 the specification of the J-m. nets previously used by the 'Discovery Investigations' as post-war 

 materials permitted. The finest bolting-silk obtainable was found to give an approximate mesh size of 

 40 x 50 fi in use. Some few large organisms are present in the catches, and have been tabulated with 

 the others in the summarized results of numerical estimations, but general sampling of the micro- 

 plankton, more especially of the phytoplankton, was the primary purpose of these hauls. 



This definition of the term microplankton agrees with the usages of Sverdrup, Johnson and 

 Fleming (1946, p. 275), based largely on that of Ekmann (1935). They relate the general terms 

 expressive of size limits to methods of capture, thus : 



Microplankton : captured by coarse or medium tow-nets. 



Microplankton: captured by the finest grade silk nets. 



Afawwoplankton : liable to escape the finest silk, hence also 'centrifuge plankton'. Also studied by 

 using fine filters, sedimentation methods, etc. 



The nannoplankton (Lohmann) may be said to include or overlap various groupings of minute 

 forms proposed by more recent specialist workers, such as '//-flagellates', 'ultra-plankton' and 

 ' hekisto-plankton ' (Cole, 1952). 



Though the usage has varied we find that the outstanding plankton workers of the past agree upon 

 the necessity of treating these generally descriptive terms in an elastic manner (cp. Steuer, 191 1; 

 Johnstone, Scott and Chadwick, 1924). At first the prefixes macro- and micro- seem to have referred 

 broadly to large plankton organisms visible to the naked eye, and to the host of smaller forms that can 

 be seen with the aid of the microscope. But Johnstone, Scott and Chadwick referred all net-caught 

 plankton to the macroplankton, so that for them microplankton became synonymous with Lohmann's 

 nannoplankton. 



Meunier (19 13) intended his use of the term microplankton to be construed in the earlier sense, but 

 with restricted application : to unicellular organisms only. This would very nearly meet our needs in 

 discussing the Benguela current material, were it not that the multicellular alga Trichodesmiiim is of 

 some importance among the phytoplankton in that region. Moreover, the relative abundance of the 

 smaller metazoa is obviously an important factor if the ' conditions of life ' of the phytoplankton are 

 to be considered. 



More recently Ekmann (1953, p. 312), while adhering to his own early definitions (as given by 



