MICROPLANKTON 255 



polluted waters, inimical to most other marine plankton organisms, in the inner Oslo Fjord. Thus it 

 even seems possible that these blooms might be the result, rather than the cause, of the mortalities. 



The seemingly conflicting statements on the nature of the discolorations so often seen near Walvis 

 Bay do seem to be partially resolved by our observations. Gilchrist's (1914) early insistence upon 

 the vast proliferations of diatoms in the area are amply confirmed. Then to the dinoflagellate blooms 

 described above we can add one of ciliate Protozoa (considerably to the northward but still within 

 the Benguela current area) which was sampled at the end of the second survey. Further we have little 

 doubt that Marchand's contention that blooms due to Noctiluca miliaris also occur there will eventually 

 be vindicated. 1 



In the past each commentator has tended to assume that the most recently identified bloom-forming 

 organism was responsible for all the other macroscopically similar visual effects reported from the 

 area. This was natural enough when direct observations were few and often obscurely recorded. The 

 point which we can claim from our results is that blooms — visible discolorations — due to widely 

 differing types of organisms may occur (mostly at different times no doubt) within the same relatively 

 small sea area. 



The estimates of area and volume of the bloom-forming organisms in Tables 18-20 have been 

 made from the means of numerous measurements and approximation to the nearest geometrical 

 figure. They do not pretend to be precise, but are unlikely to show positive error. They are given here 

 in the hope that the relative quantities of plant substance represented by counts of the various species 

 may be better appreciated with their aid. The areas in particular may interest those familiar with 

 Whipple's ([1889], 1908) concept of the International Standard Unit, a square 20 /i on the side 

 viewing the organisms in optical section, which was found very useful in practical limnological work. 

 These findings suggest that it could be very useful in marine work also, but it has to be borne in 

 mind that it is only in the study of these blooms (or in cultures) that the marine plankton worker ever 

 encounters populations approaching the densities shown in Tables 18-20. Usually he has to deal 

 with populations sparser by at least one order of magnitude than these. It is in fresh waters that such 

 concentrations are frequently met with. 



From the volumes the relative size relations of the more important plants found in these samples 

 can be computed, with the results shown in Table 21. This shows, for example, that here one cell of 

 the diatom Stephanopyxis turris was roughly eleven times as big as one cell of Chaetoceros constrictum, 

 and twenty-five times as big as one of the dominant dinoflagellates, Peridinium triquetrum. Obviously 

 it is important to establish dominance by methods going further than the counts alone when possible. 

 Lohmann (1908) made an attempt in this direction by analogous methods in Kiel Bay, but generally 

 speaking the range of size variation, even within single species of phytoplankton organisms, is so great 

 that application of the method to large series would demand recalculation of volume factors every 

 few samples, and the increase in time and labour involved would make it quite impracticable. 



An intense red discoloration observed close inshore (WS 1107) in 13 05' S. at the conclusion of 

 the second survey proved to be due to swarming of a ciliate protozoan Cyclotrichium meunieri Powers 

 (probably synonymous with Halteria rubra Lohmann = Mesodinium rubrum Lohmann (Apstein).) 

 Large-scale discolorations caused by this organism had previously been seen in the waters round 

 Cape Peninsula (Hart, 1934). 2 When swarming this animal distorts itself or bursts spontaneously in 



1 Trichodesmium thiebautii, the Cyanophycean that so often forms blooms in many parts of the tropical and subtropical 

 Atlantic, probably does so in the Benguela current area on occasions. It was dominant in some of our poorer plankton hauls, 

 but we have not yet seen it there in bloom-forming quantities. 



2 An unfortunately premature communication, missing Lohmann (1908), Apstein (1908), Paulsen (1909) and even Darwin 

 (1839). I tried to rectify these mistakes in a second letter published in 1943, but cannot blame myself for having missed 

 Powers (1932). Our ship left Europe in 1933 when his work had barely had time to reach European libraries, and being 



