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BULLETIN OF THE BUREAU OF FISHERIES 
plankton have been made by methods the reliability of which has steadily increased 
through the medium of successive trial and criticism. Inasmuch as our Gulf of 
Maine studies touch only the edge of this field, I may simply refer the reader to 
Hensen himself, to Lohmann (1903 and 1911), to Steuer (1910), and especially to the 
summaries by Johnstone (1908) and by Gran (1915), 28 for general accounts of such 
undertakings. Much of the earlier work of this sort was robbed of part of its value 
by the impossibility of determining how much of the vertical column of water fished 
through by the net was actually filtered by it. But thanks to Lohmann’s (1911) 
demonstration that satisfactory counts of many of the most important pelagic 
plants could be obtained by centrifuging a water sample obtained with an ordinary 
water bottle, and to Gran’s (1912a; 1915) discovery of a satisfactory preservative 
(Flemming’s fluid) for such samples, a simple but exact method for quantitative 
plankton work is now available, which it is to be hoped American biologists will 
soon adopt. 
While this method gives far more reliable results for the smaller planktonic 
plants, "many of the larger species,” as Lebour (1917, p. 135) points out, "do not 
get into the water samples in anything like a representative number,” and as a rule 
this method is quite worthless for the larger animal plankton. In fact, no one 
collecting apparatus can be expected to be equally satisfactory for all the members 
of the plankton, large as well as small. 29 
Horizontal hauls with ordinary tow nets yield useful information as to the 
relative abundance of phytoplankton, but only if hedged about by the same pre- 
cautions as are necessary for the zooplankton (p. 79), the need of which is now 
universally recognized. For example, we face the impossibility of insuring that 
all the tows shall fish through an equal column of water, because it is practically 
impossible to keep even a steamer moving at a uniform rate at the low speed that 
towing requires. The uncertainty introduced by imperfect filtration is much more 
serious for phytoplankton than for zooplankton, for the much finer-meshed nets 
that must be employed become clogged much sooner and to a greater degree. This 
is especially the case when Phseocystis and certain diatoms swarm (that is, just 
when information on their abundance is most to be desired), for they often clog 
the silk so thoroughly that the nets become quite impervious to water after a few 
minutes, so that the catch becomes the product of the first part of the tow only. 
There is also the problem of a method of estimating the amount of phyto- 
plankton caught, on the one hand sufficiently accurate for the results to be instruc- 
tive and on the other rapid enough to deal in a practical manner with the large 
amounts which horizontal tows at the surface often yield. The total volume — 
simplest and easiest measure — is estimated by the same method as for the zoo- 
plankton, described above (p. 81), and entails the same sources of error, the worst 
being the uncertainty as to what proportion of the measured volume represents the 
actual plankton and how much of its bulk is due to the spaces between its members. 
28 W. E. Allen (1921) has recently formulated a formidable list of sources of error inherent in all collections of plankton taken 
with tow nets. 
29 Lebour’s (1917) tables give instructive examples of the discrepancy between net hauls and collections made with the water 
bottle oil Plymouth, England. 
