PLANKTON OF THE GULF OF MAINE 



81 



The choice of a unit and of a method of measurement by which to express the 

 quantitative abundance of the zooplanktonic community as a whole, as distinguished 

 from its several component groups, is a matter of real difficulty. The 

 easiest thing to do is simply to let the .whole catch settle in suitable 

 jars or graduates until visible shrinkage ceases and to record the 

 volume of the resulting mass. Unfortunately, however, this does 

 not give a true measure of the actual content of the net, much less 

 (owing to the sources of error just mentioned) of the total column 

 of water fished through, because it likewise includes the gaps between 

 the individual animals composing it, together with any detritus that 

 may have been in suspension in the water. This introduces a serious 

 error, for plankton settles more or less closely according to the shapes 

 of the individual animals composing it, smooth, round, fish eggs, for 

 example, packing far more closely and regularly than do copepods 

 with their long appendages. Nevertheless, even such simple measure- 

 ments as this yield rough pictures of the abundance of the animal 

 plankton, hence they have been made for all our vertical tows and for 

 many of the horizontal ones. Jespersen (1924) measured the volume 

 of the catch after draining the water from it. The process may be 

 rendered more accurate if after draining a known amount of water is 

 added, when the resultant increase in the volume will correspond to 

 that of the catch plus the small amount of liquid which still adhered 

 to the plankton after the draining. I have employed this method in 

 a few cases where it seemed likely that the direct measurement of 

 volume would be seriously misleading because of the character of the 

 organisms concerned. The use of the centrifuge would be stdl better, 

 but this has not been attempted for the Gulf of Maine hauls. 38 



Counting is the most instructive method of estimating the catch 

 from most points of view, though it entails much labor and time, 

 and this is the only method by which the actual numerical strength 

 of the several groups of animals composing the zooplankton can be 

 learned. Various types of apparatus have been devised for this 

 purpose, most of them by the Kiel School of Biologists, the process 

 followed for the Gulf of Maine hauls being as follows: The catch 

 of the vertical net (its volume having been measured as above) is 

 first diluted to a volume of 150 cubic centimeters, well mixed, and 

 then, while the plankton is still in suspension, 3 cubic centimeters 

 are taken with a suitable pipette and the copepods, fish eggs, etc., 

 counted. The ordinary pipette, familiar to every biologist, will 

 seldom serve for taking this sample; but it is not necessary to em- 

 ploy the complicated "Stempel" pipette, for one of the shape shown 

 in the accompanying sketch (fig. 36), with large rubber bulb, 

 tube opening about 3 millimeters in diameter, and total volume of 



Fig. 3fi.— Volumet- 

 ric pipette used 

 for sampling cope- 

 pods for counting 



aB For an excellent account of these and of other methods of plankton estimation see Johnstone, 1908, p. 129. 



