+ 



8 PLANKTON OF WISCONSIN LAKES 



In obtaining this plankton material Hensen used nets which had 

 straining surfaces made of bolting cloth and, in the various nets, bolt- 

 ing cloth with different sizes of mesh was used, the finest being the 

 No. 20. These nets were lowered into the water to the desired depth 

 and were then hauled to the surface at as uniform a speed as possible, 

 thus passing through a definite column of water. This is known as 

 the vertical haul method. The whole column of water through which 

 the net passes is not strained, however, since the straining part of the 

 net offers some resistance to the passage of the water and a certain 

 portion of it is pushed aside and not strained. This makes it necessary 

 to determine the efficiency of the net, or the coefficient, which serves as 

 a factor for calculating the total number of organisms in the column 

 of water. This method is subject to a serious error in that this 

 coefficient varies with the age of the net and with the abundance of the 

 plankton. The silk bolting cloth is subject to a certain amount of 

 shrinkage and its meshes also become more or less clogged with organ- 

 isms which adhere to the net permanently in spite of careful washing. 

 During the progress of the haul, the meshes become temporarily 

 blocked with organisms, especially when the plankton is abundant, so 

 that the efficiency of the net decreases as it approaches the surface. 



Hensen determined the quantity of plankton in his net catches in 

 three ways, namely, (a) by allowing it to settle, (b) by enumeration, 

 and (c) by evaporating an aliquot part of the catch to dryness, weigh- 

 ing it, and then igniting it to ascertain the organic matter. In the 

 settling method the material was placed in a graduated cylinder and, 

 after standing a definite number of hours, a reading of the volume was 

 taken. Such readings, however, have little value since the different 

 organisms do not settle with the same degree of compactness. In the 

 enumeration method a definite portion of the material was placed in 

 a suitable counting tray and the organisms in a certain number of 

 definitely marked off squares were counted. From the results thus 

 obtained the total number of the various organisms in the catch was 

 estimated. The numerical method is still extensively used by plank- 

 tologists and it yields very valuable data concerning the abundance 

 and distribution of the various plankton constituents. The method of 

 drying, weighing, and igniting is also a valuable one since it gives im- 

 portant data respecting the proportion of organic material in the catch. 



As work in this field of science has progressed new types of nets 

 have been devised and new methods have been employed for the pur- 

 pose of obtaining more accurate results. In order to ascertain more 

 accurately the quantity of water that is strained for a catch the pump 

 method was introduced. By means of a pump and hose a definite 

 amount of water may be obtained from different depths and strained 



