SOME PLANKTON STUDIES IN THE GREAT LAKES. 141 



be directly compared with one another without applying the correction. It is not 

 possible to compare the measurements shown in this schedule with those given by 

 Apstein for European lakes, until the measurements here given have been reduced to 

 show the volume under a square meter. 



Stations in, v, and ix are in Lake St. Clair, while stations xix and xx were in 

 Lake Erie near the Put in Bay Islands. The following facts are evident from an 

 inspection of the schedule: 



1. There is about three times as much plankton at stations xix and xx in Lake 

 Erie in the month of September as at any of the stations in Lake St. Clair. 



2. The hauls made by letting the net down but a little way from the surface con- 

 tain nearly as much plankton as those made from the bottom at the same stations. 

 Thus at station in the two hauls from the bottom yield an average of 1-09 c. c, 

 while a haul (in, 4) made from a depth of but 1-5 meters contains 0-87 c. c. In the case 

 of station v the amount falls from an average of 1*05 for the bottom haul to -54 for the 

 1*5 meter haul. In the case of station ix the numbers are 0*64 c. c. and 0-42 c. c, respec- 

 tively. It thus appears that one-half or more than one-half of all the plankton occur- 

 ring in water 5 meters deep is in the upper 1| meters of the water. A similar result 

 was reached by Apstein. Whether the plankton maintains the same relation to the 

 surface at night or at other seasons of the .year requires further investigation. 



After having determined the volume of plankton in any locality it is possible to 

 count the number of animals and plants of each species occurring in the volume of 

 plankton taken. The method of accomplishing this has been worked out by Ilensen, 

 and is essentially as follows: Let us suppose that 1 cubic centimeter of plankton 

 has been taken. One-tenth or one-hundredth of this is spread on a glass plate upon 

 which parallel lines have been ruled with a diamond. Heusen has devised means by 

 which the one-tenth or one-hundredth part may be accurately measured. The glass 

 plate is then placed upon the stage of a specially constructed microscope. This stage 

 is provided with a suitable carriage actuated by micrometer screws, and upon this car- 

 riage the glass plate may be moved about, so that it is possible to examine all parts 

 of it. The number of each species of animal and plant upon the plate is then counted, 

 the lines serving to separate the forms counted from those still to be counted. By 

 thus counting the forms contained in a tenth part or a hundredth part of the plank- 

 ton taken the number contained in the whole plankton taken may be calculated. It 

 is then possible to calculate the number of animals of any one species occurring under 

 a square meter of surface. It is possible also to calculate the numbers of any species 

 at any depth, Thus we may find that in a haul made from a depth of 3 meters there are 

 2,000 cyclops, and in a haul made at the same time and place, but from a depth of 1£ 

 meters, there are but 500 cyclops. We have, then, the number of cyclops contained 

 in the whole column of water (3 meters long) and the number contained in the upper 

 half of this column (1.5 meters long). The difference between these two numbers 

 must be the number of cyclops contained in the lower half of the column of water. 

 In other words, then, 1,500 cyclops are living in a known volume of water, let us say 

 1 cubic meter, at a depth of from 1£ to 3 meters. By this method of counting tlie 

 number of forms in each column of water and by subtracting the contents of one 

 column from the contents of another we may know at what depth each form lives and 

 in what numbers. We may also know what its migrations are at different hours of 

 the day and at different seasons of the year. 



