N ORTH WESTERN LAKES OF THE UNITED STATES. 
65 
NET PLANKTON. 
METHOD OF OBTAINING AND ENUMERATING. 
Samples of net plankton were obtained from the various lakes for the purpose 
of ascertaining the abundance and the vertical distribution of the different kinds 
of planktonts. No data were secured for the nannoplankton organisms found in 
these bodies of water. The net catches were taken with a closing net that consisted 
of an upper truncated cone of heavy muslin and a lower straining cone made of 
No. 20 silk bolting cloth. The lower end of the straining cone bore a detachable 
bucket into which the catch was concentrated. This bucket was then removed 
and the material was transferred to a small vial with enough 95 per cent alcohol 
to preserve it. (For further description of the net see Juday, 1916, p. 573.) 
The net was usually hauled through a 5 m. stratum for each catch, but in some 
of the shallow lakes the haul was reduced to 1 m., and in the deepest lakes it varied 
from 10 to 200 m. The net was hauled at as uniform a speed as possible, the usual 
rate being about one-half a meter per second. The coefficient of the net was found 
to be 1.2; that is, the number of organisms obtained in a catch multiplied by this 
factor gave the total number for the column of water through which the net was 
drawn. 
In enumerating the organisms the catch was concentrated to a volume of 10 
cc. After shaking this material thoroughly 2 cc. were removed with a piston pi- 
pette and the Crustacea and Rotifera therein were counted with a binocular dis- 
secting microscope. The number obtained in this count multiplied by 5 gave the 
total number for the catch. The concentrated sample was again shaken, and 
1 cc. of the material was transferred to a Sedgwick-Rafter counting cell for the 
enumeration of the Protozoa and Protophyta. A compound microscope was 
used for this enumeration, and the number of organisms was ascertained in 20 
different squares on the counting cell. The area of these squares was known, so 
that the total number of organisms in the catch could be readily determined. The 
results for the Crustacea and Rotifera, as well as those for the Protozoa and Pro- 
tophyta, were finally computed to the number of individuals per cubic meter of 
PHYSICAL CONDITIONS IN LAKES. 
In considering the vertical distribution of the net plankton organisms certain 
chemical conditions must be taken into account. The Crustacea and Rotifera, 
for example, are not able to inhabit those strata of a lake that are devoid of free 
oxygen. On the basis of the amount of dissolved oxygen in the bottom water, 
lakes can be divided into three classes: (1) Those that have an abundance of dis- 
solved oxygen through their entire depth; (2) those that have only a small quantity 
of free oxygen at the bottom; and (3) those that have at the bottom a stratum of 
varied thickness that is entirely devoid of free oxygen. 
THERMAL CHANGES AND GAS CONDITIONS. 
The thermal changes in a lake during the different seasons of the year are so 
closely connected with the gas conditions that it seems best to review them to 
better understand the summer thermal and gas conditions in these lakes. 
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