METHODS AND APPAEATUS H 



Work was begun on the net plankton in the latter part of May, 1911, 

 and it was continued until July 1, 1914. Durmg the first two winters 

 no attempt was made to obtain material while the lake was covered 

 wdth ice, but during the mild winter of 1913-14 catches were made each 

 month. Owing to the late freezing of the lake the regular weekly 

 observations were continued with the launch until December 24, 1913, 

 and mild periods in the following months made it practicable to obtain 

 one catch each in January and in February, 1914, and two in March. 

 Following this the regular weekly observations were begun on April 

 18, 1914, and they were continued until July 1. In 1915 work was 

 resumed on the net plankton on April 21 and was continued until 

 June 1, 1917. In 1916 and 1917, however, only the relatively small 

 amounts of water used for the centrifuge catches were strained for 

 the net plankton. 



The observations were made at a station where the water of Lake 

 Mendota reaches a depth of 23.5 meters, a buoy being placed at this 

 point each year to mark the place. Except in late summer the catch 

 covered the entire depth of the lake down to 21 meters, either meter 

 by meter or at two meter intervals. That is, the intake end of one 

 hose was placed a few centimeters below the surface while that of the 

 other hose was lowered to one meter. They were kept at these depths 

 for a definite interval of time, usually ten to twenty minutes, while 

 the pumps were in operation. Then they were lowered to two meters 

 and three meters respectively for the same period of time, and so on 

 to a depth of 21 meters. Attempts were made to get nearer the bottom 

 than 21 meters, but bottom debris was found in the deeper catches so 

 frequently that these attempts were finally abandoned. After the 

 vernal and autumnal overturns the plankton was uniformly distributed 

 throughout the entire depth of the lake for a while and at such times 

 the depth was covered at two meter intervals. 



In late summer the lower water, that is below 15 meters, contained 

 so little net plankton that it was omitted from the catches. In all 

 such catches, however, correction has been made for this and the results 

 given in the tables are based on a depth of 21 meters. The pumps were 

 calibrated frequently and their speed was taken during each run at 

 the various depths, that is, during each ten to twenty minute period, 

 so that the quantity of water strained was readily determined with 

 a fair degree of accuracy. 



In most instances the plankton was removed from the net at the end 

 of each ten or twenty minute run in order to avoid undue clogging of 

 the net and the consequent loss of organisms while the material was 

 being concentrated in the bucket of the net. After filtering off as 

 much of the lake water as possible the material was transferred from 



