177 



In 1897-98 the lakes closed the last days of November 

 and opened again on December 12, freezing again December 

 17, and not clearing entirely until February 14. Rising water 

 continued from January 10, so that stagnation conditions did 

 not ensue. The river also closed partially early in December, 

 opening and closing again with the lakes. The first ice went 

 out with the rise on January 11. The river closed again Jan- 

 uary 27, and the ice went out February 9 and 10. Again on 

 February 21 ice was present, and for several days following. 



In 1898-99 ice again formed early in December and par- 

 tially closed the river during the month, going out about the 

 27th and reappearing on the 30th. This went out gradually 

 January 17-24, and the river froze over again on the 26th and 

 remained closed for a month. Thin ice formed March 5, re- 

 maining only three days. The lakes closed early in December, 

 the ice never entirely disappearing until the middle of March. 

 Partial breaking up occurred at the times of breaking up of the 

 river ice. These partial openings and the changes in level 

 were sufficient to prevent a period of stagnation. 



OTHER METEOROLOGICAL FACTORS. 



As indicated in Tables III.-IX., at each plankton collection 

 observations of the direction and force of the wind, with its 

 effect upon surface conditions and on the state of the sky, were 

 recorded. The relation of these factors to the plankton may 

 not seem intimate or apparent. They have more bearing on 

 the subject of vertical movements of the plankton, data upon 

 which will be found in the study of the surface and bottom col- 

 lections made with each of the vertical collections which form 

 the basis of the present paper. The surface waters affected by 

 the intensity of the sunlight and the movements caused by the 

 wind form relatively so large a part of the environment of the 

 river plankton that these factors are much more widely opera- 

 tive here than in the lakes, where the surface stratum thus 

 affected is relatively small. 



