408 



They have greater amplitudes, but are frequently followed by 

 more sudden and complete declines. Thus, in this case the 

 apex of the pulse is at 18.39 and 6.99 cm. 3 respectively in the 

 lake and river on June 7 and 14, while the decline has reached 

 2.47 and 2.88 on the 21st in the two localities — a fall of 86 and 

 59 per cent, respectively. The third instance occurs on Aug. 16 

 (lake, .45, river, .61), when a large silt content in Thompson's 

 Lake, due to roiling of the water by heavy wind, obscures the 

 actual quantity of the plankton. 



The similarity in the direction of the changes in plankton 

 content in Thompson's Lake and the other localities continues 

 in this year even to a greater degree than formerly, owing in 

 part at least to the hydrographic conditions above noted and 

 to the more complete and uniform records. In the case of 

 Thompson's Lake and the river there are 21 agreements in the 

 direction of the changes to 4 exceptions, and in the records of 

 Quiver Lake 22 to 3 in the possible 25. This is so far in excess 

 of the degree of agreement demanded by chance that we may 

 look with confidence for an emcieut cause in the common fac- 

 tors of the environment, in the similar reproductive cycles of 

 the constituents of the plankton found in common in the sev- 

 eral localities, and in the uniformity in the reactions of at 

 least a predominant portion of the total plankton assemblage 

 to the factors of the environment. 



The river levels average 8.01 ft. for the year and stood 

 above 6 ft. for 8 months of the 12. The high water increases the 

 area of the "open water," and causes a retreat of the shore-line 

 and bottom, and a decrease in the relative occupancy of the 

 bodies of water in question by the spheres of influence of the 

 immediate environment. Thus the local differentiating char- 

 acters of the several environments are in general progressively 

 less potent as the open water increases in extent. The loca- 

 tions of the 7 exceptions to the similarity in the direction of 

 the movement in production are significant. All of them lie 

 in the last five months, in the period of low water, and 2 in the 

 lowest water in August, when local influences are more potent. 



