248 



Late summer and early autumn see the decay of much of 

 the more succulent vegetation, such as the water-lilies and 

 lotus, the arrowleaf, the Potamogetons, and some of the Elodea 

 and Ceratophyllum, while the emergent and more resistant 

 rushes, sedges, and grasses yield more slowly and later to the 

 accidents of flood and ice, and do not reach the late stages of 

 decay until the following spring. 



The vegetation of Thompson's Lake is subject to consid- 

 erable fluctuations, due to other than seasonal changes. 

 These are variations in river level, the seining of fishermen, 

 and the movements caused by flood, wind, and ice. The 

 changes in level, owing to the very gentle slope of most of 

 the shore of this lake, greatly contract the littoral zone a^s the 

 spring flood recedes, and restore more or less of it with each 

 recurrent rise — changes which facilitate the decay of whatever 

 vegetation of the submerged type develops in this zone. The 

 location of the lake with its long axis in the direction (S. W. to 

 N. E.) of the prevailing winds, gives a force to the waves suffi- 

 cient at times to tear isolated patches of "moss" from their 

 slight hold on the unstable bottom and drive them toward the 

 northern end of the lake. This is an important factor in keep- 

 ing the greater part of the lake free from vegetation. 



The vegetation of Thompson's Lake is thus a considerable 

 factor in the environment of the plankton. It furnishes a con- 

 siderable quantity of decaying organic matter in fall and spring, 

 both being periods of marked plankton development. At high 

 and moderate stages of water, when a gentle current passes 

 through the lake, its influence must be generally diffused. At 

 low-water stages, when the current is cut off, its effect is much 

 more local. At such times no open channel is maintained 

 through the vegetation at the northern end of the lake (PI. II.) 

 to the outlet, as in the case of Quiver Lake. The movements 

 in the lake attending change in level tend to mingle the plank- 

 ton of regions full of vegetation with that of the open lake or 

 vice versa, thus tending to diversification. There still remains 

 at all times a large tract of open water in which for considera- 



