486 



overflowed district, and are to a varying degree traversed by 

 waters from the bottom-lands above and adjacent to them, and 

 the purely local factors of their environment, such as vegeta- 

 tion, thus become less potent. Again, it is not until the latter 

 part of this period that the vegetation attains the development 

 which continues throughout the remainder of the summer. 

 The relative barrenness (in plankton) of the vegetation-rich 

 waters is thus least striking when the vegetation is least in 

 evidence. 



During the period from August to November inclusive the 

 ratios are very much higher, rising to 1 to 16 or 20. This is the 

 low-water period, when the vegetation in the vegetation-rich 

 lakes is at its maximum development both in quantity and in 

 the relative volume of the lake occupied by it. It is also at 

 such times that these several bodies of water are more distinct 

 units of environment, with their local factors no longer merged 

 by flood conditions. The relative barrenness of the vegetation- 

 rich waters is thus greatest when the vegetation is at its maxi- 

 mum development and is most emphasized as a factor in the 

 environment. The conclusion from this comparison of the mean 

 production of plankton in vegetation-rich and in vegetation-poor 

 waters in our locality is thus inevitable that vegetation (in the 

 usual sense of the word) is inimical to the development of an 

 abundant plankton. It may also be said that the contrast 

 would be considerably heightened if it were possible to elimi- 

 nate from all the collections on which this comparison is based 

 the adventitious organisms— such as small insect larvse, mol- 

 lusks, oligochsetes, Hydra, etc., which form a considerable vol- 

 ume of many of our catches in the vegetation-rich waters. 



On the other hand, it must be maintained that the vegeta- 

 tion is only one of the factors concerned in the phenomenon pre- 

 sented by this contrast. It is quite probable that other fac- 

 tors, especially the current, tributary waters, and the chemic- 

 al constituents of the water, affect the problem in hand. Dur- 

 ing high water both Thompson's Lake and the Dogfish-Quiver 

 region are traversed by a considerable current from the bottom- 



