212 



of maximum of nitrogenous substances in the water and is fol- 

 lowed by a period of depression in these substances, and in this 

 case by a much more marked fall in the amount of plankton, 

 which does not again rise until the return of the nitrogenous 

 substances in the autumn. The unutilized minimum of nitrates 

 during the summer season is but a trifle less than that in the 

 river (cf. PI. XLV. and XLIX.), but the fact that all the 

 other forms of nitrogenous matters are not only low but are 

 lower than in the river throws some light on the slight devel- 

 opment of the plankton here as compared with that in the 

 river during this period of the summer minimum of nitroge- 

 nous substances. While the small amount of plankton seems 

 inadequate to explain the marked reduction in the various ni- 

 trogenous substances, it may be that the more permanent veg- 

 etation, the submerged aquatic flora of this lake, is an import- 

 ant factor in the reducing process. In its seasonal production 

 the plankton of Quiver Lake shows a general correlation with 

 the movement of the chemical changes, though all of its fluc- 

 tuations are not commensurate with the fluctuations of the ni- 

 trogenous materials. The operation of other factors — such as 

 the submerged aquatic flora and replacement by tributary wa- 

 ters — must be called in to throw light on all the plankton 

 changes in this lake. 



In Thompson's Lake the seasonal cycle of periods of maxi- 

 mum and minimum amounts of nitrogenous matters is almost 

 as well defined as it is in Quiver Lake. The plottings of the 

 analyses (PI. L.) from September, 1897, to March, 1899, include 

 two periods of winter maximum and one of summer minimum, 

 all of which are well defined, and affect not only the nitrates 

 but also the organic nitrogen, the albuminoid and free ammonia, 

 and the oxygen consumed. The diminished effect of floods 

 and of unusual flushes of sewage in this reservoir backwater 

 is evident in the greater regularity of its seasonal curves of 

 nitrogenous substances as contrasted with those of the river- 

 Its close dependence upon the river for its water supply is 

 shown by the similarity of its chlorine curve to that of the 



