434 



vember the production is 10- to 3- fold the average, and in De- 

 cember 9 percent, above the mean for that month. 



In Thompson's Lake the mean of the monthly averages, 

 13.31 cm.' per m,' is 61 per cent, in excess of the mean of all 

 the months of our records, though but 3 of the 9 months repre- 

 sented in the records of 1895 are above the average. The first 

 two of these, April and May, owe their predominance to a ver- 

 nal pulse of unusual volume, and the small number of collections 

 gives these months abnormally high averages, while the large 

 production in July may be attributed to the enrichment of the 

 lake in this year by an invasion of plankton-rich river water 

 from the channel (PI. XXXVL). The deficiency shown in the 

 records of the remaining 6 months, falling from 10 to 70 per 

 cent, below the averages of our records for these months, finds 

 its possible explanation in the relatively greater dominance of 

 vegetation in the lake in this season, due to two successive 

 years of low water and the prevailing low levels. Collections 

 in this year were, moreover, taken near the margin of the veg- 

 etation belt of the lake. 



Production in this, the second, year of low water, and the 

 lowest in our term of operations in all the backwater plankton 

 stations but Quiver Lake, is above the average in the year as a 

 whole, though falling below it in 57 per cent, of the time rep- 

 resented, 



The apparent suppression of the vernal pulse in the river 

 and in Quiver and Dogfish lakes may be attributed to the ab- 

 sence of spring overflow and the consequent eliuunation of vast 

 impounding and breeding areas normally present at this season, 

 and also to the direct delivery of tributary water to the chan- 

 nel and increased relative diluent action of the slight April rise 

 (PI. IX.) in both the river and Quiver Lake. The low levels 

 also serve to bring the vegetation of the two lakes named into 

 early dominance, and the relative occupancy is also increased 

 by the second year of low water and no removal of the accumu- 

 lated growth by flood action. Thompson's Lake, on the other 

 hand, owing to its great extent of open water, is less affected 



