399 



stances (cf. PI. IX. and XXXVI. ); in the case of Quiver Lake the 

 agreements number 7 out of a possible 12 (cf. PI. XXVI. and 

 XXXVI.); in the production in Dogfish Lake the correspondence 

 is found in 9 out of a possible 12 (cf. PL XXX. and XXXVI.); 

 while in Flag Lake there are 2 out of 3 (cf. PI. XXXIII. and 

 XXXVI.) . 



The agreement is lessened in this year, it seems, by the hy- 

 drographic conditions. The low water affords less opportunity 

 for a mingling of the waters of the stream and its backwaters, 

 and also serves to bring out the local environments at each of 

 the stations. Thus Thompson's Lake has but little connection 

 with other backwaters at any time during the year, and ingress 

 or egress of channel waters was but very slight during six months 

 of the twelve in this year. Vegetation also gained more ex- 

 tended possession of this lake in this year than in other seasons 

 of our operations. Low water also tends to make the channel 

 plankton more directly affected by its peculiar factors, such as 

 sewage. It is noticeable that the agreement in production is 

 most marked between Thompson's and Dogfish lakes, both back- 

 waters of somewhat similar character in respect to tributary 

 waters, relation to the channel, and vegetation. 



The most marked differences between production in this 

 lake and the channel appear in the respective amplitudes of 

 the pulses of production in April-May and June-July. In the 

 lake the rising vernal pulse attains the exceptional volume of 

 28.2 on April 10 to .52 in the river on the 9bh, a difference which 

 may in part be due to the earlier warjning up of the shoal- 

 er lake waters. The maximum (61.44 ) in the lake is 12- fold 

 that observed in the stream. The June-July production in the 

 river, on the other hand, is 3- to 5- fold that in the lake, the 

 contrast being due on the one hand, it seems, to the temporary 

 exhaustion either of the chemical substances utilized by the 

 plankton or of the reproductive capacities of the planktonts of 

 the lake waters, and, on the other, to the increased sewage con- 

 tamination in the stream as a result of low levels. The direc- 

 tion of the changes in production, however, remains the same in 



