375 



on the 2d (4.24) and the larger one in Dogfish (13.39). On the 

 8th conditions are changed ; the decline in levels (.6 ft.) has in- 

 creased the run-off, and the recent contributions of tributary wa- 

 ter have brought down into Quiver Lake an increased proportion 

 of plankton-rich impounded backwaters which increase the con- 

 tent at that point to 8.14 cm. 3 , this being, however, still below 

 that of contributing and declining Dogfish Lake (13.39 to 13.06). 



On May 16-19 the collections were not quite coincident, 

 but, such as they are, they form another exception to the simi- 

 larity of movement in the two lakes. In Quiver on the 16th the 

 decline of levels brings the proportion of tributary waters at 

 that point into greater prominence, while on the 19th in Dog- 

 fish the impounding function is greatly increased by the inter 

 vening rise in levels (1 ft.). Examination of the collections 

 also shows that the maximum of 18.4 cm. 3 in Dogfish Lake on 

 the 19th is caused primarily by an extraordinary pulse, or possi- 

 bly a local "swarm", of Melosira with some Cladocera, whose 

 apex and location the date of collection approximates. 



On the 21st the plankton content in the same locality fell 

 to .36 cm. 3 — a decline of 98 per cent, in 2 days. On the 21st and 

 22d a quantitative survey of the local distribution of the plank- 

 ton in the whole area of Quiver and Dogfish lakes was made, 

 with the result that no development commensurate with that 

 on the 19th at this point was anywhere detected. Since a sim- 

 ilar sudden decline is to be seen in Flag Lake in this same week, I 

 am inclined to the view that we are dealing here with a complex 

 biological phenonenon in which the reproductive cycles of the 

 organisms as well as external factors — such as possible tempo- 

 rary decline of food supply, or encroachment of emerging veg- 

 etation — are involved. This sudden decline is earlier and less 

 marked in Quiver Lake than in Dogfish, possibly because of in- 

 creasing differential environment, and thus occasions this tem- 

 porary dislocation of the similarity in the movement of produc- 

 tion on May 16-19, and again on May 21-22. 



On July 3 we again find lower levels reached and accom- 

 panying decline in production in Quiver Lake when tributary 



