412 



the phytoplankton by the rapidly growing aquatic vegetation. 

 These factors are not, however, potent enough to overcome the 

 effect of impounding and consequent time for breeding which 

 prevail in the lake more than in the river, and thus to lower 

 the plankton production in the lake below that in the channel. 

 In October-December we find another season marked by 

 rising water but not high lev^els, in fact, averaging only 4.22 

 ft. — a level insufficient to provide for any current through the 

 lake or any cousiderable discharge in periods of decline. It is 

 thus a season of slight and interrupted run-off. It is, however, 

 a period of increased production, reaching 10.64 in October, 

 declining to 8.08 in December, with an average of 6.70 — a trifle 

 below^ that of the midsummer period. Its relation to channel 

 production changes decidedly, risiog from a ratio of 1 to 1.54 

 in midsummer to 1 to 5. This five-fold greater plankton con- 

 tent in Thompson's Lake makes whatever run-off occurs of con- 

 siderable enriching effect upon channel plankton, though pre- 

 vailing low levels and large proportion of rising levels tend to 

 reduce the actual volume contributed in this season. The fac- 

 tors operative in increasing the relatlm production in lake 

 waters in this season are the influx of sewage-laden river 

 water, and the decay of some of the succulent vegetation of 

 the lake and its re-submerged margins at a season of plank- 

 ton pulses of an amplitude increasing by virtue of other fac- 

 tors, internal or external. Rising levels also bring about an 

 increase in current in the channel, while marked changes 

 in the bacteriological and chemical condition of channel, 

 waters attend this and the fall in temperature. The com- 

 bined effect of these factors, as shown by a comparison of the 

 records of 1897 (PI. XI.) — when low levels continued and the 

 autumnal decline in temperature was late — with those of other 

 years, is to depress production in channel waters more than it 

 falls in the lake. This fact, together with the increase in the 

 impounding function of the latter as levels rise, suffices to 

 bring about the increased relative production in lake waters in 

 the closing months of the year. 



