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 levels, in fact, averaging only 4.22 
ft.—a level insufficient to provide for any current through the 
lake or any considerable discharge in periods of decline. It is 
thus a season of shght and interrupted run-off. It is, however, 
a period of increased production, reaching 10.64 in October, 
declining to 3.08 in December, with an average of 6.70—a trifle 
below that of the midsummer period. Its relation to channel 
production changes decidedly, rising 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 relative 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 (Pl. 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. 
