411 
This is the time of greatest contrast, and also the time of 
highest levels (averaging 9.44 ft.) and therefore of continuous 
and largest run-off. It is also the season of largest plankton 
production, averaging 10.42 em.’ per m.’,—S.4 times that in 
channel waters,—and is accordingly the period of greatest en- 
richment of the channel plankton by the run-off from the lake. 
The factors operative in producing this result are the high 
levels, with resulting increase in the impounded waters of the 
lake at a season of rising temperatures favorable to plankton 
production and to the enrichment of the waters by decay of 
the vegetation of the previous year’s growth. 
In June-September we have a period of falling levels, 
maximum temperatures, lowest water, and growth predominat- 
ing over decay in the aquatic vegetation, which is relatively 
more abundant in the lake than in the river. It is therefore 
the season of greatest predominance of local environmental 
factors, and of run-off reduced to minimum yolume and fre- 
quently interrupted. It is also the season of least plankton 
production, averaging 6.74 em.’ per m.*—only 1.54-fold that in 
channel waters. The midsummer season is therefore one of 
least enrichment of channel plankton, as a result of both the 
decreased and interrupted run-off and the decrease in the relative 
production in the lake. This latter feature results both from 
the decline in production in the lake and the low-water condi- 
tions in the river, where increased fertilization by sewage and 
slackened current tend to raise its level of production at this 
season. Other factors tending to bring about the conditions of 
production prevalent in this season are possibly the greater 
relative exhaustion of the fertility in lake waters during the mid- 
summer and low water, as indicated in our chemical analyses 
by the generally lower level of the various forms of nitrogen in 
the lake than in the river. (Cf. on this point Plates XLV. and 
L.) This greater relative exhaustion maybe attributed in part 
only to reduced interchange of river and lake waters at low levels 
and consequent reduction in influx of sewage from the channel, 
and tothe utilization of some of the constituents which support 
