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 cm. 3 per m. 3 ,— 8.4 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 volume and fre- 

 quently interrupted. It is also the season of least plankton 

 production, averaging 6.74 cm. 3 per m. 3 — 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 to the utilization of some of the constituents which support 



