362 



in the channel plankton. The amplitude attained in the lake 

 is, however, but slight. 



The explanation of this marked decrease in production in 

 this year as compared with that of 1896 lies, I believe, in the 

 hydrographic conditions of the two years. In 1896 (PI. XXVII.) 

 the average height of the river for the period August-Decem- 

 ber is 5.89 ft., while in 1897 it is only 2.47 ft. The impounding 

 action of the lake was at its minimum, and there was present 

 in it in these months of 1897 only about a third the quantity of 

 water that it contained in the corresponding season of 1896, 

 and this consequently gave to vegetation in 1897 a relatively 

 greater predominance in the lake, and also made possible a 

 more frequent renewal of lake water by the contributions from 

 the creek and tributary springs, thus cutting down the time for 

 breeding. Both of these factors tend to limit plankton produc- 

 tion. We find, accordingly, that the lake produces on an av- 

 erage from August to Deceml^er but .1 cm.^ per m.^ to 4.0 in the 

 adjacent river, that is, only a fortieth of the plankton content 

 of the stream. The contrast between the lake and the river in 

 this year is heightened by the fact that owing to low water and 

 increased fertilization by sewage the production in the river is 

 much greater than usual. 



A comparison of Plates XI. and XXVIII. will reveal the 

 fact that in only 5 instances out of 24 in 1897 does Quiver Lake 

 contain more plankton than the river. These instances in Feb- 

 ruary and April attend impounding action of the lake when it 

 is not differentiated from overflowed bottom-lands as a separ- 

 ate unit of environment, while those of June 28 and July 21 

 are caused by the flushing of channel waters by floods from 

 which the lake is exempt. 



As a whole for this year, the net result of the contributions 

 from Quiver Lake is a dilution of the channel plankton with 

 which it mingles. Basing calculations upon the yearly aver- 

 ages and areas of the drainage basins, the quantitative effect 

 would be a decline in the plankton content of channel waters 

 from 3.69 to 3.65, or a loss of 1.1 per cent. 



