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 (Pl. XX VIL.) 
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 abouta 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 ay- 
erage from August to December 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 andthe 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. Theseinstances 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. 
