389 
to this rise, and consist in a fall of 2.2 ft. in channel waters 
though the depth at the station of collection changes only 1.6 
ft.—equivalent to a reduction in volume of 25 per cent. at the 
point of collection and 30-40 per cent. in the lake as a whole. 
It thus involves a considerable and rapid run-off of the rich 
plankton developed in these impounded waters. This factor 
alone is, however, quite insufficient to account for the total loss 
in plankton content in this period. Another factor which is 
correlated with this reduction in the plankton content is the 
increasing occupancy of the lake by vegetation. The decline 
in levels hastens the emergence of the emergent forms and in- 
creases the relative occupancy by submerged and floating spe- 
cies, while the vernal growth in all during these three weeks in 
May, more than any other factor, transforms the broad expanse 
of open water into a vegetation-clogged marsh in which but few 
stretches of open water are visible. This phase of the growth 
of the grosser forms of the aquatic flora robs the water of some 
of its store of nutriment and cuts off the free access of light— 
both of which might interfere with the growth of the competing 
phytoplankton. Limnetic diatoms such as Asterionl/a and Melo- 
sira are the principal synthetic organisms building up this re- 
markable pulse, and the Cladocera, principally Bosmina and Chy- 
dorus,appear in numbers with itsculmination. The composition 
of the plankton favors the inference that a temporary exhaus- 
tion of the food of the phytoplankton and zoéplankton alike con- 
tributes to the sudden reduction in plankton content, while the 
additional and perhaps related factor of reproductive cycles may 
also have a large causal relation to the phenomenon. 
1897. 
(Table VII., Pl. XXXIV.) 
There are but 7 collections in this year, at approximately 
monthly intervals in January-July. Collections were suspend- 
ed on July 16, when decline in levels made access even to the 
foot of the lake by boat impossible. With the further decline 
(Pl. XI.) in river levels the run-off from the lake soon ceased, 
