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 percent, 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 Asterionlla 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 its culmination. The composition 

 of the plankton favors the inference that a temporary exhaus- 

 tion of the food of the phytoplankton and zooplankton 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., PI. 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 

 (PI. XI.) in river levels the run-off from the lake soon ceased, 



