211 



graphs on PI. XLVIIL and XLIX.) the lake receives in addition 

 to the drainage of its own catchment-basin some access of 

 flood waters from the bottom-lands above and from the adja- 

 cent river. The water along the eastern shore, even in flood 

 conditions, is " lake " rather than river water, as a comparison 

 of the plottings of the analyses of water from the two sources 

 clearly demonstrates. Our collections of plankton and water 

 samples were taken within or near this belt of lake water, in 

 which contamination by flood waters was not usually noticea- 

 ble. Compare in this connection the chlorine curve of the 

 river and lake (PI. XLV. and XLIX.). To some slight extent, 

 then, the analyses pertain to two sources : to the waters of 

 overflow, largely belonging to the colder months and period of 

 the maximum of nitrogenous substances ; and to the waters of 

 a spring-fed lake, delimited during the period of low water and 

 of the minimum of nitrogenous substances. The data at hand 

 do not cover low-water conditions during a "maximum" period, 

 which might give evidence of a seasonal cycle in chemical 

 conditions in this lake independent of the river overflow. From 

 conditions elsewhere it seems probable that such a cycle does 

 occur here also, though the overflow and probable contamina- 

 tion may serve here to heighten somewhat the contrast be- 

 tween the maximum and minimum periods of the seasonal 

 cycle. 



In the autumn months of 1896 and 1897 covered by the 

 analyses, the rise in nitrates only is indicated (PI. XLVIIL), the 

 summer minimum continuing through the low-water period 

 of autumn. 



In 1898 and the first three months of 1899 (PI. XLIX.) the 

 period of maxii^um, November to May, is well distinguished 

 from that of the minimum. May to November, and not only in 

 the nitrates but to some extent also in all of the other substan- 

 ces, appearing most clearly in the free and albuminoid ammonia 

 and the organic nitrogen. 



As in the Illinois River, so here also the spring maximum 

 of the plankton (PI. XLIX.) comes at the close of the period 



