386 



of the year reduces greatly the value of the data for such pur- 

 poses or for comparison with other localities. 



The hydrographic conditions are such in 1896 that this lake 

 maintains, throughout, a connection with the river. This is 

 owing to the relative absence and brief duration of low levels, 

 the run-off not being completed before a new invasion occurs 

 as a result of a recurrent flood. Since falling levels prevail 

 during more than two thirds of the year, a run-off from the 

 lake continues during this portion of the time at least. The 

 lake is therefore in this year a factor in the determination of 

 production in channel waters, whose continuity is broken only 

 when levels are such that no waters are draining off from the 

 lake or passing through it during general overflow — which is the 

 case in less than one fourth of the time. The average produc- 

 tion in the lake for 1896 (13.83) is almost twelvefold greater than 

 that in the stream (1.16), and the monthly averages also (see 

 table between pp. 342 and 343) are from 2| to 218 times greater, 

 while individual collections in the lake in all Imt three in- 

 stances exceed coincident or approximate ones in the river. The 

 exception on July 30 occurred, when the invasion of flood water 

 was followed, as is usually the case in midsummer in vegetation- 

 rich backwaters, by a semi-stagnation with great development 

 of OsciUar'ui, and the formation of considerable gas with a strong 

 odor of H-iS beneath the felt of Osrll/aria which covers the bot- 

 tom. Under these presumably abnormal conditions the plank- 

 ton content reached a lower level in the lake (1.62) than in 

 the river (3.90), and this was at a time of influx rather than 

 outflow of water. With the above exceptions the lake at all ob- 

 served seasons contains a richer plankton than the channel, 

 which its run-off directly enters, and under similar hydrographic 

 conditions we are justitied in predicting at other times a similar 

 relationship, though the exact ratio of production would proba- 

 bly vary according as the vegetation by its growth or decay 

 affected the fertility of the water. 



In the absence of any satisfactory basis for determining 

 the amount of the run-off from this lake, a quantitative expres- 



