466 



the total movement (51.9) of 1895 and add it to that of 1896 

 (45.7), the resulting figures (42.1 and 55.5) will more nearly 

 characterize the hydrographic conditions in which our plank- 

 ton collections were made. After making allowances for the 



AVERAGE CHANNEL PLANKTON AND ACCOMPANYING MOVEMENT IN RIVER LEVELS. 



Year 



1894. 



1895, 



1896. 



1897. 



1898. 



Total movement in river 

 levels — in feet 



32.9 



5i.9(42.i)t 

 45-7(55-5)t 

 44.8 

 67.2 



Plankton per m.' 

 — in cm.3 * 



2.53 

 5-9' 

 1.05 

 3.28 

 2.03 



*Mean of monthly averages. tAfter transferring December rise to 1896. 



fact that our plankton averages imperfectly represent the 

 actual production, it is still apparent that plankton production 

 (per m.'') is inversely proportional, in a varying ratio, to the 

 extent of the total movement in river levels. 



In not a few of the years (PI. VII.) the hydrograph falls 

 into two periods dominated respectively by high- and by low- 

 water conditions. This is seen typically in 1897, and with less 

 contrast in 1894, 1895, and 1898, while in 1896 the recurrent 

 floods obliterate almost all traces of such a division. In their 

 totality these two hydrographic extremes present strong con- 

 trasts which must bear important relations to the plankton 

 production. They are, moreov^er, so involved with other fac- 

 tors that no simple analysis of their results seems possible. For 

 example, the high-water period is predominantly, as a rule, of 

 low temperature, considerable ice, dilution of sewage, decreased 

 light, increased access of recent water, increased current, de- 

 creased occupancy by, and lessened growth of, aquatic vegeta- 

 tion, and of greatly increased area and intimate and free con- 

 nection with reservoir backwaters. The low-water period, on the 

 other hand, is one of higher temperatures, no ice, concentration 

 of sewage, increased light, reduced volume of tributary water, — 

 which itself is of less recent origin than at high water, — slack- 

 ened current, greater domination by, and growth of, aquatic 

 vegetation, and of greatly contracted area and restricted con- 



