319 



this may be detected in the slight wave of the chlorophyll- 

 bearing organisms (PL LII.), which in comparison with the 

 wave of July and September is indeed diminutive. There is, 

 however, nothing in the chemical data to explain this suppres- 

 sion of the pulse of the chlorophyll-bearing organisms. 



The average production for this month as a whole, 3.65 

 cm.^ is large, though not so great as that of 1894 (9.67) or 1895 

 (4.03), both, like 1897, with stable conditions. It is, however, 

 greatly in excess of the production in 1896 (1.12) and 1898 (.91), 

 when August floods flushed out the stream. 



The September pulse, on the other hand, is very well de- 

 fined. It has a duration of 35 (28) days, — from Aug. 17 (24?) 

 to Sept. 21, — and a maximum amplitude of 19.80 cm.' per m.^ 

 on the 14th. Its mean falls on the 9th (10th), 30 (27) days 

 after that of the preceding pulse. As a whole, this is a month 

 of great stability. The total movement of the hydrograph is 

 only .4 ft. — the smallest monthly movement recorded at Ha- 

 vana during the years of our operations, and but rarely sur- 

 passed in many years at Copperas Creek (Table I.). Most of 

 this movement — Sept. 3-10— was due to the flash-boards placed 

 upon the LaGrange dam. The chemical conditions also are 

 in the main remarkably uniform, the only aberrant movement 

 being the constant upward tendency of the chlorine (PI. LII.) — 

 an index of the increasing proportion of sewage in the stream. 

 There is, however, no proportional increase in the various forms 

 of nitrogen, though they all exhibit a slight upward movement. 

 The temperature is sustained at summer heat (80°+) till the 

 middle of the month, dropping 10° in the third week. 



The plankton, however, exhibits considerable fluctuation, 

 rising to a maximum of 19.80 cm\ per m.^ on the 14th, and fall- 

 ing again to 3. on the 21st, at the rate of 2.4 cm.^ per day. This 

 is the maximum record for this year, though, it may be, not 

 equaling the undetected vernal maximum. It is also the lar- 

 gest volume recorded in any year after the first week in July. 

 Contributing causes for this exceptional development are to be 

 found in the stable conditions and concentration of fertilizing 



