320 



materials attending low water. This is the month of greatest 

 production in 1897, and also the one of lowest unutilized ni- 

 trates, the latter not exceeding 1 part per million during the 

 month. It is in the rising chlorine (PI. XLIV.) that we have 

 a suggestion of the degree to which sewage has made contribu- 

 tion to the stream. The unutilized nitrates do not of course 

 afford a measure of its quantity. 



The October pulse has a duration of 42 days, — from Sept. 21 

 to Nov. 2, — with a maximum amplitude of 12.92 cm.^ per m.* on 

 the 5th. Its mean also falls on the 5th, 26 (25) days after that 

 of the preceding pulse. This extended volumetric pulse is found 

 to include within its limits two of the pulses of chlorophyll- 

 bearing organisms (cf. PI. LII.),one culminating Sept. 29 and 

 the other Oct. 19. The month was one of continued hydro- 

 graphic stability. The total movement in levels was only .5 ft., 

 due mainly to the check in evaporation resulting from decline 

 in temperatures. The temperatures in this month average 

 about 64.5°, which is 6° to 8° higher than the average in other 

 years of our records. The difference between the extremes is 

 only 17° as compared with 23° in 1896 and 27° in 1898. The 

 autumnal decline in this year has come later and progressed 

 less rapidly, at least till the last ten days of the month (PI. XL), 

 than is usually the case. The curve of the October, as well as 

 that of the September, pulse is delimited on either side by 

 declines in temperature. 



The chemical conditions in this month (PI. XLIV.) are less 

 stable than in September. The nitrates and nitrites move in- 

 versely with the plankton, and both chlorine and free ammonia 

 ascend rapidly to unusual heights, suggesting the presence of 

 sewage in which decay had not yet progressed as far as was 

 wont during warmer weather. This is doubtless the result of 

 the Peoria sewage pulse, which as winter approaches extends 

 down stream toward Havana, and under the stable low-water 

 conditions of 1897 appears in exaggerated form. 



In view of the stable conditions, excessive fertilization by 

 sewage, and abnormally high temperatures, it is not surprising 



