317 



data accordingly afford opportunity to trace the cyclic move- 

 ment in production in this period. 



The Jiili/ 2)nfseh'ds a duration of 32 days, — from June 28 to 

 July 30, — with a maximum amplitude of 8.16 cm.^ per m.^ on 

 the 14th. Its mean falls on the 20th. This was a month of 

 falling river levels with slight interruptions by local rains, of 

 rising temperature and of falling nitrates, but of increasing 

 sewage contamination (PI. XLIV. ) as shown by the rising chlo- 

 rine and oxygen consumed. The pulse presents a very sud- 

 den drop in production from 6.40 cm.'' on the 16th to .92 cm.^ on 

 the 21st, followed by an immediate recovery to 6.91 on the 

 23d. I am at a loss for a satisfactory explanation of this fluc- 

 tuation. There is no change in levels at Havana (see page 

 160) which suggests flood, though there is a slight increase in 

 turbidity (Table III.) and was a rise of .1 ft. at Copperas 

 Creek on the 20th which does not appear in the Havana gage 

 readings. The chemical analysis of the sample taken on the 

 21st (Table X. and PL XLIV.) contains evidence of some dis- 

 turbance in conditions. There is a sharp decline in nitrates, 

 nitrites, and oxygen consumed, with a check in the rising 

 chlorine, while free and albuminoid ammonia and total or- 

 ganic nitrogen move upwards. Had the oxygen consumed 

 risen and the free ammonia fallen, all indices would point to- 

 ward access of recent storm water carrying silt into the stream 

 and locally diluting the plankton, though not materially af- 

 fecting the hydrograph. In any event the fluctuation in pro- 

 duction is correlated with a localized disturbance in chemical 

 conditions suggesting in some particulars restricted access of 

 recent storm waters. 



The average production for this month (4.69) is higher 

 than in any other year excepting 1894 and 1895, due it seems 

 to the somewhat stable conditions of continued decline in lev- 

 els, with slight overflow sufficiently prolonged (3 weeks at 6 ft.) 

 to afford time for breeding plankton in the waters of overflow, 

 though apparently similar floods in 1896 resulted in much light- 

 er production (PI. X.). The main difference in hydrographic 



