76 



there are three rises to overflow stages (Pt. I., PL XII. and XIII.) 

 with intervening declines of a month's duration. There is a pulse 

 of Dinobryon in each of these periods of declining flood. The pulse 

 of 275,200 on December 20 follows the November flood, and it is 

 followed by a minimum of 1,500 on the rising flood of January 10. 

 The numbers slowly increase until a meteoric rise on February 7 

 to 6,486,700 and on February 14 to 22,621,440 is followed again by 

 another decline, to 25,920 on February 28, with the sudden flood of 

 that week. During the .maximum flood stage in March (Pt. I., PL 

 XIII.) no Dinobryon was recorded, but it reappeared again on 

 March 21. The suspension of our plankton operations interrupted 

 the further tracing of the fluctuations. 



From the facts above detailed it is very evident that the pulses 

 of Dinobryon occur in channel waters at times when the run-off of 

 impounded backwaters is making its greatest contribution to the 

 river plankton. These are times of greatest stability of the en- 

 vironment in all respects save river level and its sequences. The 

 impounded waters have come from regions of slight current and 

 decaying vegetation, and there has been time in those localities for 

 the decay of sewage and debris, and for the growth of planktonts 

 such as Dinobryon. These conditions of the environment are 

 therefore favorable for the growth pulses of Dinobryon. The 

 phenomenon of pulses of growth is not, however, to be considered 

 as merely the result of declining floods. These afford a favorable 

 environment and doubtless determine within certain limits the 

 time and the extent of the pulse. The phenomenon is one common 

 to most plankton organisms, and occurs in Dinobryon of lakes where 

 floods are of little significance. 



Any evidence of recurrent minor pulses in Dinobryon at brief 

 intervals is lacking. 



Dinobryon has been found in our plankton through practically 

 the whole range of temperatures, but it disappears when maximum 

 summer heat is reached and does not return until the water cools 

 to 45 or lower. Large pulses, such as that of February 21, 1899 

 (22,621,440), have developed at temperatures approximating 32, 

 and largely under the ice. The vernal pulse of April-May has been 

 recorded at temperatures ranging from 60 to 79, but generally 

 nearer the former. No well-defined optimum temperature appears, 

 and the seasonal distribution suggests that the high temperatures 



