471 



ruary and is continued well into the early summer. Thus, 

 while in 1896 and 1897 the vernal pulses of plankton production 

 (9.89 and 5.62) are not large, in 1898 the production in this 

 season rises to 35.68. Late high water, with decay and solution 

 of organic substances increased by higher temperatures, occurs 

 at the season of rapid plankton increase, and food matters 

 which run off in the winter floods are here utilized and increase 

 the amplitude of the plankton pulse. Winter floods thus tend 

 to locally impoverish the plankton, and spring floods to in- 

 crease it. 



Enough has been said to indicate the supreme importance 

 of hydrographic conditions in the fluviatile environment in de- 

 termining the amplitude of plankton production and in differ- 

 entiating local. areas in our environment. It is the prime fac- 

 tor which distinguishes the fluviatile from the lacustrine envi- 

 ronment, stamping the former with an instability as character- 

 istic of the river as stability is of the lake. 



TEMPERATURE AND PLANKTON PRODUCTION. 



On pages 168-177 will be found a discussion of the temper- 

 ature conditions at the various plankton stations and their gen- 

 eral relations to the larger phases of plankton production. In 

 the present connection the more detailed comparison will be 

 made. 



To facilitate this comparison of production and tempera- 

 ture conditions I have prepared the accompanying table (see 

 p. 472), which gives the monthly means of production and of sur. 

 face temperatures recorded at the times of collection in the 

 river. 



This table in conjunction with the one following page 342 

 suggests the cooperation of temperature in controlling in a large 

 way the seasonal fluctuations in production. In general, in the 

 colder months less plankton is produced than in warmer months. 

 Thus in the river the mean production in the 5 months below 

 45° is but little more than 9 per cent, of that in the 7 months 

 above this temperature, in Phelps Lake, only 40 per cent., and 



