173 



cline is a gradual one; in others, as 1895 and 1898, it is subject 

 to some irregularities. With December the winter minimum 

 returns, but with less persistence than in the months which 

 follow, flood waters at this season bringing their higher tem- 

 peratures. 



The annual temperature cycle thus falls into four periods: 

 one of minimum and quite constant temperatures, including 

 December, January, and February, and a varying portion of 

 March ; one of maximum and more fluctuating temperature, 

 approaching 80° and extending, with some interruptions, from 

 the early part of June till about the middle of September; and, 

 separating these, the two shorter intervals of change. The 

 period of increase in temperature, which is also one of rapid 

 change and increase of the plankton, includes the latter part 

 of March and the months of April and May. The period of de- 

 cline, which is sometimes more abrupt than the spring rise, as 

 in 1895, 1897, and 1898, extends from the latter part of Septem- 

 ber until the end of November. This is also a period of change 

 and of frequent but not universal diminution in the plankton. 

 The average temperature for the years, as expressed approxi- 

 mately in the table, is 57.08°. This point is passed about the 

 middle of April and again about the middle of October with 

 considerable regularity. Since, however, these dates both lie 

 in periods of rapid change, the average temperatures are of 

 much less duration than the more extreme ones. The existence 

 of these well-defined periods of maximum, minimum, increase, 

 and decline of temperatures affords the basis for corresponding 

 seasonal changes in the minute life of the water as fundamental 

 and extensive as those which affect the plant and animal life of 

 terrestrial and aerial environment. This subject of the rela- 

 tion between temperature and organisms of the plankton will 

 be fully discussed in connection with the statistical study of 

 .their seasonal distribution. 



A comparison of the thermographs (PI, VIII.-XIII.) of the 

 different years and an inspection of the table on page 171, 

 reveal but few significant annual differences. The spring rise 



