Factors Determining Vertical Distribution. 421 



regulating factor, since the amount of food in that level in au- 

 tumn is more than sufficient to support the total crustacean 

 population. 



Temperature. 



Temperature may be considered under three heads: (1) the 

 rise and fall of the average temperature of the water from 

 spring to late autumn, (2) the diurnal variation of temperature, 

 (3) the vertical distribution of temperature. 



I have not been able to discover that the warming or cooling 

 of the water in spring or fall affects directly the vertical dis- 

 tribution of any species except Daphnia pulicaria. The move- 

 ments of this species are undoubtedly determined by the rise or 

 fall of the general temperature of the water. It is a sub-ther- 

 moclinal species in plankton-poor lakes and in summer it keeps 

 as near as possible to the cool water in lake Mendota. 



The diurnal variation of temperature has no noticeable direct 

 effect on vertical distribution. 



The most striking fact in the vertical distribution of temper- 

 ature is the formation in the lake during summer of the thermo- 

 cline which forms the lower limit of the Crustacea from July on. 

 The Crustacea follow accurately the position of the thermo- 

 cline. This layer has a vertical oscillation of two or even three 

 meters, being affected by the direction of the wind. In every 

 case the lower limit of the Crustacea oscillates with the posi- 

 tion of the thermocline and follows it downward as it gradually 

 descends during the summer. 



The statement made in my former paper (Birge, '95, p. 481) 

 that "during July, only the upper twelve meters are tenanted by 

 Crustacea, and over ninety per cent, are in the upper nine 

 meters" should be modified so as to read, that ninety-five per cent. 

 or more of the Crustacea are found above the thermocline, which 

 in July is situated from nine to twelve meters below the sur- 

 face. Yet, close as is this correspondence between Crustacea 

 and thermocline, the temperature is not the fact which limits 

 their downward extension. This will be shown under the next 

 head. 



I have no doubt, however, that the thermocline is always an 



